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Accepted by Journal of Oceanography (Review) 1

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Fifty years of the 137ºE repeat hydrographic section in the western North Pacific Ocean 4

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Eitarou Oka1*, Masao Ishii2,3, Toshiya Nakano3,2, Toshio Suga4,5, Shinya Kouketsu5, 7

Masatoshi Miyamoto1, Hideyuki Nakano2, Bo Qiu6, Shusaku Sugimoto7, Yusuke Takatani3 8

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1Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa 277-8564, Japan 10

2Oceanography and Geochemistry Research Department, Meteorological Research Institute, Tsukuba 305-0052, 11

Japan 12

3Global Environment and Marine Department, Japan Meteorological Agency, Tokyo 100-8122, Japan 13

4Department of Geophysics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan 14

5Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 15

Yokosuka 237-0061, Japan 16

6Department of Oceanography, University of Hawaii at Manoa, Honolulu, HI 96822, USA 17

7Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan 18

*Corresponding author. E-mail: eoka@aori.u-tokyo.ac.jp; Tel: +81-4-7136-6042; Fax: +81-4-7136-6056 19

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Keywords: 137ºE section, Western North Pacific, Repeat hydrography, 21

Physical oceanography, Biogeochemical Oceanography, Biological Oceanography, 22

Marine pollution, Seasonal to decadal variability, Long-term change 23

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December 2017 25

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Abstract 27

The 137ºE repeat hydrographic section of the Japan Meteorological Agency across the 28

western North Pacific was initiated in 1967 as part of the Cooperative Study of the Kuroshio 29

and Adjacent Regions and has been continued biannually in winter and summer. The 30

publicly available data from the section have been widely used to reveal seasonal to decadal 31

variations and long-term changes of currents and water masses, biogeochemical and 32

biological properties, and marine pollutants in relation to climate variability such as the El 33

Niño−Southern Oscillation and the Pacific Decadal Oscillation. In commemoration of the 34

50th anniversary in 2016, this review summarizes the history and scientific achievements of 35

the 137ºE section during 1967−2016. Through the publication of more than 100 papers over 36

this 50-year span, with the frequency and significance of the publication increasing in time, 37

the 137ºE section has demonstrated its importance for future investigations of physical- 38

biogeochemical-biological interactions on various spatiotemporal scales, and thereby its 39

utility in enhancing process understanding to aid projections of the impact of future climate 40

change on ocean resources and ecosystems over the 21st century. 41

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1. Introduction 44

Growing emissions of anthropogenic carbon dioxide (CO2) and other greenhouse gases 45

are causing warming, acidification, and deoxygenation of the ocean globally (IPCC 2013). 46

To understand the physical and biogeochemical changes and variations in the ocean as well as 47

their controlling mechanisms and impacts/feedbacks on climate and ecosystems, it is 48

imperative that the oceanographic community continues observing the ocean for a prolonged 49

period of time through multidisciplinary approaches. The 137ºE repeat hydrographic section, 50

which has been maintained by the Japan Meteorological Agency (JMA), marked its 50th 51

anniversary in 2016. It traverses the Kuroshio, the subtropics, and the tropics in the western 52

North Pacific between Japan and New Guinea (Fig. 1). There are other long-term time series 53

stations and lines such as Station P/Line P in the eastern subarctic North Pacific (Whitney and 54

Freeland 1999), the Hawaii Ocean Time-series program in the central subtropical North 55

Pacific (Karl and Lukas 1996), and the Bermuda Atlantic Time-series Study in the western 56

subtropical North Atlantic (Michaels and Knap 1996). However, none of them is 57

comparable to the 137ºE section in terms of the meridional and vertical extent and the variety 58

of measured variables (Bingham et al. 2002). 59

The 137ºE section was initiated in 1967 as part of the Cooperative Study of the Kuroshio 60

and Adjacent Regions (CSK) supported by the Intergovernmental Oceanographic Commission, 61

UNESCO under the leadership of Dr. Jotaro Masuzawa (Photo 1), who was a Senior 62

Scientific Officer of the JMA in 1956−1971 and the Director-General later in 1980−1983 63

(Masuzawa 1967; Kuroda 2017). Enlightened by Prof. Raymond B. Montgomery during his 64

stay at the Johns Hopkins University in 1962−1964 (Masuzawa and Nagasaka 1975), 65

Masuzawa aimed to “establish an observational framework for ocean monitoring, at least in 66

the western North Pacific as the Japanese area of responsibility”, “in order to investigate 67

common variability in phenomena on as a large scale as possible” (Masuzawa 1978). 68

Impressively, his foresight and vocation emerged long before the 1980s, during which time 69

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the world shared the recognition that it is of crucial existential importance for human society 70

to accurately understand the changes in the ocean that play an essential role in regulating 71

climate and ecosystems, followed by the establishment of the Global Ocean Observing 72

System in 1991. Masuzawa chose the 137ºE meridian “to observe major currents and water 73

masses as undisturbed by islands and submarine mountains as possible” (Mazuzawa 1967; 74

Fig. 1). It is worth noting that at that time the research community had not identified or 75

described some of the major currents/water masses appearing in the 137ºE section, such as the 76

Subtropical Countercurrent (STCC; Uda and Hasunuma 1969), the Subtropical Mode Water 77

(STMW; Masuzawa 1969), and the North Subsurface Countercurrent (NSCC; Tsuchiya 1975). 78

In addition, while global warming as a result of anthropogenic CO2 emission and its partial 79

absorption by the ocean had been surmised (Revelle and Suess 1957), the impact of the 80

resultant carbonate system changes in seawater on marine ecosystems as well as the decline of 81

dissolved oxygen (O2) had not been recognized. 82

The first historical survey of the 137ºE section was conducted in January 12−24, 1967 83

over a distance of 3860 km between 34ºN and 0º45’S in the first cruise of R/V Ryofu-maru II 84

(Mazuzawa 1967). Since then, the observational program has not been interrupted for more 85

than 50 years, in particular during the earlier period when little scientific achievement was 86

made (Fig. 2). A decade after the launch, Masuzawa (1978) expressed his concern. “There 87

has been always a doubt in my mind about what we can reveal from yearly observations, even 88

for examining large-scale, long-term variations.” “Although the section has weathered 89

hardships for more than 10 years, my concern that it can be all pain and no gain has not gone 90

away. While I secretly have an irreverent idea that the value of the section should be judged 91

in 30 years, the responsibility of planning still weighs heavily on my shoulders.” Despite his 92

doubt, more than 100 papers using the 137ºE data have been published during the past 50 93

years (supplementary Table S1) with increasing frequency (Fig. 2) and with increasing 94

significance. Among them, seven (Midorikawa et al. 2005, 2010; Nakano et al. 2007; Ren 95

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and Riser 2010; Ishii et al. 2011; Purkey and Johnson 2012; Qiu and Chen 2012) were quoted 96

by Chap. 3 of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment 97

Report on ocean observation (Rhein et al. 2013). For the half-century-long contributions to 98

the advancement of marine science through monitoring and data management, the 9th POMA 99

Award was given from the North Pacific Marine Science Organization (PICES) to the 137ºE 100

section in 2016. 101

In this review, we summarize scientific achievements accomplished through analyses of 102

the 137ºE data. In addition to these studies, many others have used the data in combination 103

with other observations for various targets, e.g., to investigate surface thermohaline variability 104

in the tropical Pacific (Ando and McPhaden 1997), to estimate global air-sea fluxes of CO2 105

(Takahashi et al. 1997 and its updates), and to calculate nitrate transport by the Kuroshio from 106

the East China Sea to the south of the Kuroshio (Guo et al. 2013). Modelling studies have 107

also used the 137ºE data for initial conditions and for validations, particularly in recent years 108

(e.g., Luo and Yamagata 2003; Fujii et al. 2009; Shibano et al. 2011; Wada et al. 2011; Yara et 109

al. 2012; Qiu et al. 2014b; Nishikawa et al. 2015; Chen et al. 2016; Tseng et al. 2016; Toyoda 110

et al. 2017). 111

Section 2 introduces the observation history of the 137ºE section. Section 3 describes 112

the typical distribution of currents and water masses in the 137ºE section. Sections 4 and 5 113

present scientific achievements in physical and biogeochemical oceanography, respectively. 114

Section 6 demonstrates achievements in the other research fields ― specifically, in terms of 115

biological properties and marine pollution. Section 7 summarizes the achievements from the 116

137ºE section, evaluates the extent to which Dr. Masuzawa’s aims were attained, and 117

discusses implications for future research and directions. 118

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2. Observation history 120

In the earlier years, the 137ºE section was visited from 34ºN to 1ºS in the latter half of 121

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January (the time period and southward progression of winter observations have been 122

unchanged for 50 years) with a typical station interval of 40’ north of 32ºN, 1º at 8−32ºN, and 123

30’ south of 8ºN (Masuzawa 1967, 1968; Akamatsu and Sawara 1969; Masuzawa et al. 1970; 124

Nagasaka and Sawara 1972; Shuto, 1996; Fig. 3). At each station, temperature (T) was 125

measured using reversing thermometers and water samples were collected by Nansen bottles 126

(Photo 2) down to 1250 m depth, except for 4000 m at every 5º between 30ºN and the equator. 127

Depth was determined by wire angle at depths less than 100 m and by the thermometric 128

method using a pair of pressure-protected and unprotected thermometers at depths greater 129

than 100 m. Water samples were analyzed for a wide variety of parameters, such as salinity 130

(S), O2, nutrients (Masuzawa et al. 1970; Sagi 1970), ammonia (Sagi 1969b), pH and total 131

alkalinity (Akiyama et al. 1968), calcium (Sagi 1969a), trace metals such as iron and 132

manganese, chlorophyll-a and pheophytin (Kawarada and Sano 1969), phytoplankton and 133

zooplankton (Kawarada et al. 1968), among others. In the equatorial region, current velocity 134

from the surface to 500 m depth relative to 600−900 m was measured by two TS-II (Roberts 135

type) current meters from the drifting vessel to supplement geostrophic calculations (Bessho 136

1995). Maritime meteorological observations were also carried out routinely, eight times a 137

day (Hayashi et al. 1968, 1970). 138

In 1972, in response to the domestic enforcement of Act on Prevention of Marine 139

Pollution and Maritime Disaster in the previous year, the JMA started the Observation for 140

Monitoring Background Marine Pollution targeting heavy metals (mercury and cadmium), 141

and made the 137ºE section biannual by adding summer observations around July1 (Sagi et al. 142

1974; Kamiya 2013; Fig. 3). The observations south of 3ºN had been continued until 143

summer 1986, but were discontinued in summer 1988; subsequently, the 137ºE section has 144

1 The Observation for Monitoring Background Marine Pollution was launched not only at the

137ºE section but also at a meridional section along 165ºE and five sections around Japan (PH,

PK, PM, PN, and PT). The five sections were named after the combination of the initial of

“Pollution” and that of the four Marine Observatories of JMA (Hakodate, Kobe, Maizuru, and

Nagasaki) and the headquarters in Tokyo (Ogawa and Takatani 1998)

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been occupied between 3ºN and 34ºN. A conductivity-temperature-depth profiler (CTD) 145

mounted with Niskin bottles and a bottom-mounted acoustic Doppler current profiler (ADCP) 146

were introduced in summer 1988, enabling high-resolution profiling of T and S with respect to 147

pressure, water sampling at precise depths, and measurements of subsurface current velocity 148

(Shuto 1996). 149

As a number of the hydrographic sections of the JMA in the western North Pacific were 150

selected as the repeat sections of the World Ocean Circulation Experiment Hydrographic 151

Program (WHP) during 1990−1994, the JMA further improved its observation skill while 152

coordinating these sections and carried out the 137ºE section in summer 1994 as the WHP P9 153

one-time section, which was characterized by full-depth, high-resolution, and multi-parameter 154

observations (Kaneko 1998; Kaneko et al. 1998). The standard observation depth of the 155

regular 137ºE section extended down to 2000 m in summer 1995 in conjunction with the 156

launch of R/V Ryofu-maru III (Kaneko 2002; Fig. 3). The observations in the Kuroshio 157

region were further augmented in summer 2001, resulting in the present station interval of 20’ 158

north of 31ºN, 30’ at 30−31ºN, and 1º south of 30ºN. In addition to the winter and summer 159

observations shown in Fig. 3, those in spring and fall were conducted between 1992 and 2009 160

(e.g., Qiu and Chen 2010a, 2012), although the observations by R/V Keifu-maru I before 161

2001 had some shortcomings, such as lower-quality S data, larger station spacing, and smaller 162

observational depth range. 163

To develop a globally coordinated network of sustained hydrographic sections after the 164

completion of WHP, the Global Ocean Ship-based Hydrographic Investigations Panel 165

(GO-SHIP), which is now part of the Global Climate Observing System and the Global Ocean 166

Observing System, was established in 2007 with distinguished coordinating services by the 167

International Ocean Carbon Coordination Project and the Climate Variability and 168

Predictability Program (CLIVAR) (Hood et al. 2009). After conforming to the higher data 169

quality standard of GO-SHIP, the JMA launched high-accuracy hydrographic observations by 170

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R/Vs Ryofu-maru III and Keifu-maru II in spring 2010, and conducted P9 revisit observations 171

in summer 2010 and summer 2016 (Kamiya 2013) covering all the level-1 variables of 172

GO-SHIP (http://www.go-ship.org/DatReq.html). In the regular 137ºE section cruises with 173

longer station intervals and limited sampling depths, most of level-1 variables including many 174

of Essential Ocean Variables for physics and biogeochemistry (http://www.goosocean.org/) 175

have been measured. Since summer 2010, dissolved O2 profiles have been collected 176

continuously by a rapid-response optical sensor RINKO-III (JFE Advantech Co., Ltd.) 177

mounted on the CTD and reported after calibrating with the dissolved O2 data from Winkler 178

titration method (Uchida et al. 2010; Sasano et al. 2011). 179

The data obtained by shipboard observations of the JMA, including measurements along 180

the 137ºE section, are publicly available online (http://www.data.jma.go.jp/gmd/kaiyou/db/ 181

vessel_obs/data-report/html/ship/ship_e.php). These include the cruise summary, the station 182

information, and data of hydrography, CTD and expendable CTD, expendable and digital 183

bathythermograph, water sampling, greenhouse gases, petroleum hydrocarbon and heavy 184

metals, floating pollutants, tar ball, maritime meteorology, and aerology. Among these data 185

sets, those from cruises that meet GO-SHIP criteria have been submitted to the CLIVAR and 186

Carbon Hydrographic Data Office (https://cchdo.ucsd.edu/). All datasets from cruises with 187

ocean interior high-precision carbonate system measurements (Sect. 5) have also been stored 188

in the Pacific Ocean Interior Carbon Database (PACIFICA; Suzuki et al. 2013; 189

https://www.nodc.noaa.gov/ocads/oceans/PACIFICA/) and the Global Ocean Data Analysis 190

Project Version 2 (GLODAPv2; Olsen et al. 2016; https://www.nodc.noaa.gov/ocads/oceans/ 191

GLODAPv2/). Data of underway CO2 measurements in surface seawater since the early 192

1980s (Sect. 5) have also been available in the Global Surface pCO2 (LDEO) database 193

(Takahashi et al. 2017; https://www.nodc.noaa.gov/ocads/oceans/ 194

LDEO_Underway_Database/) and the Surface Ocean CO2 Atlas (SOCAT; Bakker et al. 2016; 195

https://www.socat.info/). 196

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3. Typical distribution of currents and water masses in the 137ºE section 198

The 137ºE section crosses the western part of the North Pacific subtropical gyre and the 199

North and South Pacific tropical gyres, and numerous zonal currents that are components of 200

these gyres (Fig. 1). The Kuroshio, which is the western boundary current of the North 201

Pacific subtropical gyre, usually flows eastward at 33ºN along the southern coast of Japan 202

(Fig. 4), but is frequently shifted offshore as far south as 30ºN forming a large-meander path 203

from the mid-1970s to the early 1990s (Qiu and Joyce 1992; Kawabe 1995; horizontal bars at 204

the bottom of each panel in Fig. 8). To the south of the Kuroshio exists weak westward flow 205

associated with the recirculation gyre of the Kuroshio, which has been often called the 206

Kuroshio Countercurrent. Farther south, the North Equatorial Current (NEC) flows 207

westward broadly between 25º and 7ºN. Its northern part north of 15ºN reaches a depth of 208

1000 m with a subsurface current core at 200−300 m, while the southern part south of 15ºN 209

has a thickness of 300−500 m and is strongest at the surface (Masuzawa 1967). Embedded 210

in the northern NEC between 18º and 24ºN, multiple subtropical fronts and the associated 211

STCCs flow eastward in the shallow layer (Masuzawa 1967; Kimura 1982), corresponding to 212

the northward shoaling of isotherms in the upper 200 m in contrast with the northward 213

deepening in the underlying layers (Fig. 7a). Due to baroclinic instability associated with 214

the reversal of the current direction between the STCC and the NEC, the STCC region 215

exhibits large mesoscale eddy variability, being second only to the Kuroshio Extension region 216

in the North Pacific (Qiu 1999; Fig. 5). At 2−7ºN, the North Equatorial Countercurrent 217

(NECC) flows eastward in the shallow layer above the thermocline (at ~200 m; Fig. 4). Its 218

core shifts southward with increasing depth and connects to that of the eastward flowing 219

NSCC in the subthermocline layers on the long-term average (Qiu and Joyce 1992; Gouriou 220

and Toole 1993), while the two cores are observed separately in some individual sections 221

(Guan 1986; Bingham and Lukas 1995; Fig. 6). 222

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Underneath the NEC and the NECC, the recently identified North Equatorial 223

Undercurrents (NEUCs; Qiu et al. 2013a) flow eastward at depths greater than 300 dbar at 5º, 224

9º, 13º, and 18ºN (Fig. 4b). The existence of quasi-stationary NEUCs across the tropical 225

North Pacific was first suggested by geostrophic velocity distributions in the 137ºE section 226

referred to 2000 dbar after 1995 (Fig. 3) that, unlike those referred to shallower depths before 227

1995 (e.g., Fig. 4a; Qiu and Joyce 1992), captured these jets in the intermediate layer (Bo Qiu, 228

personal communication). It was then confirmed (Qiu et al. 2013a) by the subsurface 229

velocity distribution in the North Pacific based on T/S measurements by Argo profiling floats 230

(Roemmich et al. 2001). The NEUCs may be a part of quasi-zonal jet-like structures, which 231

are ubiquitous over the global ocean (e.g., Maximenko et al. 2005) and whose formation 232

mechanism has been actively investigated (see Chen et al. 2015 for reviews and extensive 233

references). Based on analyses of an eddy-resolving ocean general circulation model and a 234

1.5-layer reduced gravity model, the companion paper (Qiu et al. 2013b) also offered a 235

formation mechanism; the NEUCs are formed by breaking of wind-forced baroclinic Rossby 236

waves, which are generated annually at the eastern boundary of Pacific basin, due to nonlinear 237

triad interactions, and the subsequent eddy-eddy interactions. 238

In the equatorial region, direct current measurements by two current meters until 1986 239

(Sect. 2) yielded a consistent structure (Masuzawa 1967, 1968, 1970; Akamatsu and Sawara 240

1969; Nagasaka and Sawara 1972; Guan 1986; Bingham and Lukas 1995; Fig. 6). The 241

eastward flowing Equatorial Undercurrent (EUC; Cromwell et al. 1954), which begins north 242

of New Guinea and appears as a strong subsurface jet on the equator in the central to eastern 243

Pacific (e.g., Wyrtki and Kilonsky 1984), is observed as a relatively weak jet at 100−300 m, 244

0.5−1.5ºN in the 137ºE section. Between the equator and 1ºS, the monsoonal New Guinea 245

Coastal Current (NGCC; Wyrtki 1961) flows eastward in boreal winter and westward in 246

boreal summer along the northern coast of New Guinea at the surface, while the New Guinea 247

Coastal Undercurrent (NGCUC; Lindstrom et al. 1987; Tsuchiya et al. 1989), which is the 248

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low-latitude western boundary current of the tropical South Pacific, flows westward at depths 249

greater than 100 m throughout the year. A majority portion of the NGCUC turns east near 250

and east of the 137ºE section to feed the EUC (Tshuchiya et al. 1989; Gouriou and Toole 251

1993; Bingham and Lukas 1995), and some of the remaining portion turns east in the 252

downstream regions to feed the NSCC and the NECC (Tsuchiya 1991; Talley et al., 2011). 253

These currents transport various water masses originating from the North and South 254

Pacific to the 137ºE section (Fig. 7). The subsurface S maximum centered at 150 m, 15ºN is 255

the North Pacific Tropical Water (NPTW; Cannon 1966; Tsuchiya 1968), also referred to as 256

the North Pacific Subtropical Underwater. NPTW is formed in the central part of the North 257

Pacific subtropical gyre where evaporation dominates precipitation and is transported 258

westward by the NEC. The S minimum at 500−900 m north of 17ºN along the isopycnal of 259

σθ = 26.8 kg m-3 (θ is potential temperature and σθ is potential density) is the North Pacific 260

Intermediate Water (NPIW; Sverdrup et al. 1942; Reid 1965). NPIW originates in the 261

Okhotsk Sea, enters the North Pacific subtropical gyre through the mixed water region east of 262

Japan, and is then advected westward by the Kuroshio recirculation and the northern NEC 263

(Talley 1993; Yasuda 2004; Fujii et al. 2013). The S minimum in the 137ºE section extends 264

southward of 17ºN, beyond which its σθ decreases from 26.8 to 26.5−26.3 kg m-3. The S 265

minimum at 11−17ºN at σθ < 26.6 kg m-3 is the Tropical Salinity Minimum (TSM; Yuan and 266

Talley 1992), which is formed in the eastern North Pacific through merging of NPIW and the 267

Shallow Salinity Minimum formed in the California Current region (Reid 1973) and is 268

transported westward by the lower part of the NEC. 269

The S maximum centered at ~150 m, south of 6ºN is the South Pacific Tropical Water 270

(Cannon 1966), while the S minimum at ~700 m, south of 11ºN at σθ = 27.2 kg m-3 is the 271

Antarctic Intermediate Water (Sverdrup et al. 1942; Reid 1965). These waters formed in the 272

South Pacific are transported westward to the eastern coast of Australia by the South 273

Equatorial Current, then northward through the Coral and Solomon Seas by the Great Barrier 274

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Reef Undercurrent and the North Queensland Current, and finally northwestward through the 275

Vitiaz Strait (Fig. 1) to the western equatorial Pacific by the NGCUC (Tsuchiya et al. 1989; 276

Tsuchiya 1991; Qu and Lindstrom 2002). Subsequently, the South Pacific Tropical Water is 277

transported eastward by the EUC and the NECC (Masuzawa 1972; Tsuchiya et al. 1989). 278

Some Antarctic Intermediate Water is advected eastward by the NSCC, while most of it is 279

transported northward along the eastern coast of Mindanao by the Mindanao Undercurrent 280

(Tsuchiya 1991; Hu and Cui 1991; Fine et al. 1994). 281

Another feature seen in the 137ºE section is the weak stratification near the top of the 282

thermocline at 16−18ºC south of the Kuroshio (Fig. 7a). This structure was termed STMW 283

by Masuzawa (1969) because it produces a mode in the volume distribution on the T−S 284

diagram for the subtropical gyre. Subsequently, the moniker mode water has been applied to 285

all similar water masses characterized by weak stratification or low potential vorticity (PV) in 286

the world oceans (Hanawa and Talley 2001). 287

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4. Scientific achievements in physical oceanography 289

The data from the 137ºE section are best applied to investigations of temporal variations 290

of the meridional-vertical structures and their integrated values (e.g., dynamic height, 291

cross-sectional area, and volume transport) in relation to climate variations. During the first 292

two decades, they were analyzed to reveal interannual variations of thermohaline structures 293

and currents. This was mainly considered in the tropical region in relation to the El 294

Niño−Southern Oscillation (ENSO) (Masuzawa and Nagasaka 1975; Guan 1986; Andow 295

1987; Saiki 1987), as well as to typhoon occurrence (Nagasaka 1981), weather in Japan 296

(Kurihara 1984, 1985), and the Indian summer monsoon (Yasunari 1990). The 137ºE data 297

were also combined with other meridional transects to demonstrate that the decorrelation 298

length scale of and the necessary sampling density for subsurface temperature variability in 299

the western North Pacific is 3º (lat.)/3º (lon.) at 17.5−30ºN and 6º/10º at 5−17.5ºN (White et 300

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al. 1982), which rationalized expansion of a regional expendable bathythermograph 301

observation network of the North Pacific Experiment to the entire Pacific in 1980 (White 302

1995). 303

Since Hanawa et al. (1988) examined winter mixed layers south of the Kuroshio in 304

relation to ENSO, a number of studies have also used the 137ºE data to explore the subtropics. 305

Moreover, extension of the time series has enabled us to describe not only interannual but also 306

decadal variability (Shuto 1996), particularly in relation to the Pacific Decadal Oscillation 307

(PDO; Mantua et al. 1997), and now to separate decadal variability and long-term trends (Qiu 308

and Chen 2012; Oka et al. 2017). In the following part of this section, we review scientific 309

achievements in physical oceanography from the 137ºE section, mainly over the last 30 years 310

in terms of thermohaline structures (Sect. 4.1) and large to meso-scale currents (Sect.4.2). In 311

addition, the 137ºE data have contributed to studies on submesoscale and microscale 312

phenomena in recent years (Sect. 4.3). These studies on smaller scales have benefitted from 313

the CTD and ADCP measurements repeated along the section since summer 1988, which have 314

facilitated a reduction in the error bars of estimations and clarified temporal variability of the 315

phenomena. 316

317

4.1 Thermohaline structures 318

After its discovery (Masuzawa 1969), STMW had been left unexplored for two decades, 319

until its formation process associated with deep winter mixed layers south of the Kuroshio 320

and the Kuroshio Extension (KE) was investigated by Hanawa (1987) and Hanawa and 321

Hoshino (1988). Following these studies, Suga et al. (1989) examined STMW in the 137ºE 322

section in the winters spanning 1967−1987 and the summers spanning 1972−1986. By using 323

apparent oxygen utilization (AOU), they clarified that a half (one)-year-old STMW appeared 324

in the summer (winter) section as far south as 26ºN (23ºN), suggesting differences in the 325

formation region and the subsequent southwestward advection by the Kuroshio recirculation 326

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that were verified later by using climatological data (Suga and Hanawa 1990, 1995a) and 327

Argo profiling float data (Oka 2009). 328

As for year-to-year variability, Suga et al. (1989) demonstrated that less STMW was 329

advected from the east during large-meander periods of the Kuroshio (Fig. 8a), during which 330

anomalously warm STMW was formed locally around 137ºE, that is, in an isolated 331

recirculation gyre west of the Kuroshio large meander (Hayashi 2008; Sugimoto and Hanawa 332

2014). Furthermore, Suga and Hanawa (1995b) analyzed the 137ºE section up to 1991 to 333

indicate that during non-large-meander periods of the Kuroshio, STMW with lower PV and 334

lower AOU tended to be formed in winters with stronger monsoon expression, suggesting the 335

importance of surface cooling. On the other hand, Soga et al. (2005) examined the summer 336

occupation of the 137ºE section up to 2004 to point out that thinner and warmer STMW was 337

formed around 2000 despite moderately strong cooling. Supporting their result, Qiu and 338

Chen (2006) analyzed T profiles south of the KE during 1993−2004 to demonstrate that the 339

STMW formation is largely controlled by PDO-related decadal variability of the KE, its 340

southern recirculation gyre, and the associated eddy field2 (Qiu and Chen 2005; Qiu et al. 341

2007, 2014a). Specifically, the STMW formation weakens (strengthens) during unstable 342

(stable) periods of the KE (Fig. 8a), likely because high (low) eddy activity transports more 343

(less) high PV water north of the KE to the STMW formation region to hinder (facilitate) 344

deepening of winter mixed layers. Such importance of oceanic control on the STMW 345

formation and subduction was confirmed by analyses of decade-long Argo profiling float data 346

(Rainville et al. 2014; Oka et al. 2015; Cerovečki and Giglio 2016), although it does not 347

preclude or deny the importance of atmospheric control prior to 1991. An analysis of 348

2 The decadal variability of the KE system, which was detected by satellite altimeter

measurements initiated in 1992 (Ducet et al. 2000), is driven by PDO-related wind forcing in

the central North Pacific. When negative (positive) sea surface height anomalies generated

in the warm (cool) phase of PDO reach the area east of Japan after propagating westward for

3−4 years, the KE turns into an unstable (stable) state and is accompanied by a weakened

(strengthened) southern recirculation gyre and high (low) regional eddy activity.

15

historical T profiles over 1968−2014 indicated that the STMW formation was controlled 349

primarily by surface cooling during the late 1970s and 1980s and by oceanic processes after 350

~1990 (Sugimoto and Kako 2016). Furthermore, an analysis of the 137ºE section over 351

winters spanning 1967−2016 demonstrated that for the last 25 years, S of STMW decreased 352

(increased) during unstable (stable) KE periods, which can be also explained by the decadal 353

variability in water exchange between the saline subtropics and the fresher mixed water 354

region across the KE (Oka et al. 2017). Interestingly, such decadal S variability of STMW 355

was almost in phase along the 137ºE section against our expectation from the climatological 356

STMW circulation (Bingham et al. 2002), suggesting importance of horizontal eddy mixing. 357

Studies on low-frequency variability of NPTW based on the 137ºE section data have been 358

advanced with the development of other hydrographic data and surface flux data. Shuto 359

(1996) analyzed NPTW in the 137ºE section spanning 1967−1987, although its relation to 360

surface freshwater fluxes in the formation region, he mentioned, could not be evaluated due to 361

the absence of evaporation and precipitation data. By using available wind stress data, he 362

demonstrated that on decadal time scales, the cross-sectional area of NPTW (S > 35.0) 363

corresponded to the absolute maximum of negative wind stress curl in the subtropical region 364

with a time lag of 0−2 years. A subsequent study using the 137ºE data during 1967−1995 365

(Suga et al. 2000) exhibited that the area and S of NPTW (S > 34.9) increased remarkably in 366

association with the 1976/77 regime shift (Nitta and Yamada 1989; Trenberth 1990; Fig. 8b). 367

This change was explained only partly by the thermohaline forcing estimated from available 368

evaporation and wind stress data, but was well explained by the Ekman pumping calculated 369

from wind stress data. 370

A decade later, accumulation of Argo profiling float data and satellite altimeter sea surface 371

height data as well as development of various surface flux data enabled us to fully examine 372

variations in the NPTW formation and circulation with the underlying mechanisms. Katsura 373

et al. (2013) analyzed Argo float data spanning 2003−2011 to demonstrate that the most saline 374

16

portion of NPTW around 15ºN in the 137ºE section was formed in the western part of the sea 375

surface S maximum near 25ºN, 180º. In this part of the formation region, excess evaporation 376

over precipitation seemed to be balanced by eddy diffusion, whose strength varied in 377

association with the PDO (Qiu and Chen 2013; Sect. 4.2). As a result, the maximum S of 378

NPTW in the 137ºE during 1992−2012 lagged the PDO index by 3 years, which was 379

considered to be the sum of 1 year for the eddy activity adjustment to the PDO forcing and 2 380

years for the NPTW advection. More recently, Nakano et al. (2015) analyzed the 137ºE 381

measurements spanning 1967−2011 to reveal that the decadal variability of NPTW area (S > 382

34.9) differed between its southern (and the most saline) part at 10−18ºN and its northern part 383

at 18−25ºN, the latter of which dominated variability of the total area (Fig. 8b). The decadal 384

variability of northern NPTW area was positively correlated with both evaporation over 385

precipitation and Ekman pumping in the formation region west of 180º, and also with the 386

PDO-related eddy kinetic energy in the STCC region (Qiu and Chen 2013). It might be also 387

related to the decadal KE variability, as the area (Nakano et al. 2015; Fig. 8b) and S (Nan et al. 388

2015) of northern NPTW exhibited a similar decadal variability to the S of STMW mentioned 389

above, at least after ~1990 (Oka et al. 2017). 390

Low-frequency variability of NPIW and its cause have also been investigated, commonly 391

assuming that the distribution of NPIW in the 137ºE section depends primarily on its 392

westward advection in the subtropical gyre. Qiu and Joyce (1992) analyzed the 137ºE data 393

over 1967−1988 to demonstrate that the cross-sectional area of NPIW (S < 34.25) decreased 394

during large-meander periods of the Kuroshio (Fig. 8c), due to weakening of the Kuroshio 395

recirculation, as is the case with STMW. Shuto (1996), without considering the Kuroshio 396

path state, indicated that decadal variability of NPIW area (S < 34.2) corresponded with that 397

of the absolute maximum of negative wind stress curl with a time lag of 3−6 years. Nakano 398

et al. (2005) used the 137ºE data up to 2000 to exhibit that the NPIW area (S < 34.2), which 399

had dominant time scales of 10 and 3−4 years, was correlated with the North Pacific index 400

17

(Trenberth and Hurrell 1994) representing the strength of the Aleutian Low with a lag of 401

about 11 years on decadal timescales, and with wind stress curl in the central North Pacific 402

with a time lag of 4 years on interannual time scales. More recently, Sugimoto and Hanawa 403

(2011) analyzed the 137ºE section during 1972−2008 to demonstrate that the decadal 404

variability of NPIW area (S < 34.2) was dominated by that of its northern part north of 25ºN 405

(Fig. 8c), which corresponded with the intensity of the Kuroshio recirculation that fluctuated 406

as the first-mode baroclinic response to the Aleutian Low activity. 407

Generally speaking, it is becoming increasingly clear when reviewing recently published 408

studies (Sugimoto and Hanawa 2011; Nakano et al. 2015; Oka et al. 2015, 2017) that decadal 409

variability of the KE system commonly influences characteristics of STMW, NPTW, and 410

NPIW, whose cross-sectional area in the 137ºE section simultaneously increases (decreases) 411

during stable (unstable) KE periods characterized by strengthened (weakened) recirculation 412

gyre and low (high) eddy activity. In Fig. 8, such synchronized decadal variability seems to 413

exist to some extent for the last 25 years since 1992, during which the KE path has been 414

monitored by satellite altimeter measurements and the Kuroshio has mostly taken a 415

non-large-meander path. This is an interesting relation that needs quantification in a future 416

study because if true, the decadal variability of representative water masses in the 137ºE 417

section is controlled by oceanic processes rather than local air-sea processes, and is remotely 418

controlled by the PDO-related atmospheric forcing in the central North Pacific. 419

The 137ºE section has also contributed to clarifying long-term T and S changes. In the 420

North Pacific subtropical gyre, a freshening trend has been widely observed in the main 421

thermocline/halocline and the underlying NPIW and TSM (e.g., Lukas 2001; Wong et al. 422

2001; Ren and Riser 2010) as a part of global change (e.g., Wong et al., 1999; Hosoda et al. 423

2009; Durack and Wijffels 2010). Nakano et al. (2007) analyzed the 137ºE section during 424

1967−2005 to report an isobaric freshening trend reaching -0.0015 yr-1 from the lower 425

thermocline/halocline to NPIW/TSM. On the other hand, Qiu and Chen (2012) and Nan et 426

18

al. (2015) used more recent time series during 1993−2009 and 1992−2009, respectively, to 427

present a freshening trend several times larger than Nakano et al.’s (2007) from the upper 428

thermocline/halocline to the surface layer, which suggests interdecadal changes. The latest 429

analysis of 50-year time series during 1967−2016 (Oka et al. 2017; Fig. 9) clarified that rapid 430

freshening began in mid-1990s and persisted for the last 20 years in the upper halocline 431

corresponding to STMW and a lighter variety of Central Mode Water (CMW; Nakamura 432

1996; Suga et al. 1997; Oka and Suga 2005). Additional analyses of the repeat hydrographic 433

section along 144ºE maintained by the Japan Coast Guard demonstrated that the freshening 434

trend originated in the winter mixed layer in the KE region, although the mechanism of 435

surface freshening in that region remains unclear. In association with this long-term 436

freshening, the cross-sectional area of NPTW exhibits a trend toward decrease after the 437

1976/77 regime shift (Nakano et al. 2015; Fig. 8b), while that of NPIW shows a trend toward 438

increase throughout the 50-year period (Fig. 8c). 439

In contrast with the freshening subtropics, T and S in the upper tropical ocean presented a 440

large trend toward increase centered at 12ºN, 150 dbar (Fig. 9a, b). This is primarily related 441

to southward shift of NEC, whose thermosteric effect resulted in a local sea-level rise 442

exceeding 10 mm yr-1 that is more than three times faster than the global average (Qiu and 443

Chen 2012; Sect. 4.2). 444

445

4.2 Large to meso-scale currents 446

Qiu and Joyce (1992) analyzed the 137ºE section in winters of 1967−1988 and summers 447

of 1972−1988 to examine variations of various currents and water masses. They argued that 448

for El Niño years relative to non-El Niño years, surface dynamic height in the tropical region 449

was lower, as previously reported for summer 1972 and winter 1973 (Masuzawa and 450

Nagasaka 1975), geostrophic volume transport of both the NEC and the NECC was larger (71 451

and 69 Sv versus 57 and 42 Sv; 1 Sv ≡ 106 m3 s-1), and the boundary between the NEC and 452

19

the NECC and the southern boundary of the NECC both shifted southward by 1º. They also 453

demonstrated that the volume transport of the two currents agreed well in both amplitude and 454

phase with the Sverdrup transport estimated from wind stress data. 455

Two decades later, bifurcation of the NEC at the eastern coast of the Philippines was 456

examined by using satellite altimeter sea surface height data during 1993−2009 (Qiu and 457

Chen 2010b). Interannual variation of the bifurcation latitude, which was much larger than 458

its seasonal variation, corresponded well with the Niño-3.4 index, being higher in El Niño 459

years with larger volume transport of both the NEC and the NECC. The satellite altimeter 460

data and the quarterly 137ºE section data during 1993−2009 analyzed in a subsequent study 461

(Qiu and Chen 2012) consistently demonstrated that over the 17 years the NEC bifurcation 462

location and the boundary between the NEC and the NECC shifted southward by 2º and 1º, 463

and the volume transport of the NEC and the NECC increased by 8 and 4 Sv, respectively 464

(Fig. 10). Using a 1.5-layer reduced gravity model, Qiu and Chen (2012) concluded that the 465

southward migration and the strengthening of the NEC/NECC accompanied by the rapid 466

sea-level rise near the NEC bifurcation latitude (Sect. 4.1) were caused by the surface wind 467

stress of the recently strengthened atmospheric Walker circulation. In support of this finding, 468

Hu and Hu (2014) used ADCP data in the upper 200 m between 8º and 18ºN of the 137ºE 469

section during 1993−2008 to demonstrate that the volume transport of the NEC corresponded 470

to the Niño-3.4 index with a time lag of 6 months and increased by 5 Sv over the 16 years. 471

For longer time scales, Zhai et al. (2013) analyzed the 137ºE section during 1972−2008 to 472

indicate that the NEC transport exhibited decadal variability with a time scale of ~10 years in 473

association with vertical movements of the sea surface and the permanent pycnocline in the 474

southern part of the NEC, which was explained by decadal variability in the wind stress 475

forcing. Such interannual to decadal variations and long-term changes of the NEC and the 476

associated surface thermohaline structure likely impact the spawning of the Japanese eel 477

(Anguilla japonica) near seamounts around 14ºN, 143ºE in the West Mariana Ridge (Fig. 1) 478

20

and the subsequent larval transport to the growth habitats in East Asia (Tsukamoto 1992; 479

Kimura et al. 1994, 2001; Kimura and Tsukamoto 2006; Tsukamoto 2006; Kim et al. 2007; 480

Zenimoto et al. 2009; Hsu et al. 2017). 481

Since the STCC was discovered by Uda and Hasunuma (1969), its generation mechanism 482

has been explored by many studies (Kobashi and Kubokawa 2012). Among them, 483

Kubokawa and Inui (1999) and Kubokawa (1999) demonstrated using an idealized ocean 484

general circulation model that low-PV waters with various densities subduct from the mixed 485

layer depth front in the northern part of the subtropical gyre and pile up in the southern part of 486

the gyre, and an eastward near-surface countercurrent is generated along the southern edge of 487

this low-PV pool. In response to their numerical result, Aoki et al. (2002) analyzed four 488

meridional repeat hydrographic sections including 137ºE and the WHP one-time sections to 489

examine the relation between two STCCs in the western North Pacific (Hasunuma and 490

Yoshida 1978; Fig. 1) and mode waters (Oka and Qiu 2012). They concluded that the 491

northern STCC was located at the southern edge of STMW, while the southern STCC was 492

located at the southern edge of low-PV waters in a wide density range including CMW. 493

Such correspondence between STCCs, including a third one in the eastern North Pacific, and 494

mode waters was further demonstrated using climatology constructed from T profiles over the 495

North Pacific (Kobashi et al. 2006). 496

The intensity of the STCC varies seasonally with a maximum in spring and a minimum in 497

fall (White et al. 1978), due largely to the seasonal variation in the wind stress forcing 498

(Takeuchi 1986). This alters the strength of baroclinic instability between the STCC and the 499

NEC, resulting in the eddy kinetic energy in the STCC region (Fig. 5) being maximum in 500

April−May and minimum in December−January (Qiu 1999; Kobashi and Kawamura 2002). 501

To further reveal year-to-year variability of STCC’s mesoscale eddy field, Qiu and Chen 502

(2010a) analyzed satellite altimeter sea surface height data over 1993−2008 and demonstrated 503

that eddy kinetic energy was higher in 1996−1998 and 2003−2008 and lower in the other 504

21

years. They also examined the quarterly 137ºE data during the same period to show that the 505

vertical shear between the STCC and the NEC was larger and more favorable for baroclinic 506

instability in eddy-rich years than eddy-weak years. Their subsequent study (Qiu and Chen 507

2013) revealed that decadal variability of eddy kinetic energy was highly correlated with the 508

PDO index with a lag of 6 months and was due to the decadal variability of the vertical shear, 509

which was controlled by that of surface heat flux forcing rather than wind stress forcing 510

through convergence of Ekman heat flux. 511

Interannual variability in the volume transport of the Kuroshio and its relation to the path 512

variations south of Japan (Fig. 1; horizontal bars at the bottom of each panel in Fig. 8) has 513

also been a subject of broad interest. Qiu and Joyce (1992) analyzed the 137ºE section over 514

1967−1988 to demonstrate that the net transport of the Kuroshio, which was defined as the 515

eastward transport of the Kuroshio minus the westward transport of the Kuroshio 516

Countercurrent, was larger during large-meander periods (39 Sv) than non-large-meander 517

periods (29 Sv) on the average. This supported observational results in the East China Sea 518

(Kawabe 1980; Saiki 1982) and numerical results (Chao 1984; Yoon and Yasuda 1987; 519

Akitomo et al. 1991), while recent sensitivity experiments using a data-assimilation model 520

(Usui et al. 2013) indicated that smaller transport is favorable for maintenance of the large 521

meander, as pointed out by earlier observational studies (e.g., Nan’niti 1960; Nitani 1975). 522

For longer-term variations, Sugimoto et al. (2010) analyzed the 137ºE section over 523

1972−2007 to demonstrate that the net Kuroshio transport varied on a timescale of ~10 years 524

in association with vertical movements of the permanent pycnocline in the southern part of 525

the Kuroshio, which fluctuated as the first-mode baroclinic response to the Aleutian Low 526

activity. They also indicated that the transport of the Kuroshio recirculation also fluctuated 527

decadally, affecting the cross-sectional area of NPIW in the 137ºE section (Sugimoto and 528

Hanawa 2011; Sec. 4.1). 529

Several studies have analyzed the 137ºE section to examine abyssal circulation in the 530

22

Philippines Sea, where deep inflow through the channel at the junction of the Yap and West 531

Mariana Ridges at 12ºN, 139ºE first enters the West Mariana Basin and then spreads 532

northward to the Shikoku Basin and westward to the Philippines Basin (Mantyla and Reid 533

1983; Uehara and Taira 1990; Kawabe 1993; Fig. 1), as confirmed by the WHP full-depth 534

observations including P9 (Kaneko et al. 1998; Kaneko et al. 2001). Sudo (1986) analyzed θ 535

and dissolved O2 data at 4000 m depth at 15, 20, 25, and 30ºN during 1976−1983 and argued 536

that property anomalies propagated from 15ºN to 30ºN in 1 year at a speed of 5−6 cm s-1, 537

while Fukasawa et al. (1995) subsequently reinterpreted the same θ time series as indicating 538

that the anomalies propagated from 15ºN to 30ºN in 3 years at a speed of 2 cm s-1. Further 539

analyses of deep CTD casts continued at every 5º and the GO-SHIP P9 revisits in 2010 and 540

2016 (Fig. 3) are expected to clarify long-term variations of deep water properties and the 541

abyssal circulation in the Philippines Sea. 542

543

4.3 Smaller-scale phenomena 544

Turbulent diapycnal mixing and its spatiotemporal variations, which control transport of 545

heat, salt, and nutrients and maintain the ocean stratification, have drawn increasing attention 546

in recent years (e.g., Wunsch and Ferrari 2004; Alford et al. 2016). Jing and Wu (2010) 547

applied the Thorpe-scale (Thorpe 1977) and fine-scale parameterization (Kunze et al. 2006) 548

methods to available CTD profiles at the 137ºE section during 1998−2007 to examine the 549

time-mean structure and seasonal variation of diapycnal mixing. Time-mean diapycnal 550

diffusivity (K) exceeded 10-4 m2 s-1 around 8º, 25º, and 34ºN over the rough topography, and 551

was 7.4 × 10-5 m2 s-1 when averaged in the section at depths greater than 300 dbar, being 552

much higher than the values of O(10-5 m2 s-1) obtained by microstructure measurements in the 553

mid-latitude ocean interior (Gregg 1987). In addition, K in the Kuroshio region fluctuated 554

seasonally with a maximum in winter and a minimum in spring and summer and decreased in 555

amplitude with depth, presumably due to seasonally-varying surface wind stress forcing. A 556

23

subsequent study using CTD profiles at the JMA’s three repeat sections including 137ºE and 557

the fine-scale parameterization method (Qiu et al. 2012) demonstrated that time-mean K at the 558

137ºE section was high at 8−10ºN, 22−24ºN, 25−29ºN, and 32−34ºN (Fig. 11). The high K 559

at the former two latitude bands over the rough topography was enhanced with depth, while 560

that at 25−29ºN over the relatively featureless topography, which was also observed at the 561

other two sections, was vertically uniform and was attributable to parametric subharmonic 562

instability. Lag-correlation analyses with external forcing indicated that the time-varying K 563

at 25−29ºN corresponded to spring-neap modulated semidiurnal tidal current with a time lag 564

of 6 days and to surface wind forcing with no time lag. 565

Submesoscale processes have also drawn increasing attention in recent years (e.g., 566

Thomas et al. 2008; McWillams 2016), and are expected to be further clarified by the onset of 567

the Surface Water and Ocean Topography (SWOT) satellite mission planned for 2021, whose 568

horizontal resolution (~15 km; Fu and Ubelmann 2014) is one order smaller than that of 569

ongoing satellite observations (~150 km; Ducet et al. 2000). As satellite altimeter 570

measurements detect signals of both balanced geostrophic flow that is dominant at meso and 571

larger scales and unbalanced internal waves that are dominant at smaller scales, it is important 572

for the SWOT mission to identify the length scale at which geostrophic flow loses its 573

dominance and is overtaken by internal waves. To answer this question, Qiu et al. (2017) 574

recently analyzed ADCP current velocity data in 33 surveys at the 137ºE section to estimate 575

the transition length scale in four latitude bands. Kinetic energy levels for internal waves 576

were similar among the four bands, and those for geostrophic flow primarily determined the 577

transition scale in each band. The estimated transition scale differed greatly among latitude 578

bands, being 15 km in the eddy-abundant Kuroshio band (28−34ºN), 50 and 80 km in the 579

moderately unstable STCC (14−28ºN) and NECC (3−9ºN) bands, respectively, and 250 km in 580

the stable NEC band (9−14ºN). 581

582

24

5. Scientific achievement in biogeochemical oceanography 583

During and after the International Geophysical Year (1957−1958), it was found that CO2 584

in surface seawater was in general not in equilibrium with that in marine boundary air (e.g., 585

Torii et al. 1959; Takahashi 1961; Keeling 1968). To demonstrate the spatiotemporal 586

variations of air-sea CO2 exchange over the western North Pacific, the JMA measured the 587

partial pressure of CO2 in marine boundary air (pCO2air) and that in an aliquot of air being 588

equilibrated with surface seawater (pCO2sw) continuously along the 137ºE section using a 589

non-dispersive infrared gas analyzer and a showerhead type equilibrator (Inoue 2000) for 590

several winters from 1968 (Akiyama 1968, 1969; Masuzawa et al. 1970). After an 591

interruption during the 1970s, the Meteorological Research Institute of the JMA, which had 592

explored pCO2 distributions in the Pacific, Indian, and Southern Oceans from the late 1960s 593

to the early 1970s onboard R/V Hakuho-maru of the University of Tokyo (Miyake and 594

Sugimura 1969; Miyake et al. 1974), resumed the pCO2 measurements along the winter 137ºE 595

section for research purposes in 1981 (Fushimi 1987; Inoue et al. 1987). In the light of 596

importance of greenhouse gases in global warming (IPCC 1990), the JMA took over these 597

measurements in 1990, by initiating biannual operational measurements of CO2 in surface 598

seawater and in the air as part of the Observation for Monitoring Background Marine 599

Pollution (Hirota et al. 1991, 1992, 1993; Fushimi et al. 1993). 600

Measurements during the earlier years revealed that pCO2sw in the subtropical region 601

exhibits large seasonal variations being low in winter and high in summer, and that this region 602

serves as a strong sink of CO2 on the annual average, which are now well known to the ocean 603

carbon researchers. Along the winter 137ºE section, pCO2sw is lowest at ~30ºN in the 604

subtropical region south of the Kuroshio and tends to be higher in the lower latitudes, while 605

pCO2air shows smaller spatial variation, being nearly constant south of 15ºN and increasing 606

slightly northward north of 15ºN (Fushimi 1987; Inoue et al. 1987; Fig. 12). 607

Correspondingly, the difference between pCO2sw and pCO2

air (ΔpCO2 = pCO2sw - pCO2

air) is 608

25

largely negative, reaching a minimum of -60 μatm at ~30ºN, and increases southward to 609

around zero or even positive in the equatorial region. In summer, by contrast, pCO2sw is 610

comparable to or higher by up to 50 μatm than pCO2air, which slightly decreases northward in 611

the subtropical region (Hirota et al. 1991). The large seasonal variations of pCO2sw in the 612

subtropics were attributed mainly to the thermodynamic effect of seasonal variation of sea 613

surface temperature (SST) that is partly compensated for by the changes in carbon chemistry 614

due to biological production in summer and by entrainment of CO2-rich subsurface water in 615

winter (Inoue et al. 1987, 1995; Inoue and Sugimura 1988a; Murata and Fushimi 1996: 616

Murata et al. 1998; Fig. 13a). The net air-sea CO2 flux (positive upward), which depends on 617

ΔpCO2 and the gas transfer velocity expressed as a function of wind speed, fluctuates 618

seasonally in this region between -16 mmol m-2 day-1 in winter, associated with large negative 619

ΔpCO2 and strong wind, and 4 mmol m-2 day-1 in summer (Inoue et al. 1995; Fig. 13b). For 620

the annual average, the CO2 flux is at a minimum of -8 mmol m-2 day-1 south of the Kuroshio 621

and increases southward, changing its sign at 5−10ºN. In the equatorial region at 137ºE 622

located in the western equatorial Pacific warm pool, the annual-mean CO2 flux ranges 0.2−0.7 623

mmol m-2 day-1, which indicates a weak CO2 source. This source is much weaker than that 624

in the upwelling region in the central and eastern equatorial Pacific known as the largest 625

source in the world oceans (Tans et al. 1990). 626

In the equatorial region, pCO2sw and ΔpCO2 fluctuate interannually in relation to ENSO. 627

The pCO2 measurements along the winter 137ºE section from 1981 to 1985 revealed that in 628

winter 1983 during the 1982/83 El Niño event, pCO2sw and ΔpCO2 between 1ºS and 7ºN 629

considerably increased in association with SST decrease, presumably due to shallowing of the 630

thermocline, while those between 7º and 17ºN decreased with SST (Fushimi 1987; Inoue et al. 631

1987; Fig. 14). This contrasted with the central and eastern equatorial Pacific during the 632

same period where pCO2sw decreased to the values almost equivalent to those of pCO2

air due 633

to the cessation of upwelling (Feely et al. 1987). Subsequent measurements repeated in the 634

26

western and central Pacific during 1987−1989 using several Japanese vessels demonstrated 635

that pCO2sw in the central equatorial Pacific decreased in January and February 1987 during 636

the 1986/88 El Niño event and increased in January and February 1989 during the 1988/89 La 637

Niña event, while pCO2sw at the 137ºE section in the western equatorial Pacific warm pool 638

exhibited relatively small variations, compared to those during the strong 1982/83 El Niño 639

event (Inoue and Sugimura 1988b, 1992). Furthermore, extensive pCO2 measurements 640

conducted over the entire equatorial Pacific during the 1990s as part of the Joint Global Ocean 641

Flux Study exhibited that during the strong El Niño events, pCO2sw increased in the western 642

equatorial Pacific but decreased to the near-equilibrium values in the central and eastern 643

equatorial Pacific, resulting in the reduction of annual-mean CO2 emission from the equatorial 644

Pacific to 0.2−0.4 Pg C yr-1 that was much smaller than 0.8−1.0 Pg C yr-1 during non-El Niño 645

years (Feely et al. 2002). 646

In addition to its seasonal and interannual variations, a long-term trend toward pCO2sw 647

increase in response to the pCO2air increase associated with anthropogenic CO2 emissions has 648

been observed at 137ºE. Based on the decade-long record from the winter 137ºE section3 649

from 1984 to 1993, Inoue et al. (1995) demonstrated that pCO2sw was increasing at a rate of 650

+1.8 ± 0.6 μatm yr-1 at 15−30ºN, which was equal to the rate of pCO2air increase, and +0.5 ± 651

0.7 μatm yr-1 at 3−14ºN. Apart from a few studies indicating a long-term pCO2sw increase 652

using two observations a decade apart (e.g., Inoue and Sugimura 1988a), this was the first 653

study that corroborated the trend toward pCO2sw increase based on time-series data from 654

shipboard observations. A decade later, Midorikawa et al. (2005) reconfirmed the persistent 655

trend toward pCO2sw increase; its rate for the two decades of 1984−2003 averaged between 3º 656

and 34ºN at the winter 137ºE section was +1.7 ± 0.2 μatm yr-1 and was comparable to that of 657

pCO2air (+1.60 ± 0.03 μatm yr-1), which suggests that the western North Pacific Ocean 658

3 pCO2

sw data in 1981 and 1982 have not been used in Inoue et al. (1995) and the subsequent

studies because of a technical problem in the measurements (Inoue et al. 1995).

27

responds rapidly to the increase of CO2 in the atmosphere. 659

The carbonate system in seawater is characterized by four measurable parameters: pCO2, 660

dissolved inorganic carbon (DIC), total alkalinity (TA), and pH. In principle, data of any 661

two of these parameters and those of T and S can be used to derive the other carbonate system 662

parameters. The JMA has measured DIC from discrete water samples at the 137ºE section 663

using a CO2 extraction-coulometry method (Ishii et al. 1998) in summer 1994 (WHP P9), 664

winter 1997, and summer 2000, and every cruise since 2003. Based on the data from five 665

meridional transects in the western North Pacific during 1994−1997 including the above two 666

at 137ºE, Ishii et al. (2001) demonstrated that surface TA normalized at S = 35 (nTA), which 667

was calculated from pCO2sw, surface normalized DIC (nDIC), SST, and sea surface salinity 668

(SSS), was almost invariable in space and time. They also indicated that the seasonal 669

variations in pCO2sw were ascribed mainly to those of SST and nDIC; specifically, surface 670

nDIC decreased (increased) from winter (summer) to summer (winter), compensating for 671

~30% of pCO2sw increase (decrease) due to SST increase (decrease). Furthermore, Ishii et al. 672

(2001) estimated the monthly surface nDIC distribution in the western subtropical gyre from 673

pCO2sw and SST of Inoue et al. (1995) (Fig. 13a) assuming constant nTA. They diagnosed 674

that the nDIC decrease from winter to summer was due to biological production and was 675

partly compensated for by the air-to-sea CO2 flux, while its increase from summer to winter 676

was explained by entrainment of subsurface water associated with mixed layer deepening, and 677

that the effect of the horizontal DIC transport appears minor. Their analysis was 678

substantially applied to the measurements along the winter 137ºE section over 1983−2003 by 679

Midorikawa et al. (2006). On interannual time scales, the effects of SST and nDIC 680

variations on pCO2sw variations in the subtropical region have been compensating to each 681

other through entrainment, thereby the long-term trend toward pCO2sw increase has been 682

observed distinctly. This was also the case for the warm pool of the equatorial region where 683

SST tends to be higher and DIC tends to be lower when stratification was enhanced by the 684

28

formation of barrier layers (Lukas and Lindstrom 1991), although the relationship between 685

SST change and DIC change was not so robust there (Ishii et al. 2009). 686

The JMA also started high-precision measurements of pH in 2003 (Saito et al. 2008) and 687

TA in 2010 using indicator dye−spectrophotometry methods. Data of surface pH during 688

2003−2008 calculated from pCO2sw, SST, and SSS assuming a constant nTA agreed well with 689

those measured spectrophotometrically when using dissociation constants of carbonic acid 690

given by Lueker et al. (2000). This allowed the pH time series to be extended back to 1983 691

based on pCO2sw measurements (Midorikawa et al. 2010). The calculated surface pH at 692

3−33ºN of the 137ºE section in winter (summer) during 1983−2007 (1987−2007) exhibited a 693

long-term trend toward decrease at a rate of -0.0018 ± 0.0002 yr-1 (-0.0013 ± 0.0005 yr-1) on 694

average at ambient SST (Fig. 15) and that of -0.0015 ± 0.0003 yr-1 (-0.0014 ± 0.0004 yr-1) 695

when normalized to 25ºC. These trends were comparable to those observed at the 696

time-series stations in the central subtropical North Pacific near Hawaii and those in the 697

subtropics of the North Atlantic (Bates, 2007; González-Dávila et al. 2007: Dore et al. 2009). 698

A similar pH trend of -0.0020 ± 0.0007 yr-1, together with a trend of -0.012 ± 0.005 yr-1 for 699

the aragonite saturation state, was determined for 1994−2008 at the northernmost stations 700

(33º40’−34ºN) of the 137ºE section located off the southern coast of Japan and inshore of the 701

Kuroshio (Ishii et al. 2011). 702

Midorikawa et al. (2011) subsequently suggested the acceleration of ocean acidification 703

after the 1980s by demonstrating that, when the trend toward surface pH decrease determined 704

at the winter 137ºE section over 1983−2008 was extrapolated back to 1969−1970, the 705

extrapolated pH value at each latitude was higher than that estimated from the pCO2sw 706

measurements made in the winter of these years along 138º and 158ºE (Miyake et al. 1974; 707

Inoue et al. 1999). On the other hand, Midorikawa et al. (2012) pointed out that in spite of 708

the acceleration of pCO2air increase over the long term, the mean rate of pCO2

sw increase at 709

the winter 137ºE section was lower for 1999−2009 than for the earlier period of 1984−1997 at 710

29

most latitudes between 3º and 33ºN. This was particularly the case at 10º−20ºN where the 711

southward migration of NEC (Qiu and Chen 2012; Sect. 4.2) possibly led to thickening of the 712

warm and CO2-poor surface water and the reduction of entrainment of CO2-rich subsurface 713

water. Thus, the rate of pCO2sw increase was likely to be decadally variable, and its 714

mechanism needs to be pursued in a future study. 715

As a result of subduction from the winter mixed layer with increasing pCO2sw, DIC in the 716

ocean interior has been increasing. In the latest issue of the annual Marine Diagnosis Report 717

(Japan Meteorological Agency 2017), it is shown that nDIC at the 137ºE section exhibits a 718

long-term trend toward increase on isopycnals down to σθ = 27.2 kg m-3 for the last two 719

decades (Fig. 16). In the water column between the sea surface and σθ = 27.5 kg m-3 at a 720

depth of ~1300 m, anthropogenic CO2 has been accumulated at a mean rate of 4.1−12.3 gC 721

m-2 yr-1 over 1994−2016 (Fig. 17). The accumulation rate was highest at 30ºN, where 722

STMW that transports CO2 from the surface layer to the interior is thickest. In addition to 723

this long-term change, PDO-related decadal variability of STMW formation and subduction 724

(Sect. 4.1) influences biogeochemical structure in the upper part of the permanent thermocline 725

in the western subtropical gyre. By analyzing biogeochemical data at 25ºN in the 137ºE 726

section, Oka et al. (2015) detected consistent changes during the stable KE period after 2010 727

that pH and aragonite saturation state (AOU, nitrate, and nDIC) in the STMW layer increased 728

(decreased), particularly against long-term trends for pH, aragonite saturation state, and nDIC, 729

in association with the increasing advection of STMW from the east (Fig. 18). This result 730

indicates a new mechanism by which climate variability impacts biogeochemical properties in 731

the ocean’s interior, and potentially also impacts the surface ocean acidification trend along 732

with biological productivity. 733

Along with the ocean acidification, decrease of O2 concentration in seawater, i.e., 734

deoxygenation, associated with the ocean warming is another potential threat to marine 735

ecosystems. Takatani et al. (2012) analyzed the 137ºE section over 1967−2010 to 736

30

investigate the trend of dissolved O2 on isopycnals. They found marked decreases of O2 at 737

20−25ºN in the subtropical gyre after the mid-1980s, with a rate of -0.28 ± 0.08 μmol kg-1 yr-1 738

on σθ = 25.5 kg m-3 (corresponding to the denser limit of STMW), -0.36 ± 0.08 μmol kg-1 yr-1 739

on 26.8 kg m-3 (NPIW), and -0.23 ± 0.04 μmol kg-1 yr-1 on 27.3 kg m-3 (O2 minimum layer). 740

They demonstrated that the O2 decrease in the upper pycnocline (< 26.0 kg m-3) was due 741

mainly to the isopycnal deepening and the decline of O2 solubility associated with upper 742

ocean warming, while that in the layers below (> 26.0 kg m-3) was attributable primarily to 743

AOU changes; specifically, the O2 decrease in the NPIW originated from the NPIW formation 744

region, and that in the O2 minimum layer was explained by intensification of westward 745

transport of low O2 water from the eastern North Pacific. 746

747

6. Scientific achievement in other research areas 748

6.1 Biological oceanography 749

Phytoplankton and zooplankton sampling along the 137ºE section was conducted from 750

1967 through 2006. Phytoplankton in water samples from Nansen or Niskin bottles, mainly 751

diatoms, were concentrated by decanting and centrifugalizing, and were identified and 752

counted using a light microscope (Kawarada et al. 1968). Macrozooplankton was collected 753

by vertically towing a 0.33-mm mesh NORPAC net with a 45-cm diameter aperture and a 754

180-cm length from 150 m depth to the sea surface, and chaetognaths and some copepods 755

were identified and counted using a stereo microscope (Kawarada et al. 1968; Kitou 1974; 756

Kawashima and Nagai 1990; Nagai et al. 2015). In the CSK cruise in January−March 1967 757

including the first 137ºE section (Masuzawa 1967), a new species of Calanoid Copepoda, 758

Calanoides philippinensis (Kitou and Tanaka 1969), was described. 759

Along the 137ºE section, both diatoms and chaetognaths were abundant in the Kuroshio 760

and the equatorial regions, while they were scarce in the NEC (Kawarada et al. 1968). The 761

latest analysis for winter observations during 1967−1995 (Nagai et al. 2015) recorded 26 762

31

chaetognath species, of which 22 were epipelagic and 4 were mesopelagic, and classified 17 763

common epipelagic species into 6 groups according to their distributions: Zonosagitta nagae 764

in the Japanese coast type, Mesosagitta minima in the Japanese coast-Kuroshio type, 765

Krohnitta subtilis, Pseudosagitta lyra, Sagitta bipunctata, and Serratosagitta 766

pseudoserratodentata in the Kuroshio-subtropical type, Aidanosagitta neglecta, Ferosagitta 767

ferox, Fe. robusta, and Flaccisagitta enflata in the NEC-NECC type, A. regularis, K. pacifica, 768

Z. bedoti, and Z. pulchra in the NECC type, and Fl. hexaptera, Pterosagitta draco, and Se. 769

pacifica in the bimodal type characterized by a bimodal distribution with the minimum in the 770

NEC. 771

As with physical and biogeochemical parameters, biological properties have exhibited 772

long-term variations and trends. Sugimoto and Tadokoro (1998) analyzed the 137ºE section 773

during 1970−1992 to demonstrate that the chlorophyll-a (Chl-a) concentration in the upper 774

200 m decreased drastically around 1980 in both winter and summer in all regions at 30−32ºN, 775

21−23ºN, and 14−16ºN corresponding to the Kuroshio and its counter current, the STCC, and 776

the NEC, respectively, and the nighttime mesozooplankton biomass in the upper 150 m also 777

decreased simultaneously in the former two regions. Based on the longer 137ºE time series 778

during 1971−2000, Watanabe et al. (2005) demonstrated that the mixed layer σθ as well as 779

phosphate and nitrate averaged vertically in the mixed layer exhibited a trend toward decrease 780

in both winter and summer and in all areas at 30−34ºN, 15−30ºN, and 3−15ºN, suggesting a 781

decrease in the supply of nutrients from the subsurface (Fig. 19). Consistently, the water 782

column Chl-a and net community production also had a trend toward decrease in all areas (Fig. 783

20). Furthermore, phosphate, Chl-a, and net community production in all seasons and areas 784

fluctuated bidecadally with a period of ~21 years, lagging behind the PDO index by 2 years 785

(Figs. 19, 20). Vertical structure of biological properties is also changing; during 1972−1997, 786

the proportion of Chl-a existing in the upper 75 m of water column (the depth of subsurface 787

Chl-a maximum) in summer has decreased (increased) at a rate of -0.4 % yr-1 (0.4 m yr-1) in 788

32

the subtropical region of the 137ºE section (Ishida et al. 2009). 789

790

6.2 Marine Pollution 791

To evaluate over the global ocean the degree of petroleum contamination at the sea 792

surface, the Marine Pollution (Petroleum) Monitoring Pilot Project (MaPPMoPP) was 793

implemented by the Intergovernmental Oceanographic Commission and the World 794

Meteorological Organization under the framework of the Integrated Global Ocean Station 795

System in 1975−1980. In response to the request from the MaPPMoPP, the JMA started 796

visual observations of floating pollutants such as floating plastics (Suzuoki and Shirakawa 797

1979) and oil slicks in 1976 and measurements of floating particulate petroleum residues (tar 798

balls; Sano et al. 1979) and dissolved/dispersed petroleum hydrocarbons (Shigehara et al. 799

1979) in 1977 in the western North Pacific including the 137ºE section, as part of the 800

Observation for Monitoring Background Marine Pollution (Ogawa and Takatani 1998; 801

Takatani et al. 1999; Kamiya 2013). Observations during the earlier years demonstrated that 802

tar balls, collected by neuston net towed at the sea surface, and petroleum hydrocarbons, 803

extracted from surface water using carbon tetrachloride and determined by fluorescence 804

spectrophotometry, were abundant in the Kuroshio and its countercurrent, in the STCC, and 805

north of New Guinea, while they were scarce in the NEC (Suzuoki and Matsuzaki 1983; 806

Takatani et al. 1986). In addition, tar balls were sticky and shiny in the Kuroshio and 807

became crumbly and dull to the south and the east, implying that they had been weathered 808

while being transported from their likely source in the western margin of the western North 809

Pacific where the traffic of oil tankers was busy. At 137ºE, tar balls decreased drastically 810

around 1980, and petroleum hydrocarbons have gradually decreased with interannual 811

fluctuations, owing to the restriction of oil discharges from ships by Annex I of the 812

International Convention for the Prevention of Pollution from Ships (MARPOL) 73/78 that 813

entered into force in 1983 (Takatani et al. 1986; Takatani et al. 1999; Japan Meteorological 814

33

Agency 2013; Fig. 21). On the other hand, floating plastics do not exhibit a clear long-term 815

trend, despite the ban of dumping plastics by Annex V of the MARPOL 73/78 Convention 816

that came into force in 1988. 817

818

7. Summary 819

Hydrographic observations along the 137ºE section have been repeated by the JMA using 820

R/Vs Ryofu-maru and Keifu-maru biannually in winter since 1967 and in summer since 1972. 821

These operational observations that have persisted for more than 50 years were initiated as 822

part of the CSK (Masuzawa 1967), and have been maintained with an aim to detect long-term 823

variations of oceanic conditions in the western North Pacific connected with climate change 824

and to monitor background marine pollution (Shuto 1996) and more recently to evaluate 825

changes in the carbon cycle and the mechanistic drivers (Nakano 2013). The data collected 826

along 137ºE have been analyzed not only by the JMA officers but also by scientists from 827

various countries to quantify and understand seasonal to decadal variations and long-term 828

changes of thermohaline structures (Sect. 4.1), large to meso-scale currents (Sect. 4.2), 829

biogeochemical and biological properties (Sect. 5, 6.1), and marine pollutants (Sect. 6.2). 830

Despite the concerns expressed by Dr. Masuzawa during the early years regarding its 831

sampling frequency (Sect. 1), the 137ºE section has been quite useful because it captures 832

zonal integration of westward propagating Rossby signals during the course of which seasonal 833

and shorter-term variations are cancelled out (Qiu and Chen 2010b). 834

When the 137ºE section was initiated 50 years ago, shipboard observations were the main 835

tool of ocean research. Since then, significant development has been made with other 836

observing platforms and tools, including satellites, Argo, various datasets of the ocean interior 837

structure and surface fluxes, and numerical models. The great expansion of observing 838

platforms for oceanography over the last several decades has in fact enhanced and enriched 839

the scientific opportunities afforded by the 137°E section itself and thereby the scientific 840

34

value of the section, as new generations of researchers are able to return to the historical 841

measurement record with new processes and timescale questions. This is reflected in the 842

accelerated increase in scientific publications that incorporate measurements from 137°E (Fig. 843

2). The continued extension and maintenance of the time series has also provided unique 844

opportunities for evaluating variability and transients on decadal and longer timescales over 845

which the ocean plays a dominant role in the climate system. Furthermore, the introduction 846

of CTD and ADCP measurements in the late 1980s opened the door to explorations of 847

submesoscale and microscale phenomena (Sect. 4.3). Thus, beyond Dr. Masuzawa’s 848

expectation, the 137ºE section has contributed to understanding of variability on a wide range 849

of spatiotemporal scales, and has developed to an irreplaceable platform for integrated, 850

multidisciplinary ocean observations. 851

Recent findings on decadal variability of water masses and currents related to the PDO 852

(Sect. 4.1, 4.2) and on smaller-scale phenomena (Sect. 4.3) strongly suggest that the 137ºE 853

section continues to be an essential component of a multi-platform observing system for 854

physical oceanography. In other research areas, it is obviously a “gold mine.” For example, 855

the measurements along 137°E offer an unique opportunity for linking the more well-studied 856

surface variations on ocean biogeochemistry with subsurface variability (Sect. 5). In 857

addition, although the duration of biogeochemical and biological measurements is relatively 858

short compared to physical oceanographic measurements, the mechanistic understanding of 859

scales of variability inferred from the 137ºE section will be important in the interpretation of 860

repeat hydrographic measurements, where there are known issues with aliasing of 861

high-frequency variability in the decadal sampling strategy. Thus, analyses of continued 862

measurements along 137°E will be of value not only for clarifying long-term variations over 863

the local region, but also for clarifying scales of variability more generally. Deep 864

measurements including the WHP/GO-SHIP P9 transects in 1994, 2010, and 2016 also need 865

to be analyzed to examine decadal variability in the abyssal Philippines Sea. It is worth 866

35

noting, finally, that the JMA has also maintained other repeat hydrographic sections, including 867

the one along 165ºE corresponding to the WHP/GO-SHIP P13 section (e.g., Kawabe and 868

Taira 1998; Midorikawa et al. 2002; Oka and Suga 2005; Kouketsu et al. 2010; Qiu et al. 869

2012; Sasano et al. 2015). Integrated analyses of these sections will lead to comprehensive 870

understanding of common, large-scale variability in the western North Pacific that Dr. 871

Masuzawa targeted 50 years ago. 872

873

Acknowledgments 874

The authors are respectful and grateful to the late Dr. Jotaro Masuzawa for his foresight 875

and leadership to launch the 137ºE section and to the present and past staff in the Marine 876

Division of the Global Environment and Marine Department (and the former 877

Division/Department) and the captains and crews of the R/Vs Ryofu-maru and Keifu-maru, 878

the Japan Meteorological Agency for their long-term observation efforts. The authors also 879

thank Michio Aoyama, Chihiro Kawamura, Fumiaki Kobashi, Hideki Nagai, Naoki Nagai, 880

Hiroshi Ogawa, Keith Rodgers, Masaro Saiki, Kazuaki Tadokoro, Atsushi Tsuda, Hiroaki 881

Saito (the editor), and two anonymous reviewers for helpful comments on the manuscript. 882

This review is based on the presentations and discussion made in the symposium entitled 883

“Fifty years of the 137ºE repeat hydrographic section and the future sustained ocean 884

observations in Japan”, which was held on March 18, 2016 in Tokyo associated with the 885

spring meeting of the Oceanographic Society of Japan. 886

887

36

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Figure captions 1498

Fig. 1 The 137ºE section (thick black line) and the associated surface and subsurface 1499

currents (solid and dashed white lines). KE, Kuroshio Extension; nSTCC (sSTCC), 1500

northern (southern) STCC (Kobashi et al. 2006); NEC, North Equatorial Current; MC, 1501

Mindanao current; MUC, Mindanao Undercurrent; ITF, Indonesian Through Flow; NECC, 1502

North Equatorial Countercurrent; NSCC, North Subsurface Countercurrent; EUC, 1503

Equatorial Undercurrent; NGCUC, New Guinea Coastal Undercurrent. nNLM and tLM 1504

denote a nearshore non-large-meander path and a typical large-meander path of the 1505

Kuroshio, respectively (Kawabe 1995). Note that the width of currents is not considered, 1506

particularly for the NEC (see Fig. 4). Thin black contours with color indicate isobaths at 1507

an interval of 500 m, based on ETOPO2v2 (National Geophysical Data Center 2006) 1508

Fig. 2 Cumulative number of papers using the 137ºE section data published in all journals 1509

and books (Supplementary Table S1; circles) and those excluding three in-house journals of 1510

the JMA (Geophysical Magazine, Oceanographical Magazine, and Weather Service 1511

Bulletin; dots), plotted against year from 1967 through 2016 1512

Fig. 3 Latitude of hydrographic observations in winter and summer at the 137ºE section, 1513

plotted against year from 1967 through 2016. The color of dots indicates the maximum 1514

depth of standard layers at each station 1515

Fig. 4 a Distribution of eastward geostrophic velocity (cm s-1) relative to 1000 dbar in the 1516

winter 137ºE section, averaged between 1967 and 1987. Number on the ordinate denotes 1517

depth in meters. Negative values are hatched (after Shuto 1996) b Distribution of 1518

eastward velocity measured by ADCP (color) and potential density (kg m-3) obtained by 1519

CTD (black contours) in the 137ºE section, averaged between 2004 and 2016 (after Qiu et 1520

al. 2017) 1521

Fig. 5 Root-mean-squared sea surface height variability in the North Pacific based on 1522

high-pass-filtered satellite altimeter data from October 1992 to April 2009. Regions 1523

51

where the variability exceeds 12 cm are indicated by thin black contours. White contours 1524

denote the mean sea surface height field (cm) by Niiler et al. (2003) (after Qiu and Chen 1525

2010a) . © Copyright 2010 American Meteorological Society (AMS). Permission to use 1526

figures, tables, and brief excerpts from this work in scientific and educational works is 1527

hereby granted provided that the source is acknowledged. Any use of material in this work 1528

that is determined to be “fair use” under Section 107 of the U.S. Copyright Act or that 1529

satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108) 1530

does not require the AMS’s permission. Republication, systematic reproduction, posting in 1531

electronic form, such as on a website or in a searchable database, or other uses of this 1532

material, except as exempted by the above statement, requires written permission or a 1533

license from the AMS. All AMS journals and monograph publications are registered with 1534

the Copyright Clearance Center (http://www.copyright.com). Questions about permission to 1535

use materials for which AMS holds the copyright can also be directed to the AMS 1536

Permissions Officer at permissions@ametsoc.org. Additional details are provided in the 1537

AMS Copyright Policy statement, available on the AMS website 1538

(http://www.ametsoc.org/CopyrightInformation) 1539

Fig. 6 Distributions of eastward velocity (cm s-1) measured by two current meters in the 1540

137ºE section in a winter and b summer of 1981 (after Guan 1986) 1541

Fig. 7 Distributions of a T and b S in the 137ºE section in winter 1967. Dots indicate the 1542

observation locations. Dotted lines in b indicate thermosteric anomaly contours of 175, 1543

125, and 80 centiliters per ton (cl/t), which correspond to σt = 26.26, 26.79, and 27.26 kg 1544

m-3 (σt is potential density calculated using T instead of θ) (after Masuzawa 1967) 1545

Fig. 8 Time series of cross-sectional area of a STMW, b NPTW, and c NPIW in the 137ºE 1546

section during 1967−2016, calculated from optimally interpolated T/S data of Nakano et al. 1547

(2007). Here, STMW is defined as regions of PV < 2.0 × 10-10 m-1 s-1 and θ = 16º−19.5ºC 1548

(e.g., Suga et al. 1989), NPTW as regions of S > 34.9 north of 7ºN (e.g., Suga et al. 2000), 1549

52

and NPIW as regions of S < 34.2 at depths greater than 200 dbar (e.g., Shuto 1996). Dots 1550

(circles) represent winter (summer) observations. Red (green) plots in b and c indicate the 1551

area north (south) of 18ºN and 25ºN, respectively. Horizontal bars at the bottom of each 1552

panel denote large-meander periods of the Kuroshio. Solid (dotted) bars at the top of each 1553

panel indicate stable (unstable) periods of the KE after October 1992. 1554

Fig. 9 Average (black contour; units: ºC in a) and linear trend (white contours with color; 1555

units ºC year-1 in a and year-1 in b) of a θ and b S with respect to pressure in the 137ºE 1556

section during 1967−2016. Linear trend of S during c 1967−1996 and d 1997−2016, 1557

otherwise following b. (Modified after Oka et al. 2017) 1558

Fig. 10 Time series of a transport of the northern part of NEC circulating in the subtropical 1559

gyre, b transport of the southern part of NEC circulating in the tropical gyre, c the center 1560

latitude of the tropical gyre, and d the NECC transport based on the satellite altimeter data 1561

(left) and the quarterly 137ºE section data (right). In the left (right) panels, gray line 1562

denotes the monthly (quarterly) values. In all panels, black line denotes the 1563

low-pass-filtered time series, and dashed line denotes the linear trend. Note that the 1564

increasing trend (4.1 Sv during 1993−2009) of the NECC transport detected by satellite 1565

altimeter data was not observed in the 137ºE section data, presumably due to the local 1566

decreasing trend north of New Guinea (after Qiu and Chen 2012). © Copyright 2012 1567

American Meteorological Society (AMS). Permission to use figures, tables, and brief 1568

excerpts from this work in scientific and educational works is hereby granted provided that 1569

the source is acknowledged. Any use of material in this work that is determined to be “fair 1570

use” under Section 107 of the U.S. Copyright Act or that satisfies the conditions specified 1571

in Section 108 of the U.S. Copyright Act (17 USC §108) does not require the AMS’s 1572

permission. Republication, systematic reproduction, posting in electronic form, such as on 1573

a website or in a searchable database, or other uses of this material, except as exempted by 1574

the above statement, requires written permission or a license from the AMS. All AMS 1575

53

journals and monograph publications are registered with the Copyright Clearance Center 1576

(http://www.copyright.com). Questions about permission to use materials for which AMS 1577

holds the copyright can also be directed to the AMS Permissions Officer at 1578

permissions@ametsoc.org. Additional details are provided in the AMS Copyright Policy 1579

statement, available on the AMS website (http://www.ametsoc.org/CopyrightInformation) 1580

Fig. 11 a (top) Distribution of time-mean diapycnal diffusivity (K) inferred from 57 CTD 1581

surveys at the 137ºE section during 1997−2010. Grey shade denotes the bathymetry of 1582

Smith and Sandwell (1997). (bottom) Red curve shows the roughness of bathymetry 1583

defined as the root-mean-square bathymetric height variance in a 32 km × 32 km square. 1584

b Distribution at the ASUKA section southeast of Shikoku, Japan, otherwise following a 1585

(after Qiu et al. 2012) 1586

Fig. 12 CO2 concentration (xCO2) in surface seawater (cross: winter, plus: summer) and 1587

marine boundary air (dot: winter, circle: summer) along the 137ºE section in winter and 1588

summer 1990, plotted against latitude (modified after Hirota et al. 1992). xCO2 is related 1589

to pCO2 as, 1590

pCO2 = xCO2 (1 μatm – pH2O), 1591

where pH2O (μatm) is saturated water vapor pressure, which increases with temperature. 1592

Due to the existence of pH2O, pCO2 values in μatm are 1−4% lower than xCO2 values in 1593

ppm 1594

Fig. 13 Seasonal variation of a ΔpCO2 (μatm) and b air-sea CO2 flux (mmol m-2 day-1) in 1595

the western North Pacific between 132º and 142ºE in 1990 (after Inoue et al. 1995) 1596

Fig. 14 Interannual variation of pCO2sw (left) and ΔpCO2 (right) along the 137ºE section in 1597

winter during 1981−1985 (after Fushimi 1987) 1598

Fig. 15 Time series of surface pH at six latitudes at the 137ºE section in a winter and b 1599

summer. Red circles, 3ºN; violet triangles, 10ºN; orange diamonds, 15ºN; green squares, 1600

20ºN; light blue triangles, 25ºN; and blue circles, 30ºN (after Midorikawa et al. 2010) 1601

54

Fig. 16 Time series of preformed nDIC on the isopycnals of σθ = 25.0 (red), 26.0 (orange), 1602

26.8 (green), 27.2 (blue), and 27.5 kg m-3 (purple) at a 10ºN, b 20ºN, and c 30ºN of the 1603

137ºE section. Lines indicate the least-square fit. Values in parentheses denote the 1604

approximate depth of the isopycnals (after Japan Meteorological Agency 2017) 1605

Fig. 17 Increasing rates of water column inventories of CO2 between the sea surface and σθ 1606

= 27.5 kg m-3 at 10−30ºN of the 137ºE section. Vertical bars indicate the 95% 1607

confidence interval. Replotted from Japan Meteorological Agency (2017) 1608

Fig. 18 Time series of a AOU, b salinity-normalized dissolved inorganic carbon (nDIC), c 1609

preformed nDIC, d pH, e aragonite saturation state index (Ωarag), and f nitrate (NO3) 1610

concentration on the isopycnal of σθ = 25.0 kg m-3, based on observations at 25ºN of the 1611

137ºE section (after Oka et al. 2015) 1612

Fig. 19 Time series of the mixed layer σθ (a, b) and phosphate (PO4; c, d) and nitrate (NO3; 1613

e, f) averaged in the mixed layer at 30−34ºN (black circles), 15−30ºN (blue triangles), and 1614

3−15ºN (red squares) of the 137ºE section in winter (a, c, e) and summer (b, d, f). 1615

Vertical bars denote standard errors for three-year running mean. Solid lines and dashed 1616

curves represent the linear regression line and the nonlinear fitting curve estimated by the 1617

Fourier sine expansion, respectively (after Watanabe et al. 2005) 1618

Fig. 20 Time series of the water column Chl-a (CHL) in a winter and b summer and c net 1619

community production (NCP) at the 137ºE section, otherwise following Fig. 19. d PDO 1620

index in winter (after Watanabe et al. 2005) 1621

Fig. 21 Time series of annual-mean concentrations of a tar balls, b petroleum hydrocarbons 1622

as chrysene equivalent, and c floating plastics at the 137ºE section. Replotted from Japan 1623

Meteorological Agency (2013) with additional data in 2013−2016 (Japan Meteorological 1624

Agency 2017) 1625

Photo 1 Dr. Jotaro Masuzawa (1922−2000) (after Nakano 2016) 1626

Photo 2 Water sampling from Nansen bottles and reading reversing thermometers onboard 1627

55

R/V Ryofu-maru II (around 1969) © Japan Meteorological Agency 1628

1629

1630

56

1631

1632

Fig. 1 The 137ºE section (thick black line) and the associated surface and subsurface 1633

currents (solid and dashed white lines). KE, Kuroshio Extension; nSTCC (sSTCC), 1634

57

northern (southern) STCC (Kobashi et al. 2006); NEC, North Equatorial Current; MC, 1635

Mindanao current; MUC, Mindanao Undercurrent; ITF, Indonesian Through Flow; NECC, 1636

North Equatorial Countercurrent; NSCC, North Subsurface Countercurrent; EUC, 1637

Equatorial Undercurrent; NGCUC, New Guinea Coastal Undercurrent. nNLM and tLM 1638

denote a nearshore non-large-meander path and a typical large-meander path of the 1639

Kuroshio, respectively (Kawabe 1995). Note that the width of currents is not considered, 1640

particularly for the NEC (see Fig. 4). Thin black contours with color indicate isobaths at 1641

an interval of 500 m, based on ETOPO2v2 (National Geophysical Data Center 2006) 1642

1643

1644

1645

Fig. 2 Cumulative number of papers using the 137ºE section data published in all journals 1646

and books (Supplementary Table S1; circles) and those excluding three in-house journals of 1647

the JMA (Geophysical Magazine, Oceanographical Magazine, and Weather Service 1648

Bulletin; dots), plotted against year from 1967 through 2016 1649

1650

58

1651

1652

Fig. 3 Latitude of hydrographic observations in winter and summer at the 137ºE section, 1653

plotted against year from 1967 through 2016. The color of dots indicates the maximum 1654

depth of standard layers at each station 1655

1656

1657

59

1658

1659

1660

1661

Fig. 4 a Distribution of eastward geostrophic velocity (cm s-1) relative to 1000 dbar in the 1662

winter 137ºE section, averaged between 1967 and 1987. Number on the ordinate denotes 1663

depth in meters. Negative values are hatched (after Shuto 1996) b Distribution of 1664

eastward velocity measured by ADCP (color) and potential density (kg m-3) obtained by 1665

CTD (black contours) in the 137ºE section, averaged between 2004 and 2016 (after Qiu et 1666

al. 2017) 1667

1668

60

1669

1670

Fig. 5 Root-mean-squared sea surface height variability in the North Pacific based on 1671

high-pass-filtered satellite altimeter data from October 1992 to April 2009. Regions 1672

where the variability exceeds 12 cm are indicated by thin black contours. White contours 1673

denote the mean sea surface height field (cm) by Niiler et al. (2003) (after Qiu and Chen 1674

2010a). © Copyright 2010 American Meteorological Society (AMS). Permission to use 1675

figures, tables, and brief excerpts from this work in scientific and educational works is 1676

hereby granted provided that the source is acknowledged. Any use of material in this work 1677

that is determined to be “fair use” under Section 107 of the U.S. Copyright Act or that 1678

satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108) 1679

does not require the AMS’s permission. Republication, systematic reproduction, posting in 1680

electronic form, such as on a website or in a searchable database, or other uses of this 1681

material, except as exempted by the above statement, requires written permission or a 1682

61

license from the AMS. All AMS journals and monograph publications are registered with 1683

the Copyright Clearance Center (http://www.copyright.com). Questions about permission to 1684

use materials for which AMS holds the copyright can also be directed to the AMS 1685

Permissions Officer at permissions@ametsoc.org. Additional details are provided in the 1686

AMS Copyright Policy statement, available on the AMS website 1687

(http://www.ametsoc.org/CopyrightInformation) 1688

1689

1690

62

1691

1692

Fig. 6 Distributions of eastward velocity (cm s-1) measured by two current meters in the 1693

137ºE section in a winter and b summer of 1981 (after Guan 1986) 1694

1695

1696

1697

63

1698

1699

Fig. 7 Distributions of a T and b S in the 137ºE section in winter 1967. Dots indicate the 1700

observation locations. Dotted lines in b indicate thermosteric anomaly contours of 175, 1701

125, and 80 centiliters per ton (cl/t), which correspond to σt = 26.26, 26.79, and 27.26 kg 1702

m-3 (σt is potential density calculated using T instead of θ) (after Masuzawa 1967) 1703

1704

64

1705

1706

Fig. 8 Time series of cross-sectional area of a STMW, b NPTW, and c NPIW in the 137ºE 1707

section during 1967−2016, calculated from optimally interpolated T/S data of Nakano et al. 1708

(2007). Here, STMW is defined as regions of PV < 2.0 × 10-10 m-1 s-1 and θ = 16º−19.5ºC 1709

(e.g., Suga et al. 1989), NPTW as regions of S > 34.9 north of 7ºN (e.g., Suga et al. 2000), 1710

and NPIW as regions of S < 34.2 at depths greater than 200 dbar (e.g., Shuto 1996). Dots 1711

(circles) represent winter (summer) observations. Red (green) plots in b and c indicate the 1712

area north (south) of 18ºN and 25ºN, respectively. Horizontal bars at the bottom of each 1713

panel denote large-meander periods of the Kuroshio. Solid (dotted) bars at the top of each 1714

panel indicate stable (unstable) periods of the KE after October 1992. 1715

1716

65

1717

1718

Fig. 9 Average (black contour; units: ºC in a) and linear trend (white contours with color; 1719

units ºC year-1 in a and year-1 in b) of a θ and b S with respect to pressure in the 137ºE 1720

section during 1967−2016. Linear trend of S during c 1967−1996 and d 1997−2016, 1721

otherwise following b. (Modified after Oka et al. 2017) 1722

1723

1724

66

1725

1726

Fig. 10 Time series of a transport of the northern part of NEC circulating in the subtropical 1727

gyre, b transport of the southern part of NEC circulating in the tropical gyre, c the center 1728

latitude of the tropical gyre, and d the NECC transport based on the satellite altimeter data 1729

(left) and the quarterly 137ºE section data (right). In the left (right) panels, gray line 1730

denotes the monthly (quarterly) values. In all panels, black line denotes the 1731

low-pass-filtered time series, and dashed line denotes the linear trend. Note that the 1732

increasing trend (4.1 Sv during 1993−2009) of the NECC transport detected by satellite 1733

altimeter data was not observed in the 137ºE section data, presumably due to the local 1734

decreasing trend north of New Guinea (after Qiu and Chen 2012). © Copyright 2012 1735

American Meteorological Society (AMS). Permission to use figures, tables, and brief 1736

excerpts from this work in scientific and educational works is hereby granted provided that 1737

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use” under Section 107 of the U.S. Copyright Act or that satisfies the conditions specified 1739

in Section 108 of the U.S. Copyright Act (17 USC §108) does not require the AMS’s 1740

permission. Republication, systematic reproduction, posting in electronic form, such as on 1741

67

a website or in a searchable database, or other uses of this material, except as exempted by 1742

the above statement, requires written permission or a license from the AMS. All AMS 1743

journals and monograph publications are registered with the Copyright Clearance Center 1744

(http://www.copyright.com). Questions about permission to use materials for which AMS 1745

holds the copyright can also be directed to the AMS Permissions Officer at 1746

permissions@ametsoc.org. Additional details are provided in the AMS Copyright Policy 1747

statement, available on the AMS website (http://www.ametsoc.org/CopyrightInformation) 1748

1749

1750

68

1751

1752

Fig. 11 a (top) Distribution of time-mean diapycnal diffusivity (K) inferred from 57 CTD 1753

surveys at the 137ºE section during 1997−2010. Grey shade denotes the bathymetry of 1754

Smith and Sandwell (1997). (bottom) Red curve shows the roughness of bathymetry 1755

defined as the root-mean-square bathymetric height variance in a 32 km × 32 km square. 1756

b Distribution at the ASUKA section southeast of Shikoku, Japan, otherwise following a 1757

(after Qiu et al. 2012) 1758

1759

1760

69

1761

1762

Fig. 12 CO2 concentration (xCO2) in surface seawater (cross: winter, plus: summer) and 1763

marine boundary air (dot: winter, circle: summer) along the 137ºE section in winter and 1764

summer 1990, plotted against latitude (modified after Hirota et al. 1992). xCO2 is related 1765

to pCO2 as, 1766

pCO2 = xCO2 (1 μatm – pH2O), 1767

where pH2O (μatm) is saturated water vapor pressure, which increases with temperature. 1768

Due to the existence of pH2O, pCO2 values in μatm are 1−4% lower than xCO2 values in 1769

ppm 1770

1771

1772

70

1773

1774

Fig. 13 Seasonal variation of a ΔpCO2 (μatm) and b air-sea CO2 flux (mmol m-2 day-1) in 1775

the western North Pacific between 132º and 142ºE in 1990 (after Inoue et al. 1995) 1776

1777

1778

1779

71

1780

1781

Fig. 14 Interannual variation of pCO2sw (left) and ΔpCO2 (right) along the 137ºE section in 1782

winter during 1981−1985 (after Fushimi 1987) 1783

1784

1785

72

1786

1787

Fig. 15 Time series of surface pH at six latitudes at the 137ºE section in a winter and b 1788

summer. Red circles, 3ºN; violet triangles, 10ºN; orange diamonds, 15ºN; green squares, 1789

20ºN; light blue triangles, 25ºN; and blue circles, 30ºN (after Midorikawa et al. 2010) 1790

1791

1792

73

1793

1794

Fig. 16 Time series of preformed nDIC on the isopycnals of σθ = 25.0 (red), 26.0 (orange), 1795

26.8 (green), 27.2 (blue), and 27.5 kg m-3 (purple) at a 10ºN, b 20ºN, and c 30ºN of the 1796

137ºE section. Lines indicate the least-square fit. Values in parentheses denote the 1797

approximate depth of the isopycnals (after Japan Meteorological Agency 2017) 1798

1799

1800

74

1801

1802

Fig. 17 Increasing rates of water column inventories of CO2 between the sea surface and σθ 1803

= 27.5 kg m-3 at 10−30ºN of the 137ºE section. Vertical bars indicate the 95% 1804

confidence interval. Replotted from Japan Meteorological Agency (2017) 1805

1806

1807

75

1808

1809

Fig. 18 Time series of a AOU, b salinity-normalized dissolved inorganic carbon (nDIC), c 1810

preformed nDIC, d pH, e aragonite saturation state index (Ωarag), and f nitrate (NO3) 1811

concentration on the isopycnal of σθ = 25.0 kg m-3, based on observations at 25ºN of the 1812

137ºE section (after Oka et al. 2015) 1813

1814

76

1815

1816

Fig. 19 Time series of the mixed layer σθ (a, b) and phosphate (PO4; c, d) and nitrate (NO3; 1817

e, f) averaged in the mixed layer at 30−34ºN (black circles), 15−30ºN (blue triangles), and 1818

3−15ºN (red squares) of the 137ºE section in winter (a, c, e) and summer (b, d, f). 1819

Vertical bars denote standard errors for three-year running mean. Solid lines and dashed 1820

curves represent the linear regression line and the nonlinear fitting curve estimated by the 1821

Fourier sine expansion, respectively (after Watanabe et al. 2005) 1822

1823

1824

77

1825

1826

Fig. 20 Time series of the water column Chl-a (CHL) in a winter and b summer and c net 1827

community production (NCP) at the 137ºE section, otherwise following Fig. 19. d PDO 1828

index in winter (after Watanabe et al. 2005) 1829

1830

1831

1832

1833

78

1834

1835

Fig. 21 Time series of annual-mean concentrations of a tar balls, b petroleum hydrocarbons 1836

as chrysene equivalent, and c floating plastics at the 137ºE section. Replotted from Japan 1837

Meteorological Agency (2013) with additional data in 2013−2016 (Japan Meteorological 1838

Agency 2017) 1839

1840

1841

1842

79

1843

1844

Photo 1. Dr. Jotaro Masuzawa (1922−2000) (after Nakano 2016) 1845

1846

80

1847

1848

Photo 2. Water sampling from Nansen bottles and reading reversing thermometers onboard 1849

R/V Ryofu-maru II (around 1969) © Japan Meteorological Agency 1850

1851

1852

1

Supplementary Table S1 List of papers using data from the 137°E section. Note that this

list includes only those papers that specified use of the 137°E data, and does not include

those papers that used the 137°E data through datasets or databases.

1967–1969

Masuzawa J (1967) An oceanographic section from Japan to New Guinea at 137°E in January

1967. Oceanogr Mag 19:95–118

Akiyama T (1968) Partial pressure of carbon dioxide in the Atmosphere and in sea water over

the western North Pacific Ocean. Oceanogr Mag 20:133146

Akiyama T, Sagi T, Yura T (1968) On the distributions of pH in situ and total alkalinity in the

western North Pacific Ocean. Oceanogr Mag 20:18

Hayashi S, Miyashita I, Fujita I (1968) Maritime meteorological summary of western North

Pacific Ocean in winter. Oceanogr Mag 20:105–120

Kawarada Y, Kitou M, Furukawa K, Sano A (1968) Plankton in the western North Pacific in

the winter of 1967 (CSK). Oceanogr Mag 20:9–29

Masuzawa J (1968) Second cruise for CSK, Ryofu Maru, January to March 1968. Oceanogr

Mag 20:173–185

Akamatsu and Sawara (1969) The preliminary report of the third cruise for CSK, January to

March 1969. Oceanogr Mag 21:83–96

Akiyama T (1969) Carbon dioxide in the atmosphere and in sea water over the western North

Pacific Ocean. Oceanogr Mag 21:121127

Kawarada Y, Sano A (1969) Distribution of chlorophyll and phaeophytin in the western North

Pacific. Oceanogr Mag 21:137–146

Kitou M, Tanaka O (1969) Note on a species of Calanoides (Copepoda, Calanoida) from the

western North Pacific. Oceanogr Mag 21:67–81

Sagi T (1969) The concentration of calcium and the calcium chlorosity ratio in the western

North Pacific Ocean. Oceanogr Mag 21:61−66

Sagi T (1969) The ammonia content in sea water in the western North Pacific Ocean.

Oceanogr Mag 21:113−119

1970–1974

Hayashi S, Takano H, Yamashita A (1970) Maritime meteorological summary of the western

North Pacific Ocean in the winters of 1967, 1968 and 1969. Oceanogr Mag 22:95–123

Masuzawa J (1970) Geostrophic flux of the North Equatorial current south of Japan. J

Oceanogr Soc Jpn 26:61–64

Masuzawa J, Akiyama T, Kawarada Y, Sawara T (1970) Preliminary report of the Ryofu

Maru Cruise Ry7001 in January–March 1970. Oceanogr Mag 22:1–25

Sagi T (1970) On the distribution of nitrate nitrogen in the wesetern North Pacific Ocean.

Oceanogr Mag 22:63−74

Masuzawa J (1972) Water characteristics of the North Pacific central region. In: Stommel H,

Yoshida K (eds) Kuroshio—Its Physical Aspects. Univ Tokyo Press, Tokyo, pp 95–127

Nagasaka K and Sawara T (1972) A preliminary report of the cruise of the R/V Ryofu-Maru

in January–March 1971. Oceanogr Mag 24:25–38

Kitou M (1974) Chaetognatha. In: Marumo R (ed) Marine Plankton, Univ Tokyo Press,

Tokyo, pp 65–85 (in Japanese)

2

Sagi T, Yura T, Akiyama T (1974) The cadmium content in sea water in the adjacent regions

of Japan and in the western North Pacific. Oceanogr Mag 25:101−110

1975–1979

Masuzawa J, Nagasaka K (1975) The 137°E oceanographic section. J Mar Res

33(Suppl):109–116

White WB, Hasunuma K, Solomon H (1978) Large-scale seasonal, secular variability of the

subtropical front in the western North Pacific from 1954 to 1974. J Geophys Res

83:4531–4544

Sano A, Satoh N, Kubo N (1979) Floating particulate petroleum resideus, tar balls, in the

western North Pacific. Oceanogr Mag 30:47–54

Shigehara K, Kimura K, Ohoyama J, Kubo N (1979) Fluorescing materials in marine

environment and petroleum pollution: I. Analytical method and preliminary report of the

result. Oceanogr Mag 30:61–74

Suzuoki T, Shirakawa K (1979) Visual observation on the floating pollutants in open ocean.

Oceanogr Mag 30:55–60

1980–1984

Minami H, Kamihira E, Nishizawa J (1981) Some features of water mass on the warm eddies

south of Honshu, Japan. Umi to Sora 57:1–18 (in Japanese with English abstract)

Nagasaka K (1981) Long-term variation of sea water temperature. Tenki 28:553–556 (in

Japanese)

Kimura Y (1982) Short-term temperature variation along 137°E in the subtropical region of

the western North Pacific. Oceanogr Mag 32:1–10

White WB, Meyers G, Hasunuma K (1982) Space/time statistics of short-term climatic

variability in the western North Pacific. J Geophys Res 87:1979–1989

Suzuoki T, Matsuzaki M (1983) Tar ball distribution and dynamics of surface waters in the

western North Pacific. Oceanogr Mag 33:19–26

Kurihara K (1984) Analysis of the statistical relationship between the end of the Baiu in

Tokyo and sea water temperatures in the western tropical Pacific Ocean. Geophys Mag

41:159–171

1985–1989

Kurihara K (1985) Relationship between the surface air temperature in Japan and sea water

temperature in the western tropical Pacific during summer. Tenki 32:407–417 (in

Japanese)

Guan B (1986) Current structure and its variation in the equatorial area of the western North

Pacific Ocean. Chin J Oceanol Limnol 4:239–255

Sudo (1986) Deep Water property variations below about 4000 m in the Shikoku Basin. La

mer 24:21–32

Takatani S, Sagi T, Imai M (1986) Distributions of floating tars and petroleum hydrocarbons

at the surface in the western North Pacific. Oceanogr Mag 36:33–42

Andow T (1987) Year-to-year variation of oceanography sub-surface section along the

meridian of 137°E. Oceanogr Mag 37:47–73

Fushimi K (1987) Variation of carbon dioxide partial pressure in the western North Pacific

surface water during the 1982/83 El Niño event. Tellus 39B:214–227

3

Inoue HY, Sugimura Y, Fushimi K (1987) pCO2 and δ13C in the air and surface sea water in

the western North Pacific. Tellus 39B:228–242

Saiki M (1987) Interannual variation of the subtropical gyre in the western North Pacific. Umi

to Sora 63:113–125 (in Japanese with English abstract)

Hanawa K, Watanabe T, Iwasaka N, Suga T, Toba Y (1988) Surface thermal conditions in the

western North Pacific during the ENSO events. J Meteorol Soc Jpn 66:445–456

Inoue H, Sugimura Y (1988) Distribution and variations of oceanic carbon dioxide in the

western North Pacific, eastern Indian, and Southern Ocean south of Australia. Tellus

40B:308–320

Inoue H, Sugimura Y (1988) Distribution of the pCO2 in the surface seawater of the western

and central equatorial Pacific during the 1986/87 El Niño/Southern Oscillation event.

Geophys Res Lett 15:1499–1502

Kano Y (1988) Variation of oceanographic conditions in the western North Pacific and its

relation to the Kuroshio and El Niño events. Umi to Sora 63:159–174 (in Japanese with

English abstract)

Kendall TR (1989) Fluctuation of transports and sea level in the western boundary region of

the tropical Pacific Ocean. J Oceanogr Soc Jpn 45:279–287

Suga T, Hanawa K, Toba Y (1989) Subtropical mode water in the 137ºE section. J Phys

Oceanogr 19:1605−1618

1990–1994

Kawashima K, Nagai N (1990) Distribution of planktonic chaetognaths in the western North

Pacific. Oceanogr Mag 40:53–64

Yasunari T (1990) Impact of Indian monsoon on the coupled atmosphere/ocean system in the

tropical Pacific. Meteorol Atmos Phys 44:29–41

Fushimi K, Nishikawa M, Mitsuda H (1991) Floating pumices in the western North Pacific.

Oceanogr Mag 41:59–74

Hirota M, Nemoto K, Murata A, Fushimi K (1991) Observation of carbon dioxide

concentrations in air and surface sea water in the western North Pacific Ocean. Oceanogr

Mag 41:19−28

Hayashi K, Dokiya Y, Ohyama J, Sagi T, Murata E, Fushimi K, Tanaka S (1992) Deposition

of selenium and arsenic on the western North Pacific. Oceanogr Mag 42:43–53

Hirota M, Nemoto K, Murata A, Fushimi K (1992) The observation of greenhouse gases and

substances that deplete the ozone layer on board the Research Vessel Ryofu Maru of the

Japan Meteorological Agency. Weather Service Bull 59:145−159 (in Japanese)

Inoue HY, Sugimura Y (1992) Variations and distributions of CO2 in and over the equatorial

Pacific during the period from the 1986/88 El Niño event to the 1988/89 La Niña event.

Tellus 44B:1–22

Qiu B, Joyce TM (1992) Interannual variability in the mid-and low latitude western North

Pacific. J Phys Oceanogr 22:1062–1079

Fushimi K, Hirota M, Nemoto K, Murata AM (1993) Distribution of methane in the surface

seawater and the air/sea flux over the western North Pacific. Oceanogr Mag 43:33–46

Gouriou Y, Toole J (1993): Mean circulation of the upper layers of the western equatorial

Pacific Ocean. J Geophys Res 98:22495–22520

Hirota M, Nemoto K, Murata AM, Fushimi K (1993) Observation of methane concentrations

in air and surface sea water in the western North Pacific Ocean. Oceanogr Mag 43:21−31

4

Murata AM (1993) A note on large annual growth rate of atmospheric CO2 over the western

North Pacific in the period 1990–1991. Oceanogr Mag 43:1–20

Kimura S, Tsukamoto K, Sugimoto T (1994) A model for the larval migration of the Japenese

eel: roles of the trade winds and salinity front. Mar Biol 119:185–190

Minami H, Takatsuki Y, Matsubara T (1994) The heat flux from sea surface and heat

transport in the ocean along the 137ºE observation line. Umi to Sora 69:231–244 (in

Japanese with English abstract)

1995–1999

Bessho S (1995) Geostrophic balance of the western North Pacific tropical current. Geophys

Mag Ser 2 1:1–4

Bessho S (1995) Interannual variations in the main thermocline in the western North Pacific

along the 137ºE meridian. Geophys Mag Ser 2 1:5–11

Bingham FM, Lukas R (1995) The distribution of intermediate water in the western equatorial

Pacific during January–February 1986. Deep Sea Res II 42:1545–1573

Inoue HY, Matsueda H, Ishii M, Fushimi K, Hirota M, Asanuma I, Takasugi Y (1995)

Long-term trend of the partial pressure of carbon dioxide (pCO2) in surface waters of the

western North Pacific, 1984–1993. Tellus 47B:391–413

Suga T, Hanawa K (1995) Interannual variations of North Pacific Subtropical Mode Water in

the 137˚E section. J Phys Oceanogr 25:1012−1017

Murata AM, Fushimi K (1996) Temporal variations in atmospheric and oceanic CO2 in the

western North Pacific from 1990 to 1993: possible link to the 1991/92 ENSO event. J

Metereol Soc Jpn 74:1–20

Shuto K (1996) Internal variations of water temperature and salinity along the 137°E meridian.

J Oceanogr 52:575–595

Ando K, McPhaden M (1997) Variability of surface layer hydrography in the tropical Pacific

Ocean. J Geophys Res 102:23063–23078

Kuragano T, Shibata A (1997) Sea surface dynamic height of the Pacific Ocean derived from

TOPEX/POSEIDON altimeter data: calculation method and accuracy. J Oceanogr

53:585–599

Takahashi T, Feely RA, Weiss RF, Wanninkhof RH, Chipman DW, Sutherland SC,

Takahashi TT (1997) Global air-sea flux of CO2. Proc Natl Acad Sci 94:8292–8299

Kaneko I, Takatsuki Y, Kamiya H, Kawae S (1998) Water property and current distributions

along the WHP-P9 section (137°–142°E) in the western North Pacific. J Geophys Res

103:12959–12984

Murata AM, Kaneko I, Nemoto K, Fushimi K, Hirota M (1998) Spatial and temporal

variations of surface seawater fCO2 in the Kuroshio off Japan. Mar Chem 59:189–200

Sugimoto T, Tadokoro K (1998) Interdecadal variations of plankton biomass and physical

environment in the North Pacific. Fish Oceanogr 7:289–299

Qu T, Mitsudera H, Yamagata T (1999) A climatology of the circulation and water mass

distribution near the Philippine coast. J Phys Oceanogr 29:1488–1505

Takatani S, Oikawa K, Ogawa Kan (1999) Long-term monitoring of background marine

pollution (oil pollution) in the western North Pacific. Weather Service Bull

66(Sp):S89−S96 (in Japanese)

5

2000–2004

Inoue HY (2000) CO2 exchange between the atmosphere and the ocean: carbon cycle studies

of the Meteorological Research Institute since 1968. In: Handa N, Tanoue E, Hama T

(eds) Dynamics and Characterization of Marine Organic Matter. Terra Sci, Tokyo, pp

509–531

Suga T, Kato A, Hanawa K (2000) North Pacific Tropical Water: Its climatology and

temporal changes associated with the climate regime shift in the 1970s. Prog Oceanogr

47:223–256

You Y, Suginohara N, Fukasawa M, Yasuda I, Kaneko I, Yoritaka H, Kawamiya M (2000)

Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific Intermediate

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Aoyama M, Hirose K, Miyao T, Igarashi Y, Povinec PP (2001) 137Cs activity in surface water

in the western North Pacific. J Radioanal Nucl Chem 248:789– 793

Ishii M, Inoue HY, Matsueda H, Saito S, Fushimi K, Nemoto K, Yano T, Nagai H,

Midorikawa T (2001) Seasonal variation in total inorganic carbon and its controlling

processes in surface waters of the western North Pacific subtropical gyre. Mar Chem

75:17–32

Kaneko I, Takatsuki Y, Kamiya H (2001) Circulation of intermediate and deep waters in the

Philippine Sea. J. Oceanogr 57:397–420

Kimura S, Inoue T, Sugimoto T (2001) Fluctuation in the distribution of low-salinity water in

the North Equatorial Current and its effect on the larval transport of the Japanese eel.

Fish Oceanogr 10:51–60

Xie SP, Liu T, Liu Q, Nonaka M (2001) Far-reaching effects of the Hawaiian Islands on the

Pacific Ocean-atmosphere. Science 292:2057–2060

Aoki Y, Suga T, Hanawa K (2002) Subsurface subtropical fronts of the North Pacific as

inherent boundaries in the ventilated thermocline. J Phys Oceanogr 32:2299−2311

Feely RA, Boutin J, Cosca CE, Dandonneau Y, Etcheto J, Inoue HY, Ishii M, Quéré CL,

Mackey DJ, McPhaden M, Metzl N, Poisson A, Wanninkhof R (2002) Seasonal and

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Hirose K, Kamiya H (2003) Vertical nutrient distributions in the western North Pacific

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Luo JJ, Yamagata T (2003) A model study on the 1988–89 warming event in the northern

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Nagai N (2004) Middle and long-term variations of marine plankton along JMA’s 137ºE

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Soga T, Takatsuki Y, Hayashi K (2005) Variability of the North Pacific Subtropical Mode

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Watanabe YW, Ishida H, Nakano T, Nagai N (2005) Spatiotemporal decreases of nutrients

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concentration of various phosphorus pools and their possible biogeochemical roles in the

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Fujii M, Chai F, Shi L, Inoue HY, Ishii M (2009) Seasonal and interannual variability of

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carbonate parameters in the western North Pacific. Tellus Ser B 62:649–659

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Sugimoto S, Hanawa K (2011) Quasi-decadal modulations of North Pacific Intermediate

Water area in the cross section along the 137ºE meridian: impact of the Aleutian Low

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