SST Gradients and the East Asian Early-Summer Monsoon

January 3, 2017 15:58 The Global Monsoon System: Research and Forecast (3rd Edition) - 10.25in x 7.5in b2503-ch01 page 3

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Chapter 1

SST Gradients and the East Asian Early-Summer Monsoon

Richard H. Johnson∗ and Michael D. Toy†∗Department of Atmospheric Science,

Colorado State University, Fort Collins, CO 80523, USA†National Center for Atmospheric Research,

Boulder, CO 80303, [email protected]

www.colostate.edu

The East Asian summer monsoon is characterized by strong, moist southwesterly flow over a broadexpanse that includes the seas adjacent to China and the Pacific Ocean south of Japan. Relativelystrong sea surface temperature (SST) gradients, particularly at the time of summer monsoon onset,exist over these waters as a result of the previous winter’s cold outbreaks. The flow of warm airover cooler water can lead to low-level convergence through the suppression of vertical momentumtransport in the boundary layer as a result of increased stability. Given sufficient moisture andinstability, deep convection can break out.

Studies have shown that this process is operative off the coast of East Asia during the summermonsoon and frequently contributes to extensive bands of precipitation. Most of the precipitationenhancement occurs over the open ocean; however, an important exception is in the area of Taiwanwhere the SST gradient intersects the island during May and June. A recent study using data fromthe 2008 SoWMEX/TiMREX suggests that the SST gradient upstream of Taiwan can increaserainfall over the island by locally enhancing low-level convergence offshore.

1. Introduction

Flow of air across gradients in sea surface tem-perature (SST) can have a significant impact onthe structure of the atmospheric boundary layer.There are several ways in which the impactscan be felt. First, flow across SST gradients canlead to changes in near-surface stability, surfacestress and fluxes, such that the wind profile ismodified (Sweet et al. 1981; Businger and Shaw1984). A second effect is the modification of tur-bulent kinetic energy, leading to changes in thevertical transfer of momentum and boundarylayer depth (Hayes et al. 1989; Wallace et al.1989). In addition to these effects, there can behydrostatic pressure changes across SST gradi-ents that can drive low-level convergence in the

tropics (Lindzen and Nigam 1987). These pro-cesses are reviewed in Xie (2004), Small et al.(2008), and Chelton and Xie (2010).

To illustrate how SST gradients can modifythe vertical transfer of momentum, a schematicdiagram is presented in Fig. 1 (from Cheltonand Xie 2010). It shows flow of air from coolto warm SSTs across a meandering SST frontalboundary. Winds are stronger on the warm sideof the front due to enhanced buoyancy fluxesand downward mixing of higher momentum airfrom aloft. Where the flow is perpendicular tothe front, there is low-level convergence of theflow and the sea surface stress. Where the flowis parallel to the front, positive vorticity is gen-erated as well as a positive curl of the surfacestress. Changes in the surface wind stress can

The Global Monsoon System: Research and Forecast (3rd Edition)Edited by C. P. Chang et al.c© 2016 by World Scientific Publishing Co.


4 R. H. Johnson and M. D. Toy

Fig. 1. Idealized depiction of surface flow across ameandering SST front (adapted from Chelton and Xie2010). Green shaded area indicates convergence of boththe surface flow and surface stress τ . Red shaded areaindicates positive vorticity and positive curl of thesurface stress.

feed back and alter the properties of the upperocean (Small et al. 2008).

The flow configuration depicted in Fig. 1is representative of that climatologically occur-ring over East Asian coastal seas and north-west Pacific Ocean during the summer monsoon.Namely, southwesterly monsoon flow passes overwarm waters in the south toward cooler watersin the north. This situation for the month ofJune is illustrated in Fig. 2. Airflow extendsfrom the southern South China Sea and westernPacific Ocean to the Japan/Korea region, wherethe SST is considerably lower. Although there isnot a sharp frontal boundary like that depictedin Fig. 1, there is a flow across an SST gradientsuch that air mass transformation and modifi-cation of the boundary layer occur. The condi-tion of warm-to-cool climatological airflow firstbegins in mid-May with the onset of the EastAsian summer monsoon. The air-sea interactionprocesses just described will be reviewed for theregion of the East Asian summer monsoon.

2. Previous Investigations of Air-SeaInteraction Impacts on Rainfall

The advent of global, satellite-based measure-ments of surface winds, SST, and precipitation

Fig. 2. 1971–2000 climatological mean June SST fromLamont-Doherty Earth Observatory website (http://iridl.ldeo.columbia.edu/) based on 1◦ Reynolds et al.(2002) SST data. White arrows schematically representtypical near-surface monsoonal airflow.

over the past two decades has permitted thedocumentation of a myriad of fascinating andcomplex interactions between the ocean andatmosphere (Xie 2004; Small et al. 2008; Cheltonand Xie 2010). Particularly prominent are situ-ations where flow across relatively strong SSTgradients modifies the boundary layer, cloudi-ness, and precipitation, such as the easternPacific cold tongue (de Szoeke and Bretherton2004), Tropical Instability Waves in the PacificOcean (Chelton et al. 2001; Liu et al. 2000;Hashizume et al. 2002), the Gulf Stream(Minobe et al. 2008), and the Kuroshio and itsextension (Nonaka et al. 2003; Xu et al. 2011;Miyama et al. 2012).

An investigation has been carried out byTokinaga et al. (2009) of air-sea interaction pro-cesses associated with warm air flowing overcooler water in the northwest Pacific Ocean,as depicted in Fig. 2. They used ship andsatellite observations, and reanalysis data toshow that strong SST gradients associated withthe Kuroshio Extension impact clouds andprecipitation over an extensive area. Results


SST Gradients and the East Asian Early-Summer Monsoon 5

Fig. 3. June–July climatology: (a) QuikSCAT surfaceconvergence (color in 10−5 s−1) and AMSR-E SST (thincontours at 1◦C intervals) superimposed on negativepressure vertical velocity of JRA-25 at 700 hPa (thickcontours greater than 2×10−2 Pa s−1 at 1×10−2 Pa s−1

intervals) and (b) vertical section of negative pressurevelocity (color in 10−2 Pa s−1) and horizontal conver-gence (contours at 0.5 × 10−6 s−1) along 145◦E. FromTokinaga et al. (2009).

of their study are shown in Fig. 3. Strongconvergence and upward motion at 700 hPacan be seen at the southern edge of thestrong SST gradient (Fig. 3a). As evidentfrom Fig. 3b, the surface convergence producesdeep upward motion in this convectively unsta-ble region of the western Pacific. In anotherstudy, Tanimoto et al. (2009) showed thatwarm southerlies across the Kuroshio Exten-sion can lead to fog formation over the coolerwater.

3. SST Gradients Over theNorthern South China Sea

The onset of the East Asian summer monsoonover the South China Sea (SCS) typically occursaround mid-to-late May (Wu and Zhang 1998;Ding and Chan 2005). It is marked by a reversalof the winds from north-easterlies to southwest-erlies over the northern SCS, where a strong SSTgradient exists as a remnant of the winter mon-soon (Chu and Chang 1997). Xie et al. (2002)showed that for the East China Sea, the coolingis enhanced due to the shallow offshore waters,which should also be a factor for the northernSCS. The mean conditions in this region for Mayare shown in Fig. 4. A fairly strong SST frontexists over the northern SCS with convergenceof the surface winds quite noticeable along theboundary. Although the SST moderates some-what as the monsoon season progresses, an SSTgradient still exists in June, as shown Fig. 5.

The mean surface winds for June are gen-erally southerly and a deceleration of the flowis evident as air crosses the strong SST gra-dient southwest and northeast of Taiwan. Thereduction in wind speed, and hence increasein low-level convergence, associated with thepassage of the air from warm to cool water

Fig. 4. Ten-year (2000–2009) May mean QuikSCATwinds and SST (1◦ data; Reynolds et al. 2002). Noteconvergence along SST front over northern SCS.



Fig. 5. As in Fig. 4, but for June.

is consistent with the mechanism proposed byWallace et al. (1989). The existence of enhancedlow-level convergence upstream of Taiwan dur-ing the early summer monsoon or Mei-yu periodwhen the atmosphere is convectively unstablesuggests a possible impact of strong SST gradi-ents upwind of Taiwan on island rainfall. TheMei-yu period is a time of very heavy rainfallover Taiwan (Chen et al. 2007). The possibil-ity of air-sea interaction affecting Taiwan rain-fall has been recently investigated by Toy andJohnson (2014). This work is the subject of thenext section.

4. Effects of SST Gradients onTaiwan Rainfall during TiMREX

In May-June 2008, a field campaign —SoWMEX (Southwest Monsoon Experiment)/TiMREX (Terrain-influenced Monsoon Rain-fallExperiment) — was conducted over Taiwan andthe surrounding region to investigate the mech-anisms for heavy rainfall over Taiwan (Jou et al.2011). Instrumentation deployed in the experi-ment included aircraft, special soundings, ships,research radars, and surface stations. Theseobservations and a numerical model have beenused in Toy and Johnson (2014) to investi-gate the potential impact of the SST gradientupstream of Taiwan on island rainfall.

The focus of the Toy and Johnson (2014)study is on the 13–18 June period of heavyrainfall over southern Taiwan when there wasmoist, southerly flow impinging on the island(Davis and Lee 2012; Xu et al. 2012). In orderto determine the effects of the SST gradient,the Advanced Research Weather and Forecast-ing Research (WRF) model (ARW; Skamarocket al. 2008) was used to carry out two simula-tions: (1) a control run with the observed SSTfield (Fig. 6a, CTRLSST, from the daily real-time global SST (RTG SST) analyses of theNational Centers for Environmental Prediction(NCEP) on a 0.5◦ grid) and (2) a companion runwith a smoothed SST field (Fig. 6b, SMTHSST).The SMTHSST field was constructed in sucha way so as to reduce the gradient upstream(to the southwest) of the island thereby exam-ining its impact on Taiwan rainfall. The SSTdistribution upwind of Taiwan in Fig. 6b isnot unlike that observed in a number of pastyears during the early summer monsoon. TheSST difference plot (Fig. 6c) shows that com-pared to SMTHSST, the CTRLSST distribu-tion is characterized by cooler water over thenorthern Taiwan Strait and warmer water offthe south tip of Taiwan.

The WRF is used with a convection-permitting horizontal grid spacing of 3 km witha stretched vertical grid having 60 levels fromthe surface to 50 hPa. The following parame-terizations are used: the WRF Single-Moment6-Class Microphysics scheme (Hong and Lim2006), Yonsei University boundary layer (Honget al. 2006, Dudhia shortwave radiation, andrapid radiative transfer model (Mlawer et al.1997) for longwave radiation. The control runwas initialized with the NCEP Global Opera-tional Analysis fields on the boundaries shownin Fig. 6 starting at 0000 UTC on 12 Junewith boundary values updated every six hours,and run up until 18 June 2008. An additionalsix runs were initialized with random perturba-tions using the WRF Model data assimilation(WRFDA) system (Barker et al. 2012). Results



Fig. 6. (a) Control SST for 13–18 Jun and(b) Smoothed-SST used in simulations, and (c) SSTdifference between the two. Arrow in (a) depicts direc-

tion of near-surface flow.

shown are the means from the 7-member ensem-ble runs. Simulations were carried out with andwithout the island of Taiwan in the model inorder to separate out the effects of the SST gra-dient from flow blocking by the high terrain ofthe island. For the sake of brevity, the no-Taiwanresults will not be shown here.

The accumulated rainfall over the five-day13–18 June period from the control simulation isshown along with the Tropical Rainfall Measur-ing Mission (TRMM) rainfall estimate in Fig. 7.In addition to the large rainfall totals along thesouth coast of China and over the north-centralSCS, the model shows a rainfall maximum alongthe southwest coast of Taiwan (Fig. 7a). The dis-tribution of the rainfall over the entire domainagrees quite well with TRMM 3B42 estimates(Fig. 7b); however, TRMM places the rainfallmaximum offshore whereas the model places itonshore. An operational rainfall product basedon composite radar data prepared by Taiwan(not shown) gives better agreement with themodel results in having the maximum rainfalljust inland of the coastline. The TRMM prod-uct, which uses a merger of microwave and out-going longwave radiation data, likely places themaximum rainfall too far to the southwest as aresult of northeasterly flow aloft advecting cir-rus from the coastal precipitation offshore. Thenorth end of Taiwan received considerably lessprecipitation than the south end during thisperiod of the experiment.

The simulated rainfall from the SMTHSSTrun, along with the rainfall difference(CTRLSST–SMTHSST), is shown in Fig. 8.The general pattern of rainfall over the largedomain in the SMTHSST (Fig. 8a) is simi-lar to that for the CTRLSST (Fig. 7a), asmight be expected. However, there are impor-tant differences between the two simulations inseveral areas related to the different SST dis-tributions (Fig. 8b). With respect to Taiwan,there is an increase in rainfall up to ∼100mmalong the southwest coast in the control simula-tion, indicating that the stronger SST gradient



Fig. 7. Accumulated rainfall over model domain from0000 UTC 13 Jun through 0000 UTC 18 Jun from

(a) WRF CTRLSST simulation and (b) TRMM 3B42data. The SST (◦C) is overlaid (contours).

is enhancing rainfall there. On the other hand,there is a reduction in rainfall over the cen-tral and northern Taiwan Strait as a result ofthe cooler water in the control simulation. Theincreased rainfall in the CTRLSST simulationover and just offshore the southwest coast ofTaiwan is associated with increased convergencein that location (not shown).

In order to understand how the SST gra-dient increases precipitation in this region, itis necessary to look at the modification of the

Fig. 8. (a) Accumulated rainfall (color shading) over

model domain from 0000 UTC 13 Jun through 0000 UTC18 Jun from WRF SMTHSST simulation. SST (◦C) isoverlaid (contours). (b) Rainfall difference (color shad-ing) between CTRLSST and SMTHSST simulations.SST difference between CTRLSST and SMTHSST simu-lations is overlaid (0.25◦C contour interval with negativevalues indicated by dashed lines).

boundary layer by the changing conditions of theunderlying ocean. To do so, plots of the verticalstructure of the lower atmosphere are presentedfor the CTRLSST and SMTHSST simulations(Fig. 9). The longitude of this plot is 120◦E,

May 19, 2017 11:24 The Global Monsoon System: Research and Forecast (3rd Edition) - 10.25in x 7.5in b2503-ch01 page 9


Fig. 9. Vertical cross sections along 120◦E of mean 0000 UTC 13 Jun–0000 UTC 14 Jun (a),(b) wind speed, (c),(d)horizontal divergence (color shading) and potential temperature (K) (contours, with 0.2 K interval for θ ≤ 302K),and difference (CTRLSST–SMTHSST) in (g) wind speed and (h) horizontal divergence. (e),(f) Mean 0000 UTC13 Jun–0000 UTC 18 Jun buoyancy flux (red curve) and SST (black curve) along 120◦E. (i) Difference (CTRLSST–SMTHSST) in accumulated rainfall along 120◦E from 0000 UTC 13 Jun through 0000 UTC 18 Jun. Letters “S” and“M” indicate latitude of ship (99810) and Makung (46734) radiosonde stations referred to in Toy and Johnson (2014).The bold brown line indicates the latitude range of the island of Taiwan for reference.



just off the west coast of Taiwan (Fig. 8). Thewind speed profiles (Figs. 9a,b) are broadly sim-ilar for both simulations with a low-level jetdeveloping within the latitude band of Taiwan.This feature is a south-westerly barrier jet thatforms to the west of Taiwan owing to flowblocking by the island (Li and Chen 1988; Yehand Chen 2002; Wang et al. 2005). However,within the latitude band encompassed by theTiMREX research ship (S) and Makung (M),there is an important difference in the flow veryclose to the surface. Namely, there is a decel-eration of the flow in CTRLSST that is notpresent in SMTHSST. This feature is reflectedin a localized convergence maximum between Sand M in CTRLSST (Fig. 9c), which is weakerin SMTHSST (Fig. 9d). These two plots alsoshow a much stronger near-surface frontal zonein this location in CTRLSST than SMTHSST.This frontal zone is anchored by the sharp pole-ward decline of the surface buoyancy flux acrossthe SST front. This effect has been shown tobe operative on the synoptic scale by Hottaand Nakamura (2011). Figures 9e,f illustratethe differing SST gradients and buoyancy fluxesacross the span of Taiwan. The increased SSTsnorth of the front in the SMTHSST simula-tion serve to enhance the downward mixing ofmomentum from the barrier jet, thereby increas-ing the surface buoyancy fluxes and weakeningthe near-surface frontal boundary near 23◦N.This process, in turn, leads to weaker conver-gence and less rainfall than in the CTRLSSTcase. The model difference fields of wind speed,divergence, and rainfall (Figs. 9g,h,i) show thatthe increased convergence and rainfall in theCTRLSST occurs at the leading (southern) edgeof the SST front. The greater low-level stabilityand reduced downward mixing of momentum inthe CTRLSST simulation near 23–24◦N supportthe arguments of Hayes et al. (1989) and Wallaceet al. (1989) for SST gradient impacts on bound-ary layer convergence.

The above findings illustrated for 13–14 Junein Fig. 9 (a period prior to the breakout of deep

convection) have been repeated for a longer, con-vectively active period (13–17 June) with sim-ilar results obtained (Toy and Johnson 2014).These simulations were designed to demonstratethat the effects of the SST gradient on thelow-level flow were independent of the exis-tence of precipitating systems. In addition, sim-ulations were carried out with the island ofTaiwan removed and the results were qualita-tively unchanged (not shown), implying thatthe SST gradient impacts on rainfall oversouthern Taiwan are essentially independent oftopographic effects. Furthermore, the mecha-nism proposed by Lindzen and Nigam (1987)involving hydrostatic effects of air residing overcooler/warmer water was also investigated byToy and Johnson (2014) and found to addition-ally contribute to some extent to the greaterlocalized convergence over southern Taiwan inCTRLSST vs. SMTHSST.

5. Summary and Conclusions

Strong SST gradients exist off the coast of EastAsia during the early summer monsoon as aresult of the previous winter’s cold air outbreaksover the ocean. Such gradients have been shownto influence low-level divergence, clouds, andprecipitation, particularly near strong SST gra-dients like those found in the Kuroshio Exten-sion. There is typically a strong SST gradientover the northern South China Sea around thetime of the onset of the summer monsoon in mid-to-late May, a period referred to as the Mei-yuin Taiwan.

The effect of this SST gradient on rain-fall over Taiwan during the 2008 SoWMEX/TiMREX field campaign has been investigatedusing the Advanced Research WRF (Toy andJohnson 2014). Through a comparison of rain-fall distributions in two experiments, one withthe observed SST and one with a smoothedSST distribution, it is found that for theSoWMEX/TiMREX mid-June heavy rainfall



period that the stronger SST gradient increasesrainfall over southern Taiwan by enhancing low-level convergence along the SST front. A reduc-tion in vertical mixing of higher-momentum airaloft to the surface occurs as the south-to-southwesterly flow crosses from warm to coolerwater, consistent with the mechanism proposedby Wallace et al. (1989). Since there is year-to-year variability in the strength of the SSTgradient over the northern South China Seaas a result varying intensity of previous wintermonsoons and other factors, these findings haveimportant implications regarding seasonal pre-diction of the intensity and location of Mei-yuprecipitation over Taiwan.

Acknowledgments

This research has been supported by theNational Science Foundation under Grant AGS-0966758. We acknowledge high-performancecomputing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR’s Compu-tational and Information Systems Laboratory,sponsored by the National Science Foundation.We also thank Brian McNoldy for creating sev-eral of the figures, Rick Taft for assistance withpreparation of the article, and three anonymousreviewers for their helpful comments on themanuscript.

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SST Gradients and the East Asian Early-Summer Monsoon