sea surface salinity and barrier layer variability in the

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Sea Surface Salinity And Barrier Layer Variability InThe Equatorial Pacific As Seen From Aquarius And

ArgoT D Qu y T Song C Maes

To cite this versionT D Qu y T Song C Maes Sea Surface Salinity And Barrier Layer Variability In The EquatorialPacific As Seen From Aquarius And Argo Journal of Geophysical Research Oceans Wiley-Blackwell2014 119 (1) pp15-29 1010022013jc009375 hal-00998663

Sea surface salinity and barrier layer variability in the equatorialPacific as seen from Aquarius and Argo

Tangdong Qu1 Y Tony Song2 and Christophe Maes3

Received 23 August 2013 revised 20 November 2013 accepted 9 December 2013 published 7 January 2014

[1] This study investigates the sea surface salinity (SSS) and barrier layer variability in theequatorial Pacific using recently available Aquarius and Argo data Comparison betweenthe two data sets indicates that Aquarius is able to capture most of the SSS featuresidentified by Argo Despite some discrepancies in the mean value the SSS from the twodata sets shows essentially the same seasonal cycle in both magnitude and phase For theperiod of observation between August 2011 and July 2013 Aquarius nicely resolved thezonal displacement of the SSS front along the equator showing its observing capacity of thewestern Pacific warm pool Analysis of the Argo data provides further information onsurface stratification A thick barrier layer is present on the western side of the SSS frontduring all the period of observation moving back and forth along the equator with itscorrelation with the Southern Oscillation Index exceeding 080 Generally the thick barrierlayer moves eastward during El Ni~no and westward during La Ni~na The mechanismsresponsible for this zonal displacement are discussed

Citation Qu T Y T Song and C Maes (2014) Sea surface salinity and barrier layer variability in the equatorial Pacific as seenfrom Aquarius and Argo J Geophys Res Oceans 119 15ndash29 doi1010022013JC009375

1 Introduction

[2] As the warmest open-ocean water in the globalocean the western Pacific warm pool plays an importantrole in the worldrsquos climate [eg Lukas et al 1996] Thiswarm water pool characterized by sea surface temperature(SST) warmer than 28Cndash29C lies between about 10Nand 15S west of 160W (Figure 1a) covering a global sur-face area equivalent to about two thirds of the continentalUnited States Although SST variability within the westernPacific warm pool is relatively small its impact on theglobal atmosphere is large [eg Palmer and Mansfield1984] because of the vigorous atmospheric deep convec-tion there The western Pacific warm pool has been shownto migrate zonally on an interannual time scale rangingfrom 2 to 7 years in response to the atmospheric and oce-anic fluctuations associated with El Ni~no-Southern Oscilla-tion (ENSO) [eg McPhaden and Picaut 1990]

[3] The western Pacific warm pool is also a fresh waterpool and its eastern edge is characterized by a zonal seasurface salinity (SSS) front which has a mean equatorialposition near the international dateline (Figure 1b) The

zonal SSS front separates the fresh western Pacific waterfrom relatively salty central Pacific water and the associ-ated SSS gradient can reach as large as 1 psu (the valuesaccording to the 1978 practical salinity scale are given inpsu for simplification in the rest of the text) in 1 longitude[Rodier et al 2000 Delcroix and McPhaden 2002 Maes2008] Such a large SSS gradient is believed to play a rolein the warm pool heat balance [eg Shinoda and Lukas1995 Chen 2004] According to previous studies this SSSfront can move eastward by up to 8000 km during an ElNi~no event [eg Maes et al 2004] consistent with therevised theory for the oscillatory nature of ENSO by focus-ing on the advective feedback alone [Picaut et al 1997]

[4] Another important feature of the western Pacificwarm pool is the presence of the barrier layer Due to alarge excess of precipitation over evaporation (P-E) salin-ity in the upper western equatorial Pacific is low (lt345psu) compared with that in the central equatorial Pacific(gt350 psu Figure 1) The importance of this SSS contrastin surface stratification was first noted by Godfrey andLindstrom [1989] and Lukas and Lindstrom [1991] Theirhydrographic observations revealed a unique vertical pro-file of temperature and salinity a shallow fresh surfacelens is embedded at the top of a deep isothermal layer Theshallow halocline marks the depth of the surface mixedlayer and the layer between the bottom of the mixed layerand the top of the thermocline was termed the lsquolsquobarrierlayerrsquorsquo [Lukas and Lindstrom 1991]

[5] The barrier layer is believed to play a role in themaintenance and temporal evolution of the western Pacificwarm pool Results from a coupled ocean-atmosphere gen-eral circulation model have clearly demonstrated that byisolating the mixed layer from the entrainment cooling atdepth and by confining the response of westerly wind

1International Pacific Research Center SOEST University of Hawaii atManoa Honolulu Hawaii USA

2Jet Propulsion Laboratory California Institute of Technology Pasa-dena California USA

3Institut de Recherche pour le Developpement Laboratoire drsquoEtudes enGeophysique et Oceanographie Spatiales Toulouse France

Corresponding author T Qu International Pacific Research CenterSOEST University of Hawaii at Manoa 1680 East-West Rd HonoluluHawaii 96822 USA (tangdonghawaiiedu)

copy2013 American Geophysical Union All Rights Reserved2169-9275141010022013JC009375

15

JOURNAL OF GEOPHYSICAL RESEARCH OCEANS VOL 119 15ndash29 doi1010022013JC009375 2014

events to a shallow mixed layer the barrier layer favors theeastward displacement of the warm pool during the onsetof El Ni~no [Maes et al 2002] In the absence of salinitystratification the eastward displacement of the warm poolwas reduced which in turn led to a reduced El Ni~no or areturn to the mean seasonal cycle of the model Theseresults seem to suggest that the barrier layer is a potentiallyimportant process influencing ENSO and merits carefulconsideration in climate research

[6] The SSS and barrier layer variability in the equatorialPacific has been investigated by previous studies based onoceanographic cruise data reconstructed salinity profilesand Voluntary Observing Ship thermosalinograph measure-ments [eg Lukas and Lindstrom 1991 Sprintall andTomczak 1992 Rodier et al 2000 Delcroix and McPha-den 2002 Fujii and Kamachi 2003 Maes et al 20022004] Recently by combining available Argo data withthe Tropical Atmosphere-OceanTriangle Trans-OceanBuoy Network (TRITON) measurements Maes et al[2006] found a tight relationship between the SSS front andthe eastern edge of the warm pool for the period 2002ndash2004 This relationship was updated later by Bosc et al[2009] using Argo profiles for the 2000ndash2007 period Thesestudies also noted the presence of a thick barrier layer onthe western side of the SSS front Closely related to thisthick barrier layer was an anomalously warm SST showingadditional evidence for the importance of salinity stratifica-tion in the regionrsquos ocean-atmosphere interaction and con-sequently the zonal displacement of the warm pool

[7] However due to the lack of sufficient salinity obser-vations the relationship between the SSS variability andENSO reported by these earlier studies has not been con-firmed on time scales longer than several years and a com-prehensive description of the barrier layer variabilitywithin the interannual context is still lacking The rapidadvance in space-based remote sensing is revolutionizingocean observations In particular the successful launch ofAquarius is providing a global observing capability of theocean from space generating near-synoptic SSS maps onglobal scale at a spatial resolution of 150 km every7 days [Lagerloef et al 2008] The Aquarius data offer aunique opportunity which has never been possible by con-ventional observations to monitor the zonal displacement

of the SSS front near the eastern edge of the western Pacificwarm pool The combined use of these SSS measurementswith the ongoing collection of Argo profiles allows for fur-ther investigation of the relationship between the SSS andbarrier layer variability providing a potential monitoringof the barrier layer in the western equatorial Pacific usingthe SSS measurements from Aquarius The results of theanalyses are reported in this paper

[8] The rest of the paper is organized as follows A briefdescription of the data and method of analysis is presentedin section 2 Comparison between the two data sets is firstpresented in section 3 to assess the quality of the Aquariusdata in the equatorial Pacific In section 4 we examine thezonal displacement of the SSS front along the equator andin section 5 we discuss the stratification associated withthis zonal displacement The link of SSS and the barrierlayer variability to ENSO is discussed in section 6 Resultsare finally summarized and discussed in section 7

2 Data Description

[9] Two data sets are used in this study One is thesatellite-based SSS measurement from Aquarius and theother is the in situ temperaturesalinity profiles from ArgoThe details of these two data sets are described below

21 Aquarius

[10] The first validated geographically gridded data setfrom Aquarius was recently released by the Ocean SalinityScience Team (httppodaacjplnasagovaquarius) Thisdata set called the AquariusSAC-D version 20 data wasbased on measurements by three separate microwave radio-meters that measure brightness temperature along anapproximately 390 km wide swath using three separatebeams with elliptical footprints of dimension 76 3 94 km84 3 120 km and 96 3 156 km The three nonoverlappingbeams sample the ocean differently from one another geo-graphically and with different incidence angles that affectthe salinity retrieval algorithm The retrieved SSS data forascending and descending tracks were bias-adjusted andmapped to a 1 3 1 grid on a monthly time scale TheAquarius mission requirement is that the global salinity root-mean-square error is no more than 02 psu on 150 3 150 km

Figure 1 Long-term (January 2005 to July 2013) mean sea surface (a) temperature (C) and (b) salin-ity (psu) from Argo The thick white lines represent the 29C isotherm in Figure 1a and 348 psu isoha-line in Figure 1b and the black dot-dashed lines show the equator and the international dateline

QU ET AL SEA SURFACE SALINITY AND BARRIER LAYER

16

and monthly average As yet the version 2 data set partiallyachieves this requirement See Lagerloef et al [2013] formore details The smoothed monthly SSS data from allthree beams for the period from August 2011 through July2013 are used in the present analysis

22 Argo

[11] In the last decade a large number of Argo floats havebeen deployed and more than 3500 of them are currentlyprofiling over the global ocean (httpwwwargoucsdedu)[cf Argo Steering Team 1998] The Argo floats record tem-perature and salinity at a vertical resolution of 1ndash5 m in theupper ocean and somewhat coarser at depth (25 m as thestandard) The measurements extend from a typical top levelaround 5 m to about 2000 m depth Based on these measure-ments the Asian Pacific Data Research Center of the Inter-national Pacific Research Center University of Hawaiirecently created a near-real-time monthly temperaturesalin-ity product for the global ocean at a 1 3 1 grid (data avail-able at httpapdrcsoesthawaiiedudodspublic_dataArgo_Productsmonthly_mean) This product has 26 (standard)levels in the upper 2000 m and spans from January 2005 topresent The vertical (standard) levels are the same as thoseused for the World Ocean Atlas [Levitus 1982] In preparingthis data set a variational analysis technique was used tointerpolate temperature and salinity onto a 3-D spatial gridFor more details about this technique and other informationabout the data see the documentation presented at httpapdrcsoesthawaiieduprojectsArgoindexphp This SSSproduct provides a baseline data set to validate the Aquariusmeasurements Its longer time series also allows us to inves-tigate the SSS and barrier layer variability in the equatorialPacific on the interannual time scale

3 Comparison Between the Two Data Sets

31 Annual Mean

[12] Before proceeding to the analysis of Aquarius datawe first compare the data with Argo to assess their qualityon the tropical Pacific basin scale For the period fromAugust 2011 to July 2013 the two data sets show similarSSS patterns (Figures 2a and 2b) Both the subtropicalsalinity maxima and the equatorial fresh water pools arewell captured by Aquarius Negative discrepancies withmagnitude larger than 02 psu are mainly found along theeastern boundary (Figure 2c) where upwelling conditionsprevail and where the Argo sampling is known to be lessadequate The largest positive discrepancies (02 psu)between the two data sets take place in the tropical Pacificalong the Intertropical Convergence Zone (ITCZ) and theSouth Pacific Convergence Zone (SPCZ) These discrepan-cies are consistent with the regionrsquos high-precipitation con-ditions As Aquarius only measures the skin surfacesalinity its measurement can be easily affected by the freshwater lens resulting from the high precipitation in theregion [Lagerloef et al 2008 Henocq et al 2010 Boutinet al 2013] In the Argo standard the conductivity-temperature-depth (CTD) pump is usually turned off as thefloat ascends through the depth of 5 m to avoid possiblecontamination in the conductivity cell [Riser et al 2008]Thus as a consequence Argo profilers measure salinityaround the 5 m depth that is assimilated to be the SSS but

it can be significantly different from the skin surface salin-ity in the precipitation dominated region like the westernequatorial Pacific as one can see from Figure 2c

[13] Along the equator the SSS discrepancies betweenthe two data sets are generally small (lt01 psu) well belowthe root-mean-square errors (02 psu) of the Aquarius data[Lagerloef et al 2008] The absence of structures along theequator indicates that errors of the Aquarius data are not sen-sitive to the presence of the warm pool and its front (Figure2c) For the period of observation (August 2011 to July2013) the mean SSS front from Aquarius as determined bythe maximum zonal SSS gradient takes place at 169Eshowing a good agreement with that (174E) from ArgoThis result suggests that despite some uncertainties in themean value (Figure 2c) Aquarius is able to capture the SSS

Figure 2 Mean sea surface salinity from (a) Argo and(b) Aquarius and (c) their difference (Argo-Aquarius) forthe period August 2011 to July 2013 Unit is psu The thickwhite lines in Figures 2a and 2b represent the 348 psu iso-haline and the black dot-dashed lines show the equator andthe international dateline


17

front along the equator thus providing a potentially usefultool to monitor the eastern edge of the western Pacific warmpool The details will be discussed in section 4

32 Seasonal Cycle

[14] Harmonic analysis of Aquarius data provides abroad consistent view of the SSS seasonal variation for theperiod from August 2011 to July 2013 in the tropicalPacific (Figure 3a) In general the seasonal variation ofSSS in the tropical Pacific is dominated by the annualcycle A large portion of the tropical Pacific has an annualcycle of SSS less than 02 psu The maximum amplitude(06 psu) of SSS annual cycle takes place in the easternequatorial Pacific similar to what has been discussed forSST [eg Kessler et al 1998] Outside of the equator acoherent band of SSS annual amplitude greater than 04psu is seen roughly coinciding with the ITCZSPCZ Basedon the World Ocean Atlas 1998 (WOA98) Boyer and Levi-tus [2002 Figure 3a] showed a similar pattern They attrib-uted the large SSS annual cycle in the tropical Pacific tothe annual fluctuation of precipitation

[15] For the same period of observation (August 2011 toJuly 2013) Argo shows essentially the same SSS annualcycle as Aquarius with the tropical Pacific being domi-nated by a high-amplitude (gt03 psu) band associated withthe ITCZSPCZ (Figure 3c) Compared with those fromAquarius and WOA98 [Boyer and Levitus 2002] the SSSannual cycle from Argo is somewhat weaker presumablydue to the insufficiency of sampling during the period of

observation By comparing the Aquarius SSS with Argodata and results from ocean models Song et al [2013]showed similar results for the global ocean suggesting aglobal observing capability of the SSS annual cycle

[16] The semiannual cycle is typically about two timesweaker than the annual cycle (Figures 3b and 3d) In muchof the region studied the semiannual cycle from both datasets is less than 01 psu Areas with SSS semiannual cyclegreater than 02 psu include the eastern equatorial Pacificand the ITCZSPCZ in the tropics So the two data setsshow essentially the same spatial patterns of the semian-nual amplitude A close inspection of these spatial patternshowever indicates that Aquarius captures more small-scalefeatures than Argo which may reflect the difference in datacoverage between the two data sets

[17] Figure 4 shows the phase of SSS variation on bothannual and semiannual time scales Despite some quantita-tive discrepancies the two data sets show essentially thesame seasonal cycle in the tropical Pacific Along the equa-tor SSS approaches its annual maximum in JuneJuly eastof the dateline and in MarchApril west of the dateline (Fig-ures 4a and 4c) A phase difference of up to 3 months isalso seen for the semiannual variation between the easternand central equatorial Pacific (Figures 4b and 4d) No prop-agation signal is obvious along the equator as can bederived from the nearly constant phase on both sides of thedateline This result differs from those observed in the SSTand sea surface height fields in which propagation of bothannual and semiannual variations has been identified [eg

Figure 3 Amplitude of (top) annual and (bottom) semiannual variations of sea surface salinity fromAquarius (left) and Argo (right) for the period August 2011 to July 2013 Note that the amplitude of thecolor bars in the bottom panels has been divided by a factor of 3 from the color bars in the upper panelsThe black dot-dashed lines show the equator and the international dateline Unit is psu


18

Kessler 1990 Qu et al 2008] The lack of propagationsignal in the SSS field is not completely understood at pres-ent and needs to be investigated further

[18] To demonstrate the SSS seasonal variation along theequator Figure 5 shows the amplitude and phase of theannual and semiannual cycles averaged in the 3Sndash3N

Figure 4 Phase of (top) annual and (bottom) semiannual variations corresponding to the day of theyear when SSS is maximum from Aquarius (left) and Argo (right) for the period August 2011 to July2013 The black dot-dashed lines show the equator and the international dateline

Figure 5 (top) Annual and (bottom) semiannual (left) amplitude and (right) phase of averaged sea sur-face salinity from Argo (red) and Aquarius (black) along the 3Sndash3N equatorial band Note that therange of the semiannual amplitude is smaller than the range of the annual amplitude by a factor of 2Units are psu on the left panels and days on the right panels


19

latitude band The comparison between Aquarius and Argois fairly good with the amplitudes from the two data setsbeing almost identical As has already been noted by previ-ous studies [eg Alory et al 2012] the SSS seasonal vari-ation is largest in the eastern equatorial Pacific where itsannual and semiannual amplitudes exceed 06 and 03 psurespectively The amplitudes drop as we progress west-ward falling below 02 psu for the annual cycle and below01 psu for the semiannual cycle in the central equatorialPacific Both the annual and semiannual amplitudesincrease in the western equatorial Pacific west of the date-line (Figures 5a and 5b)

[19] The phases of the SSS seasonal variation also showa good agreement between the two data sets (Figures 5cand 5d) except in the central equatorial Pacific near thedateline where the phases of the annual cycle from Argoare much more smoothed than those from Aquarius Somequantitative discrepancies are also evident for the semian-nual variation These include a phase difference of up to 3months in the far eastern Pacific (east of 130W) and aphase jump resolved by Aquarius in a narrow longitudeband between 170W and the dateline These discrepanciesare possibly related to the spatial sampling of the two datasets The consistency of the two data sets is relatively goodin the far western Pacific suggesting that Aquarius is ableto capture the western Pacific warm pool variation on boththe annual and semiannual time scales

4 Zonal Displacement of SSS Front

[20] To illustrate the zonal displacement of SSS frontalong the equator Figure 6 shows the time-longitude distri-

bution of SSS averaged over the latitude band between 3Sand 3N for the period from August 2011 through July2013 It is clearly shown that the zonal displacement ofSSS front is well resolved by both data sets Defined by the348 psu isohaline the SSS front is seen to migrate alongthe equator from month to month over a longitude bandlarger than 40 consistent with the previous resultsreported by Maes et al [2004] During the 20112012 LaNi~na event the SSS front moved westward to as far asabout 140E from its mean location near 170E For theperiod of observation (August 2011 to July 2013) the SSSfront resolved by Aquarius is highly correlated with thatresolved by Argo with their correlation coefficient reach-ing 085 that satisfies the 99 confidence level by t test

[21] It is worthwhile to note that with a global coverageof every 7 days Aquarius is able to capture more featuresthan Argo In the region defined by 30Sndash30N and 120Endash80W approximately 3000 Argo profiles become availableeach month which are roughly better than one profile permonth in each 2 3 2 grid on the average With such asparse coverage Argo cannot resolve all the detailed fea-tures of the SSS front In this regard Aquarius appears tooffer a good opportunity to monitor the SSS front in theequatorial Pacific (Figure 6) This is very promising for fur-ther investigations as a longer time series of Aquarius databecomes available

[22] The Argo data allow for further investigation of thezonal displacement of SSS front on the interannual timescale Figures 7a and 7b show the time evolution of SSTand SSS along the equator for the period from January2005 to July 2013 respectively From this figure one cansee a good correspondence between the SST and SSS fields

Figure 6 Time-longitude distribution of sea surface salinity along the 3Sndash3N equatorial band in thewestern central Pacific from (a) Aquarius and (b) Argo for the period August 2011 to July 2013 Thewhite lines are the 348 psu isohalines representing the SSS front


20

The eastern edge of the warm pool represented here by the29C isotherm migrates from year to year against its meanlocation near 176E The SSS front as indicated by the 348psu isohaline lies at 163E in the mean about 13 fartherwestward than the 29C isotherm This result is consistentwith the previous studies suggesting that the presence of theSSS front is inside the 29C warm pool [eg Maes et al2004] During the period of observation (January 2005 toJuly 2013) the SSS front and the eastern edge of the warmpool migrate in about the same phase both showing a signif-icant interannual variability (Figure 7)

[23] The correspondence of SSS front with the easternedge of the warm pool has been investigated by earlierstudies [eg Picaut et al 2001 Maes et al 2004 2006Bosc et al 2009] The newly available Argo data allow usfor a more detailed assessment of this relationship on theinterannual time scale (Figure 8) During the period ofobservations (January 2005 to July 2013) the correlationbetween the two time series reaches as high as 09 satisfy-ing the 99 significance level by t test (Table 1) This highcorrelation confirms that the eastern edge of the westernPacific warm pool is well characterized by the SSS frontalong the equator As indicated by the heavy dotted red linein Figure 8 the SSS front from Aquarius shows consistentresult with Argo for the period from August 2011 to July2013

5 Stratification

[24] Also included in Figure 7 is the time evolution ofthe barrier layer thickness (BLT) along the equator fromArgo In this study we define the barrier layer in the sameway as Sprintall and Tomczak [1992] by using a densitydifference from the surface value that is equivalent to adecrease (05C) in temperature Different methods can beused to define the barrier layer [eg de Boyer Montegutet al 2004] but the results remain essentially the same Inthe mean the barrier layer is thick in the western equatorialPacific (Figure 9a) Its maximum thickness (gt35 m) liesaround 160E along the equator roughly on the westernside of the SSS front (Figure 2) The mean BLT falls below15 m at about 15 off the equator Along the equator aBLT front is seen near the dateline and east of this frontthe BLT drops rapidly to less than 5 m Given the verticalresolution (gt5 m) of the Argo product values less than 5 mare not shown and excluded from the discussion belowStandard deviations or root-mean-square variations of theBLT are also included in Figure 9b In the equatorialPacific the largest (gt15 m) BLT variability occurs near thedateline roughly coinciding with the mean BLT front (Fig-ure 9a) The zonal migration of thick barrier layer is appa-rently responsible for this large BLT variability Variabilitynear 160E where the mean BLT exceeds 30 m is rela-tively weak (10 m) suggesting that most of the mean

Figure 7 Time-longitude distribution of (a) sea surface temperature (b) sea surface salinity and (c) bar-rier layer thickness from Argo along the equator in the western central Pacific Units are C in Figure 7apsu in Figure 7b and m in Figure 7c The white lines indicate the contours of 29C 348 psu and 15 mrespectively


21

BLT features shown in Figures 7c and 9a for the equatorialPacific are robust

[25] Over much of the region studied the BLT annualcycle is weaker than 6 m (Figure 10a) In the equatorialregion the annual amplitude exceeds 8 m near the coast ofNew Guinea and falls below 2 m around 160E It thenincreases toward the east approaching its maximum (gt8 m)at about 5 off the equator near the dateline Interestinglythe annual amplitude near the dateline is actually verysmall (lt4 m) on the equator (Figure 10a) which onlyexplains about a quarter of the total (gt16 m) variabilitythere (Figure 9b) The semiannual cycle of the BLT (Figure10b) shows a similar pattern as the annual cycle (Figure10a) while its amplitude is about two times smaller Ingeneral the BLT has a tendency to follow the SSS seasonalcycle (Figures 4c and 10c) In the western equatorialPacific the SSS reaches its annual maximum in MarchApril (Figure 4c) which is in phase with the E-P annualfluctuation [eg Boyer and Levitus 2002] This SSSannual maximum is consistent with a BLT annual mini-mum in much of the western equatorial Pacific (Figure10c) suggesting a good correspondence between SSS andsurface stratification Similar correspondence is also evi-dent for the semiannual variation (Figures 4d and 10d)

[26] While displaying a weak seasonal cycle most of theBLT variability along the equator is due to the zonal dis-placement of the thick barrier layer on the interannual timescale For the period of observation the zonal displacementof the thick barrier layer concurs with that of the easternedge of the warm pool in almost all the cases (Figure 7) Its

year-to-year zonal displacement along the equator canexceed 4000 km Using the 15 m contour as its easternedge (Figure 7c) the presence of the thick barrier layer ishighly correlated with the 29C isotherm and 348 psu iso-haline (Figure 8) and their overall correlation reaches 076and 079 respectively (Table 1) Note that the 15 m thick-ness of the barrier layer is above the root-mean-squareerrors of the BLT (Figure 9b) and is significant in the viewof sensitivity for the background mean state in the westernequatorial Pacific [eg Maes and Belamari 2011] Follow-ing the tiltingshearing mechanism suggested by Croninand McPhaden [2002] this result seems to suggest that thezonal displacement of the SSS front can affect the surfacestratification and consequently the presence of the thickbarrier layer in the western equatorial Pacific The proc-esses that maintain this relationship are not completelyclear to us As a dominant climate mode of the regionENSO likely plays a role [eg Picaut et al 1997] Thedetails are discussed below

6 Link to ENSO

[27] Following the pioneering works of Lindstrom et al[1987] and Lukas and Lindstrom [1991] the barrier layervariability in the western Pacific has been investigated bymany studies Based on CTD measurements along 165Efrom 1984 to 1988 Delcroix et al [1992] reported that thebarrier layer in the western Pacific was destroyed duringepisodes of eastward surface flow and equatorial upwellingdriven by easterlies Later studies focused on the barrierlayer variability on the interannual time scale [eg Andoand McPhaden 1997 Maes 2000 Delcroix and McPha-den 2002 Cronin and McPhaden 2002] and found a clearcorrespondence between the barrier layer variability andENSO [eg Fujii and Kamachi 2003 Maes et al 20022005 2006] Based on the early development of the Argodata Bosc et al [2009] noted that the barrier layer in thewestern equatorial Pacific represents a quasi-permanentfeature for both El Ni~no and La Ni~na events But the pre-cise relationship between the BLT and ENSO was notdetermined by these earlier studies due to the short periodof their observations

[28] The correspondence between the barrier layer vari-ability and ENSO is supported by the present analysis InFigure 7 we see eastward displacements of the thick barrierlayer during the 20062007 and 20092010 El Ni~no eventsand westward displacements during the 20072008 2010

Table 1 Correlations Among the Eastern Edge of the Warm Pool(WP) (29C) SSS Front (348 psu) Thick Barrier Layer (BL) (15 m)From Argo and SOI for the Period From January 2005 to July 2013a

CorrelationEastern

Edge WP SSS Front Thick BL SOI

Easternedge WP

100 (0) 090 (0) 076 (1) 2082 (1)

SSS front 090 (0) 100 (0) 079 (1) 2084 (1)Thick BL 076 (21) 079 (21) 100 (0) 2081 (0)SOI 2082 (21) 2084 (21) 2081 (0) 100 (0)

aAll correlations satisfy the 99 confidence level Numbers in thebrackets indicate the time (in months) that variables in the first column lagthose in the first row

Figure 8 (a) Time series of the eastern edge of the warmpool (black) SSS front (red) and thick barrier layer (green)compared with (b) the Southern Oscillation Index Theheavy dotted red line in Figure 8a indicates the SSS frontmeasured by Aquarius for the period from August 2011through July 2013 Here the eastern edge of the warm poolSSS front and thick barrier layer are represented by thecontours of 29C 348 psu and 15 m respectively Alltime series are normalized by their respective standarddeviations The SOI in Figure 8b has been multiplied by21 before plotting


22

2011 and 20112012 La Ni~na events These zonal displace-ments are in phase with those of the eastern edge of thewarm pool and the SSS front along the equator as indicatedby the 29C isotherm and 348 psu isohaline respectively(Figures 7a and 7b) During the period of observation thesetime series corresponded well with ENSO (Figure 8) withtheir correlation with the Southern Oscillation Index (SOI)all exceeding 080 (Table 1) During the 20092010 ElNi~no event for example the thick (gt15 m) barrier layermoved eastward to about 160W and in the same timeboth the eastern edge of the warm pool and the SSS front

reached their easternmost positions along the equatorInterestingly the thick barrier layer appears to lead boththe eastern edge of the warm pool and the SSS front alongthe equator by 1 month (Table 1) The implication of thisresult is that the response of the western equatorial Pacificto ENSO first occurs in the subsurface Then the corre-sponding changes in subsurface stratification may furtheraffect the sea surface and play a role in the zonal displace-ment of the warm pool The existing data are apparentlynot sufficient for a detailed investigation of this phase dif-ference and we will leave it for future studies

Figure 9 (a) Time mean and (b) standard deviations of the barrier layer thickness for the period fromJanuary 2005 to July 2013 from Argo The black dot-dashed lines show the equator and the internationaldateline The contour intervals are set every 5 m for BLT and 2 m for the standard deviations

Figure 10 (top) Annual and (bottom) semiannual (left) amplitude and (right) phase of the BLT (m)variation from Argo for the period August 2011 to July 2013 Note that the amplitude of the color bars inthe bottom panels has been divided by a factor of 2 The phase indicates the day of the calendar yearwhen BLT reaches its maximum The black dot-dashed lines show the equator and the internationaldateline


23

[29] To further illustrate how the eastern edge of thewarm pool the SSS front and the thick barrier layer arerelated to ENSO we conduct a composite analysis of thetwo El Ni~no events (20062007 and 20092010) and threeLa Ni~na events (20072008 20102011 and 20112012)that took place during the period of observation (Figure11) The SOIs during these events all exceeded one stand-ard deviation of its variability (Figure 8b) During El Ni~noconditions as one has already known SST gets warmer inthe central and eastern equatorial Pacific while SSS getsfresher in the western equatorial Pacific roughly between160E and the dateline The situation is reversed during LaNi~na conditions [eg Singh et al 2011] Corresponding tothese SST and SSS variations the mixed layer depth alsochanges from El Ni~no to La Ni~na conditions (Figure 12)The differences between the two composite events show ashallower mixed layer west of the dateline and a deeper

mixed layer east of it (Figure 12e) Most of the BLT varia-tions are confined in the western central equatorial Pacificroughly to the west of 160W (Figure 12f) During ElNi~no conditions the barrier layer generally gets thicker inthe longitude band between 160E and 160W on the equa-tor but becomes thinner west of 160E off the equator

[30] Figure 13 shows the composite wind stress and P-Efrom the National Centers for Environmental Prediction(NCEP)National Center for Atmospheric Research Re-analysis Project [Kalnay et al 1996] As noted by manyprevious studies mentioned above the largest wind stressdifferences between El Ni~no and La Ni~na are the westerlywind anomalies in the western equatorial Pacific (Figure13e) Associated with these westerly wind anomalies arepositive P-E anomalies (Figure 13f) The spatial pattern ofthe positive P-E anomalies essentially follows the ITCZand SPCZ with their maximum values (gt10 mm d21)

Figure 11 Composites of (left) sea surface temperature in C and (right) salinity in psu during themature phase of (top) El Ni~no (middlre) La Ni~na and (bottom) their differences in the tropical PacificThe composite of El Ni~no conditions includes 122006ndash022007 and 122009ndash022010 and the compos-ite of La Ni~na conditions includes122007ndash022008 122010ndash022011 and 122011ndash022012 The solidwhite lines represent the 29C isotherm the 348 psu isohaline and the zero isoline The black dot-dashed lines show the equator and the international dateline


24

lying between 160E and 170E near the equator In theeastern equatorial Pacific the P-E anomalies are weak andthey are generally positive north of the equator and nega-tive south of it The P-E anomalies in the eastern equatorialPacific are inconsistent with the SSS anomalies which aremostly positive east of the dateline (Figure 11) This resultclearly demonstrates that P-E is not the only process influ-encing SSS In addition to the P-E anomalies oceandynamics also plays a significant role in generating the SSSvariability [eg Johnson et al 2002]

[31] Regarding the formation of thick barrier layer sev-eral processes may be at work [Bosc et al 2009] Forexample the eastward shift of heavy rainfall associatedwith the westerly wind anomalies during El Ni~no (Figures13a and 13b) corresponds with an eastward shift of thewarm pool by up to 2000 km (Figures 11a and 11b) relativeto its mean position (Figure 1a) The eastward shift of thewarm pool then generates a shallower isohaline surfacelayer in the western equatorial Pacific (Figure 12a) whichin turn may directly contribute to the presence of the thickbarrier layer there (Figure 12b) [eg Sprintall and Tomc-

zak 1992] At the same time the meridional wind anoma-lies north of the equator (Figure 13e) force the surfacefreshwater near the ITCZ (Figure 11b) to flow equatorward[eg McPhaden et al 1992] These processes togetherwith the surface water convergence forced by the westerlywind bursts along the equator [eg Cronin and McPhaden2002] may enhance the surface stratification providing afavorable condition for the eastward displacement of thethick barrier layer The situation is reversed during La Ni~naconditions

[32] Figure 14 shows the temperature and salinity alongthe equator to further illustrate the differences in verticalstructures between the two events (Figure 14) Based onCTD measurements collected by three oceanographiccruises Maes [2008] noticed an increasing east-west gradi-ent in salinity stratification within the isothermal layerwith the higher values lying to the west of the eastern edgeof the warm pool This east-west gradient in salinity stratifi-cation is also well captured by Argo (Figure 14) shiftingfrom about 160E during El Ni~no (Figure 14b) to the coastof New Guinea (west of 130E) during La Ni~na (Figure

Figure 12 Same as Figure 11 except for (left) mixed layer depth and (right) barrier layer thicknessUnit is meters


25

14d) In both cases the maximum salinity stratification isconfined above the thermocline (black solid lines) withinthe density range less than 225 kg m23 During El Ni~noconditions the eastern edge of the warm pool as indicatedby the 29C isotherm reaches about 170W on the equator(Figure 14a) Immediately to the west of the eastern edgeof the warm pool is a salinity front whose maximum zonalgradient coincides with the 348 psu isohaline near thedateline (Figure 14b) A thick (gt15 m) barrier layer con-curs with this salinity front and covers a longitude bandroughly between 160E and170W During La Ni~na condi-tions as the eastern edge of the warm pool approaches itswestern most position near 160E the salinity front shiftswestward by nearly 3000 km from its El Ni~no conditionwhich in turn results in a westward displacement of thethick barrier layer (Figures 14c and 14d)

[33] What we would like to emphasize here is the pres-ence of minimum stratification lying to the east of the west-ern Pacific warm pool where mixed layer depth can reachas deep as 100 m (Figure 14a) Closely related to this mini-

mum stratification is the vertical entrainment of high-salinity (gt352 psu) water from below along density surfa-ces near 225 kg m23 (Figure 14b) This high-salinity waterrepresents a mix of subtropical water from both hemi-spheres [eg Lindstrom et al 1987 Bingham and Lukas1995 Qu et al 1999] During El Ni~no conditions the ther-mocline shoals in the western equatorial Pacific and deep-ens in the central and eastern parts of the basin Thesechanges generate large temperature and salinity anomaliesalong the equator with higher values lying around the ther-mocline and as a consequence potential density increasesin the western and decreases in the central and easternequatorial Pacific by up to 1 kg m23 from El Ni~no to LaNi~na (Figures 14e and 14f) Such large density changes arealso evident in the TRITON mooring data [eg Hasegawaet al 2013] suggesting an eastward transport of warm andlow-salinity water in the upper ocean With the deepeningof the thermocline in the central equatorial Pacific the ver-tical entrainment of high-salinity water is much reducedduring El Ni~no conditions and its location moves to about

Figure 13 Same as Figure 11 except for (left) wind stress and (right) precipitation minus evaporation(P-E) from NCEP reanalyses The black lines indicate the mean 29C isotherm Units are N m22 forwind stress and mm d21 for P-E The black dot-dashed lines show the equator and the internationaldateline


26

160W along the equator (Figure 14b) During La Ni~naconditions the entrainment of high-salinity water reachesits western most position near 170E and high-salinity(gt354 psu) water is seen to extend all the way to the seasurface suggesting enhanced subsurface influence (Figure14d) Since the high-salinity water is of subtropical originits entrainment in the equatorial Pacific cannot be forced bylocal processes (eg P-E and wind) alone Then what isresponsible for the zonal displacement of the SSS front andits associated thick barrier layer How much of this zonaldisplacement is due to the entrainment of high-salinitywater from below These questions will be addressed byfuture studies using results from high-resolution oceangeneral circulation models

7 Summary and Discussion

[34] Using recently available Aquarius and Argo datathis study provides a detailed description of the SSS andbarrier layer variability in the equatorial Pacific For theperiod of Aquarius observation (August 2011 to July2013) the two data sets agree reasonably well in both themean value and seasonal variability In the mean the larg-est discrepancies between the two data sets take place alongthe ITCZ and SPCZ These discrepancies may represent thedifferences between the skin surface salinity and surfacelayer salinity They may also represent the insufficiency ofArgo sampling in resolving high-frequency fluctuations ofSSS fronts [eg Juza et al 2012 Song et al 2013] Forexample Aquarius can clearly see the Tropical Instability

Figure 14 Same as Figure 11 except for vertical distributions of (left) temperature and (right) salinityalong the equator from Argo The white contours indicate potential density and the black lines representthe top of the thermocline (solid) and the base of the mixed layer (dotted) The units are C for tempera-ture psu for salinity and kg m23 for potential density


27

Wave fronts while Argo is less effective [Lee et al 2012]As the ITCZSPCZ region is dominated by low-wind high-precipitation conditions the fresh water lens resulting fromheavy rainfall may not be mixed away immediately and asa consequence the skin surface salinity measured byAquarius may be significantly lower than that at 5 m depthfrom Argo floats To our knowledge the Aquarius OceanSalinity Science Team is deploying more Argo floats withthe capacity of skin surface salinity measurement so thatthe Aquarius data can be better calibrated and validatedSimilar efforts are also being made with drifter buoys [egReverdin et al 2013]

[35] Both the Aquarius and Argo data can nicely resolvethe SSS front along the equator With a better resolution inboth space and time Aquarius is able to capture moredetailed features of the SSS front than Argo demonstratingits observing capacity of the western Pacific warm poolwhile the longer time series of Argo data allows for a moreprecise investigation of its interannual variation The Argodata also provide information on stratification of the westernPacific warm pool of which an important feature is the pres-ence of a thick (gt15 m) barrier layer With these Argo datawe are able to show that the thick barrier layer in the equato-rial Pacific varies both seasonally and interannually Whiledisplaying a weak seasonal variability the thick barrier layercan move eastward by up to 4000 km during an El Ni~noevent As Aquarius continues to provide the time series ofocean surface salinity we anticipate that the SSS front thethick barrier layer and the eastern edge of the westernPacific warm pool will become adequately resolved andtheir relationship with ENSO will be better understood

[36] The formation mechanism of the thick barrier layerin the equatorial Pacific has been discussed by many earlierstudies In addition to the surface processes proposed bythese earlier studies we emphasize the effect of high-salinity water from below representing a mix of subtropi-cal water from both hemispheres Based on available Argoand TRITON data Hasegawa et al [2013] noted that pre-cipitation and salinity changes are not consistent in someparts of the equatorial Pacific Apparently local effect ofprecipitation cannot explain the salinity changes thereThey speculated that oceanic advection and upwelling areprobably of more importance in such regions In a recentstudy using a simulate passive tracer Qu et al [2013] fur-ther demonstrated that a large portion of the high-salinitySouth Pacific tropical water makes its way into the equato-rial region in the depth of the thermocline From theresome of this water mass is entrained into the surface mixedlayer in the central equatorial Pacific The vertical entrain-ment of high-salinity water in the equatorial Pacificdepends on local Ekman pumping and basin-scale gyre cir-culation both of which have a strong ENSO signature Soone may have reason to believe that the vertical entrain-ment of high-salinity water from below is also governed byENSO and its interannual variability may directly contrib-ute to the zonal displacement of the SSS front and its asso-ciated thick barrier layer The 1 month lead of the thickbarrier layer to the eastern edge of the warm pool and theSSS front along the equator could be regarded as evidencefor this subsurface influence How this subsurface influenceplays a role in the ENSO cycle and in particular in thedevelopment of Central Pacific El Ni~no or Modoki [eg Yu

and Kao 2007 Takahashi et al 2011] is an interestingtopic for future studies

[37] Acknowledgments T Qu was supported by NSF through grantOCE11-30050 and by NASA as part of the Aquarius Science Team inves-tigation through grant NNX12AG02G Y T Song was supported by theJet Propulsion Laboratory California Institute of Technology under con-tracts with NASA C Maes is supported by IRD The authors are gratefulto N Schneider and I Fukumori for useful discussion on the topic to KYu for assistance in processing the Aquarius data and to two anonymousreviewers for valuable comments on this manuscript School of Ocean andEarth Science and Technology contribution number 9054 and InternationalPacific Research Center contribution IPRC-1033

ReferencesAlory G C Maes T Delcroix N Reul and S Illig (2012) Seasonal

dynamics of sea surface salinity off Panama The far eastern Pacificfresh pool J Geophys Res 117 C04028 doi1010292011JC007802

Ando K and M J McPhaden (1997) Variability of surface layer hydrog-raphy in the tropical Pacific Ocean J Geophys Res 102 23063ndash23078 doi10102997JC01443

Argo Steering Team (1998) On the design and Implementation of ArgomdashAn initial plan for the global array of profiling floats Int CLIVAR ProjOff Rep 21 32 pp

Bingham F M and R Lukas (1995) The distribution of intermediatewater in the western equatorial Pacific during January-February 1986Deep Sea Res Part II 42 1545ndash1573

Bosc C T Delcroix and C Maes (2009) Barrier layer variability in thewestern Pacific warm pool from 2000 to 2007 J Geophys Res 114C06023 doi1010292008JC005187

Boutin J N Martin G Reverdin X Yin and F Gaillard (2013) Sea sur-face freshening from SMOS and ARGO salinity Impact of rain OceanSci 9 183ndash192 doi105194os-9-183-2013

Boyer T P and S Levitus (2002) Harmonic analysis of climatologicalsea surface salinity J Geophys Res 107(C12) 8006 doi1010292001JC000082

Chen D (2004) Upper ocean response to surface momentum and freshwaterfluxes in the western Pacific warm pool J Trop Oceanogr 23 1ndash15

Cronin M F and M J McPhaden (2002) Barrier layer formation duringwesterly wind bursts J Geophys Res 107(C12) 8020 doi1010292001JC00117

de Boyer Montegut C G Madec A S Fischer A Lazar and D Iudicone(2004) Mixed layer depth over the global ocean An examination of pro-file data and a profile-based climatology J Geophys Res 109 C12003doi1010292004JC002378

Delcroix T and M J McPhaden (2002) Interannual sea surface salinityand temperature changes in the western Pacific warm pool during 1992ndash2000 J Geophys Res 107(C12) 8002 doi1010292001JC000862

Delcroix T G Eldin M-H Radenac J Toole and E Firing (1992) Vari-ation of the western equatorial Pacific Ocean 1986ndash1988 J GeophysRes 97 5423ndash5445 doi10102992JC00127

Fujii Y and M Kamachi (2003) Three-dimensional analysis of tempera-ture and salinity in the equatorial Pacific using a variational method withvertical coupled temperature-salinity empirical orthogonal functionmodes J Geophys Res 108(C9) 3297 doi1010292002JC001745

Godfrey J S and E J Lindstrom (1989) The heat budget of the equato-rial western Pacific surface mixed layer J Geophys Res 94 8007ndash8017 doi101029JC094iC06p08007

Hasegawa T K Ando I Ueki K Mizuno and S Hosoda (2013) Upper-ocean salinity variability in the tropical Pacific Case study for quaside-cadal shift during 2000s using TRITON buoys and ARGO floats JClim 26 8126ndash8138 doi101175JCLI-D-12-001871

Henocq C J Boutin G Reverdin F Petitcolin S Arnault and P Lattes(2010) Vertical variability of near-surface salinity in the tropics Conse-quences for L-band radiometer calibration and validation J Atmos Oce-anic Technol 27 192ndash209 doi1011752009JTECHO6701

Johnson E S G S E Lagerloef J T Gunn and F Bonjean (2002) Sur-face salinity advection in the tropical oceans compared with atmosphericfreshwater forcing A trial balance J Geophys Res 107(C12) 8014doi1010292001JC001122

Juza M T Penduff J-M Brankart and B Barnier (2012) Estimating thedistortion of mixed layer property distributions induced by the Argosampling J Oper Oceanogr 5(1) 45ndash58


28

Kalnay E et al (1996) The NCEPNCAR 40-year reanalysis projectBull Am Meteorol Soc 77 437ndash471

Kessler W S (1990) Observations of long Rossby waves in the northerntropical Pacific J Geophys Res 95 5183ndash5217 doi101029JC095iC04p05183

Kessler W S L M Rothstein and D Chen (1998) The annual cycle ofSST in the eastern Tropical Pacific diagnosed in an ocean GCM JClim 11 777ndash799

Lagerloef G et al (2008) The AquariusSAC-D Mission Designed tomeet the salinity remote-sensing challenge Oceanography 21 68ndash81

Lagerloef G et al (2013) Aquarius salinity validation analysis Data ver-sion 20 Aquarius Proj Doc AQ-014-PS-0016 36 pp [Available athttpaquariusnasagovpdfsAQ-014-PS-0016_AquariusSalinityDataVali-dation Analysis_DatasetVersion20pdf]

Lee T G S E Lagerloef M M Gierach H Kao S Yueh and K Dohan(2012) Aquarius reveals salinity structure of tropical instability wavesGeophys Res Lett 39 L12610 doi1010292012GL052232

Levitus S (1982) Climatological Atlas of the World Oceans NOAA ProfPap 13 190 pp US Gov Print Off Rockville Md

Lindstrom E R Lukas R Fine E Firing SJ Godfrey G Meyers andM Tsuchiya (1987) The western equatorial Pacific Ocean circulationstudy Nature 330 533ndash537

Lukas R and E Lindstrom (1991) The mixed layer of the western equato-rial Pacific Ocean J Geophys Res 96 3343ndash3358 doi10102990JC01951

Lukas R T Yamagata and J P McCreary (1996) Pacific low-latitudewestern boundary currents and the Indonesian throughflow J GeophysRes 101 12209ndash12216 doi10102996JC01204

Maes C (2000) Salinity variability in the equatorial Pacific Ocean duringthe 1993ndash98 period Geophys Res Lett 27 1659ndash1662 doi1010291999GL011261

Maes C (2008) On the ocean salinity stratification observed at the easternedge of the equatorial Pacific warm pool J Geophys Res 113 C03027doi1010292007JC004297

Maes C and S Belamari (2011) On the impact of salinity barrier layer on thePacific Ocean mean state and ENSO Sci Online Lett Atmos 7 97ndash100

Maes C J Picaut and S Belamari (2002) Salinity barrier layer and onsetof El Ni~no in a Pacific coupled model Geophys Res Lett 29(24) 2206doi1010292002GL016029

Maes C J Picaut Y Kuroda and K Ando (2004) Characteristics of theconvergence zone at the eastern edge of the Pacific warm pool GeophysRes Lett 31 L11304 doi1010292004GL019867

Maes C J Picaut and S Belamari (2005) Importance of salinity barrierlayer for the buildup of El Ni~no J Clim 18 104ndash118

Maes C K Ando T Delcroix W S Kessler M J McPhaden and DRoemmich (2006) Observed correlation of surface salinity temperatureand barrier layer at the eastern edge of the western Pacific warm poolGeophys Res Lett 33 L06601 doi1010292005GL024772

McPhaden M J and J Picaut (1990) El Ni~no-Southern Oscillation dis-placement of the western equatorial Pacific warm pool Science 2501385ndash1388

McPhaden M J F Bahr Y Du Penhoat E Firing S P Hayes P PNiiler P L Richardson and J M Toole (1992) The response of the

western equatorial Pacific ocean to westerly wind bursts during Novem-ber 1989 to January 1990 J Geophys Res 97 14289ndash14302 doi10102992JC01197

Palmer T N and D A Mansfield (1984) Response of two atmosphericgeneral circulation models to sea surface temperature anomalies in thetropical East and West Pacific Nature 310 483ndash485

Picaut J F Masia and Y du Penhoat (1997) An advective-reflective con-ceptual model for the oscillatory nature of the ENSO Science 277 663ndash666

Picaut J M Ioualalen T Delcroix F Masia R Murtugudde and JVialard (2001) The oceanic zone of convergence on the eastern edge ofthe Pacific warm pool A synthesis of results and implications for ENSOand biogeochemical phenomena J Geophys Res 106 236322386doi1010292000JC900141

Qu T H Mitsudera and T Yamagata (1999) A climatology of the circu-lation and water mass distribution near the Philippine coast J PhysOceanogr 29 1488ndash1505

Qu T J Gan A Ishida Y Kashino and T Tozuka (2008) Semiannualvariation in the western tropical Pacific Ocean Geophys Res Lett 35L16602 doi1010292008GL035058

Qu T S Gao and R Fine (2013) Subduction of South Pacific tropicalwater and its equatorward pathways as shown by a simulated passivetracer J Phys Oceanogr 43(8) 1551ndash1565 doi101175JPO-D-12-01801

Reverdin G S Morisset D Bourras N Martin A Lourenco J BoutinC Caudoux J Font and J Salvador (2013) Surpact A SMOS surfacewave rider for air-sea interaction Oceanography 26(1) 4857 doi105670oceanog201304

Riser S L Ren and A Wong (2008) Salinity in Argo A modern view ofa changing ocean Oceanography 21 56ndash67

Rodier M G Eldin and R Le Borgne (2000) The western boundary ofthe equatorial Pacific upwelling Some consequences of climatic vari-ability on hydrological and planktonic properties J Oceanogr 56 463ndash471

Shinoda T and R Lukas (1995) Lagrangian mixed layer modeling of thewestern equatorial Pacific J Geophys Res 100 2523ndash2541 doi10102994JC02486

Singh A T Delcroix and S Cravatte (2011) Contrasting the flavors of ElNi~no-Southern Oscillation using sea surface salinity observations JGeophys Res 116 C06016 doi1010292010JC006862

Song Y T S Yueh J-H Moon and T Qu (2013) Assessing theseasonal variability of the global sea surface salinity J Geophys Resin press

Sprintall J and M Tomczak (1992) Evidence of the barrier layer in thesurface layer of the tropics J Geophys Res 97 7305ndash7316 doi10102992JC00407

Takahashi K A Montecinos K Goubanova and B Dewitte(2011) ENSO regimes Reinterpreting the canonical and ModokiEl Ni~no Geophys Res Lett 38 L10704 doi1010292011GL047364

Yu J-Y and H-Y Kao (2007) Decadal changes of ENSO persistencebarrier in SST and ocean heat content indices 1958ndash2001 J GeophysRes 112 D13106 doi1010292006JD007654


29

Sea surface salinity and barrier layer variability in the equatorialPacific as seen from Aquarius and Argo

Tangdong Qu1 Y Tony Song2 and Christophe Maes3

Received 23 August 2013 revised 20 November 2013 accepted 9 December 2013 published 7 January 2014

[1] This study investigates the sea surface salinity (SSS) and barrier layer variability in theequatorial Pacific using recently available Aquarius and Argo data Comparison betweenthe two data sets indicates that Aquarius is able to capture most of the SSS featuresidentified by Argo Despite some discrepancies in the mean value the SSS from the twodata sets shows essentially the same seasonal cycle in both magnitude and phase For theperiod of observation between August 2011 and July 2013 Aquarius nicely resolved thezonal displacement of the SSS front along the equator showing its observing capacity of thewestern Pacific warm pool Analysis of the Argo data provides further information onsurface stratification A thick barrier layer is present on the western side of the SSS frontduring all the period of observation moving back and forth along the equator with itscorrelation with the Southern Oscillation Index exceeding 080 Generally the thick barrierlayer moves eastward during El Ni~no and westward during La Ni~na The mechanismsresponsible for this zonal displacement are discussed

Citation Qu T Y T Song and C Maes (2014) Sea surface salinity and barrier layer variability in the equatorial Pacific as seenfrom Aquarius and Argo J Geophys Res Oceans 119 15ndash29 doi1010022013JC009375

1 Introduction

[2] As the warmest open-ocean water in the globalocean the western Pacific warm pool plays an importantrole in the worldrsquos climate [eg Lukas et al 1996] Thiswarm water pool characterized by sea surface temperature(SST) warmer than 28Cndash29C lies between about 10Nand 15S west of 160W (Figure 1a) covering a global sur-face area equivalent to about two thirds of the continentalUnited States Although SST variability within the westernPacific warm pool is relatively small its impact on theglobal atmosphere is large [eg Palmer and Mansfield1984] because of the vigorous atmospheric deep convec-tion there The western Pacific warm pool has been shownto migrate zonally on an interannual time scale rangingfrom 2 to 7 years in response to the atmospheric and oce-anic fluctuations associated with El Ni~no-Southern Oscilla-tion (ENSO) [eg McPhaden and Picaut 1990]

[3] The western Pacific warm pool is also a fresh waterpool and its eastern edge is characterized by a zonal seasurface salinity (SSS) front which has a mean equatorialposition near the international dateline (Figure 1b) The

zonal SSS front separates the fresh western Pacific waterfrom relatively salty central Pacific water and the associ-ated SSS gradient can reach as large as 1 psu (the valuesaccording to the 1978 practical salinity scale are given inpsu for simplification in the rest of the text) in 1 longitude[Rodier et al 2000 Delcroix and McPhaden 2002 Maes2008] Such a large SSS gradient is believed to play a rolein the warm pool heat balance [eg Shinoda and Lukas1995 Chen 2004] According to previous studies this SSSfront can move eastward by up to 8000 km during an ElNi~no event [eg Maes et al 2004] consistent with therevised theory for the oscillatory nature of ENSO by focus-ing on the advective feedback alone [Picaut et al 1997]

[4] Another important feature of the western Pacificwarm pool is the presence of the barrier layer Due to alarge excess of precipitation over evaporation (P-E) salin-ity in the upper western equatorial Pacific is low (lt345psu) compared with that in the central equatorial Pacific(gt350 psu Figure 1) The importance of this SSS contrastin surface stratification was first noted by Godfrey andLindstrom [1989] and Lukas and Lindstrom [1991] Theirhydrographic observations revealed a unique vertical pro-file of temperature and salinity a shallow fresh surfacelens is embedded at the top of a deep isothermal layer Theshallow halocline marks the depth of the surface mixedlayer and the layer between the bottom of the mixed layerand the top of the thermocline was termed the lsquolsquobarrierlayerrsquorsquo [Lukas and Lindstrom 1991]

[5] The barrier layer is believed to play a role in themaintenance and temporal evolution of the western Pacificwarm pool Results from a coupled ocean-atmosphere gen-eral circulation model have clearly demonstrated that byisolating the mixed layer from the entrainment cooling atdepth and by confining the response of westerly wind

1International Pacific Research Center SOEST University of Hawaii atManoa Honolulu Hawaii USA

2Jet Propulsion Laboratory California Institute of Technology Pasa-dena California USA

3Institut de Recherche pour le Developpement Laboratoire drsquoEtudes enGeophysique et Oceanographie Spatiales Toulouse France

Corresponding author T Qu International Pacific Research CenterSOEST University of Hawaii at Manoa 1680 East-West Rd HonoluluHawaii 96822 USA (tangdonghawaiiedu)

copy2013 American Geophysical Union All Rights Reserved2169-9275141010022013JC009375

15

JOURNAL OF GEOPHYSICAL RESEARCH OCEANS VOL 119 15ndash29 doi1010022013JC009375 2014






2 Data Description


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Easternedge WP

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sea surface salinity and barrier layer variability in the

Documents