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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: [email protected]; 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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43
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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
4
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
5
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
119
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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197
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
10
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
288
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
13
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
14
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 [email protected]. 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
[email protected]. 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 [email protected]. 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
the source is acknowledged. Any use of material in this work that is determined to be “fair 1738
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
[email protected]. 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)
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2000–2004
Inoue HY (2000) CO2 exchange between the atmosphere and the ocean: carbon cycle studies
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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
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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
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75:17–32
Kaneko I, Takatsuki Y, Kamiya H (2001) Circulation of intermediate and deep waters in the
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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
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Aoki Y, Suga T, Hanawa K (2002) Subsurface subtropical fronts of the North Pacific as
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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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humic-like fluorescent dissolved organic matter in the subtropical western North Pacific
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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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