Evidence of the Barrier Layer in the Surface Layer ... - Semantic Scholar

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. C5, PAGES 7305-7316,MAY 15, 1992

Evidenceof the Barrier Layer in the SurfaceLayer of the Tropics JANET SPRINTALL1 Department of GeologyandGeophysics, Universityof Sydney, Sydney, Australia

MA'ITH/AS TOMCZAK2 OceanSciences Institute,Universityof Sydney,Sydney,Australia

Comparisons betweenisothermal depthto thetopof thethermocline, andthemixedlayerdepthbasedon a ot criterionwereundertaken for thetropicalworldoceans. In threeequatorial regions, a shallower mixedlayer thanisothermal layeroccurs,implyingthepresence of a stronghaloclineabovethethermocline. This distance separating thetopof thethermocline andthebottomof themixedlayeris referredto asthe"barrierlayer",in relationto its impedimentto verticalheatflux out of thebaseof the mixedlayer. Differentmechanisms are responsible for maintaining thebarrierlayerin eachof thethreeregions. In thewesternequatorial PacificOcean a salinitybudgetconfirmedthatheavylocalprecipitation mostlikely resultsin theisothermal but salt-stratified layer. In the northwestequatorialAtlantic,it is hypothesized that high salinitywatersare subducted at the subtropics duringwinterandadvectedwestwardas a salinitymaximumin the upperlayersof the tropics, resultingin thebarrierlayer.In theeasternequatorialIndianOcean,monsoonal relatedrainfallandriverrunoff contribute significantly to thefreshwater flux,producing saltstratification in thesurface. Theseresultssuggest theneedto includetheeffectsof salinitystratification whendetermining mixedlayerdepth.

1. INTRODUCTION

Thecreation of thebarrierlayerexertsanimportant influence on mixedlayerdynamics and,in particular, theinterplaybetweenthe

The ocean surface mixed layer generally denotes aquasikinetic energy and potential energy processes mentioned above. The

homogeneous layerwith little variationin temperature, salinity,and

termbarrierlayeris indicative of itseffecton themixedlayerheat

density. The zone owes itshigh degree ofvertical uniformity tobudget. Inasteady state situation, one would expect the formation mixingcausedby turbulence. The turbulence, whichmayresultin of a temperature gradientin response to the salinitygradient,in themixedlayerdeepening or stratifying,requiresan energyinput order to balance the surface heating with the mixing from thatmaybe generated by kineticenergyand/orby potentialenergy entrainmentof cooler water from below the halocline.However, at the sea surface.Kinetic energy, from the transferof the wind thisdoesnotseemto occurin thewestern equatorial Pacific,where momentumto the sea,resultsin mixing processessuchas wave

in isothermaland isohalinedepthshas been action,entrainment,andhorizontaladvectionby currents.Potential the discrepancy Thepresence of thebarrierlayermeansthatanywater energyis measured by buoyancyflux thatreflectschangeof density observed. entrainedfrom the isothermal layerinto themixedlayerhasthe

due to heat and freshwater fluxes.

Itisthepycnocline, where thewater ishighly stable, that defines same temperature asthewater inthemixed layer. There istherefore

through thebottomof themixedlayer.Thusthe thelowerboundaryof themixedlayer.That is to say,it represents a noheattransferred vertical limit to air-sea interaction, and the associatedturbulence is

heatinputatthesurface wouldperpetually raisethetemperature of

advected awayfromthe lessableto penetratethroughthislayer.Generallythe pycnocline theupperoceanif it werenothorizontally

coincides withboth thehalocline andthethermocline; however, region, orinfactif thenetheatinput through thesurface ofthe recent investigations [Delcroix etal.,1987; Lukas andLindstrom, mixed layer was notclose tozero. Astheseasurface temperature 1991] have noted thepresence ofa shallower halocline than(SST) gradients intheregion arethought tobetoosmall for advective processes tobesignificant [Enfield,1986],the thermoclinein the upperlayer of the westernequatorialPacific horizontal

Ocean. Thesharp gradient ofthehalocline wasreflected inthecase forthenear zero heat fluxisbeing strengthened [forinstance, pycnocline (asillustrated, forexample inLukas and Lindstrom's see Godfrey andLindstrom, 1989]. However, Lewis etal.[1990] [1991] Figure 3),and werefer tothedistance separating thebottom suggest that there may beanetsurface heat fluxoftheorder -15-25 Pacific, thatmaythenbelostthrough of themixedlayerfromthetopof thethermocline(i.e., thebottom W m-2intothewestern

oftheisothermal layer)as the"barrier layer".

penetration ofsolar radiation through thetransparent waters to below the mixed layer. Furtheroceanographic studiesunder differingoceanicconditionsthat will directlymeasurevertical

1Now atNOAA/Pacific Marine Environmental Laboratory, Seattle, mixing ofheat mayberequired toconf'um theinteraction between

Washington.

thebarrierlayer,SST,andheatflux in theregion.

2NowatSchool ofEarth Sciences, Flinders University, Adelaide, Differences betweentheisohalineandisothermal depthshave Australia. beenobservedin the westernequatorialPacific [Delcroixet al., 1987;LukasandLindstrom,1991];however,little hasbeendoneto

Copyright 1992by theAmerican Geophysical Union.

identifyotherregionsof theworldoceans wherethisdiscrepancy may exist.The major aim of thispresentanalysisis to map the large-scale distribution of thesedifferences in thetropicaloceans

Papernumber92JC00407. 0148-0227/92192.1C-00407505.00 7305

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SPRINTALLAND TOMCZAK: EVIDENCE OF BARRIERLAYER IN THE TROPICS

and to determine the horizontal and vertical processesthat are contributing to themismatchin verticalscalesbetweenmixedlayer andisothermaldepth. Historicallymixed layer depthhasbeenconsidered synonymous with thermoclinedepth.This assumptiondevelopedmore out of necessity(though of courseit can still be correctlyphysically based),asobservations consistingof salinityandhencedensitydata weremuchlesscommon.The existenceof thebarrierlayerindicates that determinationof a mixed layer depth basedon a temperature criterion alone is not sufficient to indicate the layer affected by surfacemixing processes.The isothermallayer doesnot always correspond to a verticallyuniformlayerbecausesalinitycangovern the stratificationof the water column.The mixed layer has to be der'reedrelativeto bothtemperatureandsalinity.The improveddata baseavailabletodaywill enableus to confirmtheisothermal/mixed layer assumption,thereby gaining a better understandingof characteristics of the mixed layerßRecentlySprintalland Tomczak [1990] undertook a comparison of mean depth and standard deviationfor sevendifferentcriteriaof mixed layer depthusingthe CTD cruisedataof theWesternEquatorialPacificOceanCirculation Study (WEPOCS) I and II expeditionsin the westernequatorial Pacific. The der'tuitionsfor mixed layer were basedon differences in temperature,salinity, and density using a variety of critical gradientcriteria and net decreasecriteria from the surface.Their studysuggests the suitabilityof mostof thesecriteriain determining eithermixedlayeror isothermallayerdepth.The two criteriausedin this studyare discussedin the analysisof section2. One, basedon a 0.5øCchangefrom theseasurfacetemperature, givesanindication of the isothermallayer to the top of the thermoclineand hasbeen usedextensivelyas a proxy to mixed layer depth.The othermethod is based on a variable ot criterion that accountsfor the thermal expansionrequiredto obtaina net temperature differenceof 0.5øC. Hence, this techniqueaccountsfor both salinity and temperature effectin determiningthedepthof themixedlayer.

and 106 m over the two WEPOCS cruises, while the isothermal

layerwas~ 64 m deep,implyinga meanbarrierlayerof 35 m in thickness.With this in mind, the resolution of the standardvertical

levelsgivenabove,for thesurfacelayerof theLevitus[ 1982]data set,appears to be quitesufficientfor computations of tropicalmixed layerdepth,andfor determining barrierlayersof greaterthan10-m thickness.

The representativeness of theoriginaldatausedin theobjective analysisin termsof spatialandtemporalbiaseshasbeendiscussed by Levitus[ 1982],who notedthatthe distribution of observations in thetropicalregionswasadequate for defininglarge-scale features thatarerepresentative of therealocean.It is seasonal datagrouped according to borealseasons, althoughherewe will referonlyto the monthlygroupingsto avoid confusionwhen referringto crossequatorial regions.Thesegroupings areFebruary, March,andApril (FMA); May, June, and July (MIJ); August, September,and October(ASO) andNovember,December,andJanuary(NDJ). Definitionof theMixedLayer The definition of the mixed layer depth (mld) is basedon a variable sigma-tcriterion,and determinesthe depthwhereot is equalto the seasurfaceot plusthe incrementin ot equivalentto a desirednet decreasein temperature.This incrementin ot usesthe coefficientof thermalexpansion, calculatedasa functionof surface temperature andsalinity.Thusthe techniqueaccounts for boththe salinityandtemperature effectin determining thedepthof themixed layer.We wishto interpolateto thedepth(z--mld)at which

Ot, ml d=ot, 0+ATDT Dt

(1)

whereot,0 is thesurfaceot value,A T is the desiredtemperature differenceand &Or/&T is the coefficientof thermalexpansion

Threeequatorial regionsare foundto displaya significant

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positivedifference(10-50 m) betweenthe top of the thermocline

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andthemixed layerdepth calculated fromthe{Itbased criterion.

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t,

This phenomenon implies the presence of a strong halocline

occurring abovethethermocline. Thedynamics of thethreeregions

30 50

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that showa barrierlayer, namelythe westernPacific,theequatorial

Atlantic, andtheBayofBengalin theIndianOcean, differdistinctly•

-75

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from each other. In section3 we develop a salinity budget and invoke different surfaceforcing mechanismsfor eachof the three •. 100 regions in order to explain the salt-stratified isothermal layer. -o Section4 givesa summaryandconclusions. 125 2. DATA AND METHODOLOGY Data

The Levitus [1982] world ocean data set will be used to

determinethe extentof salinityinfluenceon themixedlayerdepth. The data set consistsof objectively analyzed temperatureand salinityfieldsat standardoceanographic observation depthlevelson a 1o latitude-longitudegrid for the world ocean.These standard depthlevels are given at 0 m, 10 m, 20 m, 30 m, 50 m, then thereafterevery 25 m to 150 m, every 50 m to 300 m, andevery 100 m to a maximin depthof 1500m. In thetropicalocean,mixed layer depthsgreater than 150 m are rare, and more often are substantiallyshallower than this. For instance,in the western equatorialPacificOcean,LukasandLindstrom[1991] determined the averagemixedlayerdepthto be 29 m, with a rangebetween1 m

,,

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Fig. 1. Temperature, salinity,andsigma-tat 158.5øE,2.5øSduringFMA from theLevitus[1982] dataset.(Standarddepthlevelsof the Levimsdata are given on the fight axis.) The differencebetweenmixed layer depth (solidline) andisothermaldepth(dashedline) is referredto asthebarrier layer.

SPRINTALL AND TOMCZAK: EVIDENCE OF BARRIER LAYER IN THE TROPICS

3. SEASONAL STRUCTURE IN THE THICKNESS

evaluatedwith surfacevalues of temperatureand salinity. This definitionallowsfor comparison of mixedlayerdepthson a world ocean basis, as it accountsfor the difference in surface salinities and

temperatures andtheireffecton theseawaterexpansioncoefficient. A verticalprofile of temperature,salinity,and sigma-tfrom the Levitus data for the equatorialPacific Ocean at 158.5øE,2.5øS duringFMA is shownin Figure1. The mixedlayerdepthcalculated from (1) is 37 m, assuminga temperature differenceof A T = 0.5øC. If salinitystratification withinthesurfacelayerwasnegligible,then equation(1) wouldgive a mixed layerdepthas calculatedby the depthof thisdesiredtemperaturedifference(A T) from the surface isotherm.In Figure 1 the depthof the isotherm,which is 0.5øC colderthanthesurfaceisotherm,is 79 m, thatis, 42 m deeperthan the mixed layer depth.This 42-m differencebetweenthe c•tbased mixedlayerandthedeeperisothermal layerrepresents thethickness of the barrier layer for this profile. Note that the depthsof the isothermal andmixedlayersdonotcoincide withthestandard depth levelsusedby Levitus [1982], as we have linearly interpolated betweenthesedepthsto satisfythecriteriaused.

Thispaperconsiders thedistribution of thebarrierlayerin the

7307

OF THE BARRIER LAYER

Three equatorialregionsconsistentlydisplayed,throughoutall seasons,a significantpositivedifference(10 - 50 m) betweenthe isothermallayer and c•tbasedmixed layer criterion,implyingthe presenceof a stronghaloclineocctmSngabovethe thermocline.As mentionedpreviously, the difference between the two depthsis referredto asthebarrierlayer.Here, we will determinethedifferent mechanismsof surfaceforcingresponsiblefor the maintenanceof the barrier layer in each of the three regions.In the following, a Peter'sProjection[Peters, 1989; Tomczakand Krause, 1989] has beenusedfor all figuresof the equatorialoceans,asthisprojection combinesfidelity of areawith a rectangular latitude-longitude grid, both appropriatefeatures for large-scalemapping of tropical oceanographic data. TheWesternEquatorialPacificOcean The westernequatorialPacificregion(Figure2) hasin its core, generallycentredat-•160øE, a 25-m differencein depthbetween isothermal and isohaline layers (i.e., a 25-m barrier layer

tropical(i.e., 30øS-30øN)regionsof the world ocean,that is, the differencewhenisohalinelayerswere foundto be shallowerthan isothermallayers. Seasonalchartsof the absolutevaluesfor c•t basedmixedlayer,depthof theisothermal layer,andthedifferences betweenthetwo, are availablein Sprintalland Tomczak[1990] for

thickness).It stretchesfrom the northeasterncoastof New Guinea, straddlesthe equatorbetween10øSand 10øN, andreachesits most

alongthe isopycnals.This featurehas been investigatedby J.

The importanceof salinityin the upperlayersof thisregionhas beenrecognizedby many authors.The WEPOCS cruisesof 1985

extensiveeasternlimit at 170øW during MJJ (Figure 2b) of the southwestmonsoonseason.Barrier layersof 50-m thicknessare evidentin the southernhemisphereduringNDJ (Figure 2d) and the world ocean between 60øS and 60øN. The reader is referred to FMA (Figure 2a). There is a 10-m contourpresentduring all thisreportfor furtherdifferences, suchasin thesubtropics, wherea seasons,enclosing the 25-m barrier layer thickness,running deeperisopycnal thanisothermal layerexistsdueto theconvergence directly east of the Philippinesto --150øW then westwardto the of the Ekmanlayer transportandthe subduction of surfacewaters Coral Sea. Sprintall andM. Tomczak (On the formationof CentralWater and

thermoclineventilationin the southernhemisphere,submittedto Deep SeaResearch,1992).

and 1986 [Lindstrom et al., 1987; Lukas and Lindstrom, 1991] determined differences between isothermal and isohaline layers

30øN

20øN

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,•,.25 10øN ,

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10øS

20øS

(a) 30øS 120øE

150øE

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120øW

90øW

Fig.2. Differtraces (meters) betweentheisothermal andmixedlayerdepthfor thePacificOceanduring(a) February-April, (b) MayJuly, (c) August-October, and (d) November-January. Positivedifferences(solidlines)indicatea shallowerisohalinelayer than isothermal layer,andnumbers givethethickness of thebarrierlayer.

7308

SPRINTALL ANDTOMCZAK: EVIDENCE OFBARRIER LAYERINTHETROPICS 30øN

20øN

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Fig. 2.

(continued)

whichwereof approximately thesamemagnitude aswhathasbeen determined herefrom theclimatologicseasonal datasetof Levitus [1982]. Lukas [1988] suggestedthat the isothermalbut salt stratifiedlayerwasproducedby a net flux of freshwater into the region,creating thedistinctstablebarrierlayerseparating thebottom of themixedlayerfromthetopof thethermocline. The amountof freshwater requiredto inducesurfacefreshening in orderto maintaina barrierlayer can be obtainedby applying salinityandmassconservative principlesto theregionwherethe

barrierlayer exists.In the westernequatorialPacificOcean,the regionbounded by 10øS- 10øNand150øE- 170øEdi•lays abarrier layerof at least25-mthickness in all seasons. Usinga simplebox modelfor theuppersurfacelayer,massandsaltconservation are givenby

AlulSlPl + A3u3S3P3 = A2u2S2P2 +A4u4S4P4 (2) Alul +A3u3 +RB = A2u2+A4u4 whereAi is thecross-sectional areawith a salinitySi, densityPi,

SPRINTALL ANDTOMCZAK: EVIDF2qCE OFBARRIER LAYERIN THETROPICS

7309

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Fig. 2. (continued)

andvelocity normalui,fortheupstream (i=1),downstream (i=2), Thus in the calculations undertaken here we will assume an SEC andsideboundaries (i=3,4).Thefreshwater gain(R) through the velocity withUl = -50cms-1withameridional boundary between surfacearea(B) of the box represents the differencesbetween 10øSto3øN.Finally,meridional movement isthought toplayonlya precipitation (P) andevaporation (E). smallpart in the dynamicsof the mixedlayer of this region Further refinement of themodel(2) iscarried outbymaking a [McPhaden andPicaut,1990];hencewewill consider onlythetotal numberof assumptions whichmaybe appliedin general,and divergence throughthemeridionalsidesof thebox (i.e., A5u5= specifically in applying themodelto thewestern equatorial Pacific A3u3-A4u4) withnochange in themeridional valueforsalinityof

region. First, we will assumethat the differencesin densitiesare

34.8 (S5) [Levitus, 1982].

negligible. Second,watermaybe transported in themodeleither Arethesereasonable assumptions forourmodelin anattempt to throughthesidesor throughthesurface. Entrainment frombelowis explainthe existenceof the barrierlayer within the western neglected. Thismayatfirstappear a gross assumption; however, in equatorial Pacificto be dueto ocean-atmosphere freshwater flux? view of thelightmeaneasterlytradewinds experienced in the Theassumption thatentrainment processes arenegligible for this westernequatorial Pacific[WyrtkiandMeyers,1975],thisclosed regionis supported in therecentobservations of Godfreyand bottomboundary condition maybejustified.Certainlyoff the Lindstrom[1989]andLukasandLindstrom[1991].As discussed equator wecanexpectthewindto inducea surface convergence. already,the very presenceof the barrierlayer impliesthat Thevalidity oftheassumption ontheequator where divergence may entrainment cooling,through heatbudgetimplications, cannotbe resultis discussedfurtherbelow. Further,if the box is orientated significant in the watersof the westernPacificregion.Ekman alongthemeanzonalequatorial current in theregion,thenthismass transport divergence andupwelling attheequator areproportional to

maybeusedtodilutethesalinity fromS1attheeastern boundary to thelocalzonalwindstress [Gill, 1975].As stated previously, the S2atthewestern boundary, assuming nochange in eitherthezonal zonalwindstress forthewestern Pacificis closeto zeroalongthe flow(i.e.,Ul=U2)or theinflowing andoutflowing cross-sectional equator in theannualmean,andis weakduringmostmonthsof the

areas(i.e., A 1=A2). In the regionof interestin the western year [Wyrtkiand Meyers,1975].Indeed,Levitus [1982] shows equatorial Pacifican averagemixedlayerof 35 m wasestimated littlevariation in seasurface salinityacross theequatorial region [Sprintall andTomczak, 1990]withanaverage salinity at 170øE of here,perhaps implyingupwellingof thesaltierwaterof thebarrier

34.9(S1)andat 150øE of 34.6(S2)according totheLevitus [1982] layerto be m'mimal. In accordance withthis,ourneglecting of atlas.Zonalflow(Ul) in theupper50m of theregionistoward the entrainment processes on the equatorin thisregionis not an

westin theSouthEquatorial Current(SEC)witha meridional extent unreasonable assumption. between10øSto 3øN.Delcroixet al. [1987]madedirectcurrent In bothstudiesby Delcroix et al. [ 1987] andLindstromet al. measurements along165øEduringsixhalf-yearlycruises between [1987]it wasobserved thatif thezonalcomponent of thewind 7ø-10øNand20øSfromJanuary 1984to June1986andobserved a changedto westerlies,within a week the zonal flow in the SEC at

maximum speed in theSECof -50cms-1.Similarly, Lindstrom et

thesurface reverses. Thiscouldchange boththe signandthe magnitude of our Ul value.However,with thearrivalof strong mooring at 0%150øœ fromAugust1985through January 1986. westerlywindbursts,LukasandLindstrom[1991]alsoobserved al. [1987]founda peakspeedof -50 cm s-1in theSEC fromtheir

7310

SPRINTALL AND TOMCZAK: EVIDENCE OF BARRIER LAYER IN THE TROPICS

that the halocline was eroded, the barrier layer broken down, entrainmentprocessesinitiated,and strongmixing occurred.Thus in a modelwherewe areexplainingtheexistence of thebarrierlayer per sein a large-scaleclimatologicdataset,theseconditionsholdno relevance to us, and hence the assumptionsseem reasonable. Correspondingly,after the proposedmodifications,the model (2) may be writtenas A5u5S5 + Alul(S1-S2) = 0

A5u5 +RB= 0

(3)

and solvedfor the above outlinedvaluesto give a net freshwater

flux (R) of 2140 mm yr-1 (positivevalue denotesexcess precipitation)and a meridionalmasstransport(A5u5) of 0.2 Sv

whichimplies ameannorth-south velocity of2.8x 10-3 ms-1.As expected,this velocity is much smaller than the zonal flow of the SEC substantiating the hypothesisof smallmeridionalinfluencein the mixed layer dynamicsof the region. So, approximately2140

mm yr-1 of freshwater is required for theformation of the freshwaterlens at the upper surfaceof the westernequatorial Pacific. Is this value in accord with the estimates of freshwater flux

acrosstheseasurfacebasedon theexistingdistribution maps? Weare et al. [1981] establishedan annual E-P balanceby comparingthe annualestimationof rainfall in the tropicalPacific from Taylor [1973] to their computedlatent heat transfer.The distributionof thiscomputedE-P balanceindicatestwo m'mimaof

indicatesthatthe SPCZ is lesspronounced thantheITCZ, although it still exhibitschangesin bothpositionandmagnitude[Goldenburg and O'Brien, 1981]. During January,maximum convergenceis observed near 15øS in the western equatorial Pacific when associatedconvectiveactivity brings maximum rainfall [Taylor, 1973].By Septemberthe SPCZ is lessmarkedandhasshiftedmore northerly,reachingabout10øS.The seasonality in zonalmovement of the SPCZ is not as pronouncedin the distributionof the barrier layer in the southwestern Pacific Oceanas it is for seasonalITCZ movement.However, it is observedin Figure2 that the maximum valuesof thebarrierlayer lie alonga southeast/northwest axis,asis characteristic of the mean S PCZ location.

Eastof 160øW(Figure2), thenegativeisopleths in thesubtropics indicatethat the equatorialbarrierlayer is maintainedby seasonal subduction of the surfacewater.This processalternatesbetweenthe northern hemisphere in FMA (Figure 2a) and the southern hemispherein ASO (Figure 2c). It is describedmore fully in the following Atlantic Oceansection,whereit is shownto be the main sourcein maintainingthebarrierlayer structure. TheNorthwestern EquatorialAtlanticOcean

The equatorial northwest Atlantic is also characterizedby a shallower isohaline than isothermal layer (Figure 3). In NDJ (Figure3d) a 25-m discrepancyin scalesis foundin thenorthfrom theWest Indies(~20øN) out to ~50øWthenalongthenortheastern SouthAmericancoastlineto Cabo de S•o Roquejust below the over-1500mmyr-1, oneon theequatornorthof New Guinea,the equator.It containsa maximumof 50 m in the depthof thebarrier other at 10øS,170øE.This excessprecipitationover evaporation layer at ~55øW, 15øN.In FMA (Figure3a) the 25-m discrepancy coincidesextremelywell with theregionwherethe mixedlayeris reachesits most eastwardextent to 30øW at ~15ø-20øN, and the 50shallower than the isothermallayer (Figure 2). Indeed, recent m maximum has extendedto lie between 12øN-20øN. By MJJ estimatesby Oberhuber[1988] of thenet downwardfreshwaterflux (Figure3b) the 25-m isoplethhasdiminishedin area,andthe 50-m in the twominimaputsthevalueat morelike -2400 mm yr-1. The barrierlayer hasbecomeonly of the orderof a few degreessquare differencelies in the precipitationdataused.The rainfall analysis to lie directlyadjacentto the SouthAmericancoastat ~8øN.Finally usedto preparethe fluxesof Oberhuber[1988]werethoseof Shea in ASO (Figure3c) threesmall25-m maximaremainin thenorthern hemisphere,while a new region of more than 25-m barrierlayer [1986] derivedfrom landor islandstationrecordsandcompletedby satellite observations. Uncertainties of oceanic rainfall calculations thicknessdevelopsin the south. Updatedannualandseasonalmapsof E-P for the northAtlantic in boththeTaylor [1973] andtheShea[1986] precipitation datasets leadto a meanestimateof approximately -2000mm yr-1 in theE-P Oceanwere producedrecentlyby Schmittet al. [1989]. The data balance for the western equatorial Pacific. The value is in good sets used for this calculation are the heat flux estimates of Bunker agreementwith the computedamountof freshwaterrequiredto [1976] and the precipitation estimatesof Dorman and Bourke maintainthe shallowisohalinelayer. [1981]. Both of these studiesprovide an increasedspatial and The result is not surprisingconsideringthe coincidenceof the temporalresolutionfor estimationof E-P overpreviouswork.Their two negativeE-P extremesandthe maximumrainfall of over 5000 map of annual E-P shows that in the region west of 40øW, mm yr-1 in Taylor's [1973] annualrainfall map. These,in turn, correspondingto maximum discrepanciesin isothermal and correspond to the mean point of intersection between the isohaline depths (Figure 3), there is a net water loss to the IntertropicalConvergenceZone 0TCZ) of thenorthernhemisphere, atmosphere (E-P>0) of 1000-1250mm yr-1. Localfreshening of and the SouthPacific ConvergenceZone (SPCZ) of the southern the surface layer by excessprecipitation, the mechanismthat hemisphere.Both zonesareknown tropicalrainfallbands,andtheir producesthe barrierlayer in the westernequatorialPacificOcean influence in producing the barrier layer at their confluenceand andmodeledby equation(3), canthereforenot be responsible here. acrosstheentiretropicalPacificcanbe followedin Figure2. Clearly, we must look to other processes,such as horizontal The meanpositionof theITCZ is nearthe equatorin the western advectionfrom adjoiningregionsor continentalrunoff, as a source Pacific,rising to ~10øN in the centralPacific [Barnett, 1977]. The for dilutionof thesurfacelayers. 10-m differencecontour(Figure2) betweenmixedlayerdepthand The regionof net evaporation lies immediatelyadjacentto a band thermoclinedepthfollows the ITCZ acrossthe equatorialPacific, of net precipitationin the tropicsfoundbetweenthe equatorand emphasizingthe importanceof rainfall in saltstratifyingthemixed 10øNbut eastof 40øW [Schmittet al., 1989].The greatestseasonal layer. The seasonalvariationin ITCZ movementis alsoreflectedin variability in E-P in terms of both magnitudeand location of the distributionof the barrier layer thickness(Figure 2). During gradientsis foundin thisequatorialregion.It is associated with the February the ITCZ reachesa northernextreme at ~15øN, then migrationof theAtlanticOceanITCZ andits accompanying rainfall. retreatsto ~10øNduringAugust.The 10-m isoplethof barrierlayer There generally appearto be two modesof variability. During thicknessfollows this seasonality,as evidencexl in Figures2a and December-Maya net precipitationbandextendsacrossthe Atlantic 2c, respectively. between0ø and 10øN,anda netevaporation maximumliesbetween The average annual cycle of the surface wind divergence 10øNand20øNwith a strongzonalgradientbetweenthetwo.Both

SPRINTALL AND TOMCZAK: EVIDENCE OF BARRIER LAYER IN THE TROPICS

30øN

7311

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25,

20øN

10

10øN

10øS

20øS

30øS 80øW

30øN

60øW

40øW

20øW

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80øW

60øW

40øW

20øW

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10øN

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20øS

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30øS

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60oW

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80øW

60øW

40øW

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Fig.3. Differences in mixedlayerdepth(meters) betweentheisothermal andmixedlayerdepthfor theAtlanticOceanduring(a) Febmary-April,(b) May-July,(c) August-October, and(d) November-January. Positivedifferences (solidlines)indicatea shallower isohaline layerthanisothermal layer,andnumbers givethethickness of thebarrierlayer.

featureshaveextremain excessof 120001mm yr-1. This period correspondsto when the barrier layer is thickest(Figures 3a and 3d); however,as its distributionlies in the region of evaporation excess,some northward advectionof the freshwatergain at the equatoris required.In fact, Schmittet al. [1989] do estimatethat

northwardtransportof freshwaterflux, includingriver runoff, is maximum at 10øN; however, this represents the meridional divergence acrossthe entire ocean basin. Furthermore the U.S. Navy OceanographicOffice [ 1965] atlasindicatesthat the Ariantic Ocean South Equatorial Current (SEC) strengthensand extends

7312

SPRINTALL AND TOMCZAK: EVIDF•CE

OF BARRIER LAYER IN THE TROPICS

thesetwo westwardequatorialcurrents,andusuallyextendsnorth of theequatorto convergewith theNEC at -•7øN,30øW [U.S.Navy OceanographicOffice, 1965]. From here,both currentsproceed into the CaribbeanastheGuianaCurrent,or duringsummer,some flow may deflecteastwardto feed the AtlanticEquatorialCounter Current (ECC). The salinity of the maximum varies little and maintainsaboutthesamethicknessoverits longcoursewithinthe thebarrierlayersuffersa reduction in magnitude to -•500mmyr-1. NEC andSEC to well withintheGulf of Guinea.It thusrepresents The barrierlayerduringthisperiod(Figures3b and3c) is generally an inlxusionof high-salinitywater beneaththe surfacelayersof shallowerand more diffuse and extendsdown to the equator,thus lower salinity in the equatorialregions.The salinity maximum corresponding in partwith theregionof netprecipitation-1500 mm appearsto be presenteverywherein the tropicalAtlanticexceptin yr-1 in theE-P balance.Thisindicates thatduringsometimesof the two narrowbands;onebetween10ø and 15øNandextendingfrom year at the equator there is an excess of precipitation over 40øW eastwardto Africa, and a second,more narrow band between evaporation, whichmaybe sufficientto allowfor theformation of a 2øand3øSandextendingfrom 30øto 10øW.The two bandswithout fresh isohaline layer above a deeper isothermallayer. In all salinity maximum mark the southern and northern limit, however,the seasonaleast-westanisotropyin precipitationallows respectively, of thesubtropical CentralWatermassesastheyextend evaporationto dominatein the annualaveragein a narrowband towardthe equator[Worthington,1976]. Betweenthe two bands extendingto the equatorfrom the SouthAmericancoastto -•45øW. the maximumappearsagainandmay be very defineddue to the of theECC, beingfedby theNEC andSEC fromregions The major area of freshwatergain remainseast of 40øW. This presence evaporative dominance in thewesternequatorial Arianticimpliesthat west of 35ø-40øW.The distributionof the salinitymaximumis someother mechanismthan rainfall must be maintainingthe salt imitatedin thedistribution of thebarrierlayerevidentin Figure3. Negativeisoplethsindicatea shallowerisothermalthanisopycnal stratifiedisothermallayer. It is likely that the low salinityoutflowfrom the Amazonand layer, and these regions are coincident with where maximum downwardvelocityin the Ekmanlayertransport occurs[Sprintall Orinoco Rivers contributes to the freshwater flux in the surface watersof theequatorialwesternAtlantic.The OrinocoRiver hasits and Tomczak,1990], andcan thereforebe interpretedas a region mouth at-•7øN and contributes845 mm of freshwaterper annum, where CentralWater massis formedthroughsubductionalong and the Amazon, with its estuarylocated on the equator,has a isopycnals.J. SprintallandM. Tomczak(submittedmanuscript, freshwateroutflow of 835 mm yr-1 [Baumgartnerand Reichel, 1992)showthattheregion30ø-45øSis a formationregionof South for ASO 1975].This may be sufficientto accountfor someof the freshwater AtlanticCentralWater.Figure3c showsactivesubduction

north of the equatorto convergewith the Atlantic OceanNorth EquatorialCurrent(NEC) in the regionof maximin barrierlayer thickness,thusprovidinga mechanismof transport.By Juneto November, the adjacent bands of net precipitation and net evaporation havemovednorthward,associated with thenorthward intensification of the equatorialrainfallband.As a result,theE-P maximumin the regioncoincidingwith the maximumthicknessin

excessrequiredto maintainthe isohalinelayer. Thesedischarge values represent outflow at the river's estuary and have been calculatedby Baumgartnerand Reichel [1975] usingthe ratio of total water volume over the catchmentarea.Interestinglythough, Baumgartnerand Reichel [1975] assumedthatwith Coriolisforce, one-thirdof the Amazonflow wouldbe to thenorthernhemisphere, with two-thirdsto the southernhemisphere.However, it is now recognizedthat the GuianaCurrentstronglydeflectsall Amazonian flow to the north and the considerabledischargepossiblyeven strengthensthe flow northwardat its outlet [Landis, 1971]. Peak flow of the Amazonoccursduringthemonthsof May-July,andthis is evidentin the 50-m barrierlayerduringthesemonths(Figure3b) huggingthe coastnorth of the river'smouth.There is no doubtthat river runoff must have a contributoryeffect on the freshwater budgetof the tropicalwesternAtlantic; however,Neumann[ 1969] suggeststhat local precipitationalongwith advectionof extremely low or high salinitywatermassesmust alsobe considered. The presenceof a shallowerisohalinethanisothermallayerin the tropicalAtlantic Oceanhas alsobeenobservedby Defant [1961] in the Meteor cruises of 1936, and also during the Barbados MeteorologicalandOceanographic Experiment(BOMEX) cruisesin May, June,and July of 1969 [e.g., seeElliott, 1974]. During the Meteorcruises,almostall stationsin thetropicsandsubtropics were characterized by a nearlyisohalinelayerwith a thinlayercontaining a well-developed salinitymaximumat itsbase,locatedabovethetop of the thermoclineand a deeperlayer of lower salinity.This saline layer originates at the surface in the subtropicsand is formed midgyre in both hemispheres,partially in response to high evaporation[Luytenet al., 1983]. This water is thensubductedas the shallow subtropicsalinity maximum [Worthington,1976] and spreadsout southwardand westwardin the NEC of the northern hemisphere, and northward and westward in the SEC of the southernhemisphere.The SEC is the strongerandmoreconstantof

southof 15øS,coupledwith formationof a barrierlayer to the north. During FMA (Figure 3a) the sameprocessoccursin the northernhemispherein the vicinity of 20øN. The two sources alternate in renewingthebarrierlayerstructure in thewestequatorial AtlanticOcean,wherethebarrierlayeris foundduringall seasons. The thicknessof the barrierlayer is generallymaximin in the regionof confluencebetweenthe NEC andSEC (Figure3). The seasonality of thepresence of theECC is alsostronglyreflectedin the distributionof the barrierlayer thickness.During FMA the countercurrent dissipates andis leastevident,while the SEC widens from

the

South

American

coast

eastward

to

-•10øW.

Correspondingly, thebarrierlayer(Figure3a) is almostnonexistent betweenthe latitudesof the ECC duringthisperiod;however,it

extends farto thesouthandwestin thesouthern hemisphere relative to other seasons.During MJJ and ASO the countercurrent strengthensand developsinto a continuouseastwardflow, with

Figures3b and3c showinga corresponding response in thebarrier layer. By NDJ, the ECC has weakenedconsiderably,and the presence of the barrierlayer (Figure3d) is onceagainconfinedto the northwestern side of the Atlantic.

The spreadingof the water containingthe salinitymaximin is mainlyby advection. The salinitymaximin fixesthepositionof the pycnocline,and the corresponding stabilitymay stronglysuppress turbulence effects,sothatthereis almosthorizontalspreading of the salinity surface.In this case, a simple model may be devised wherebythe advectionof the salinitymaximumwaterbalancesthe surfacenet heatflux (QN) suchthat z

QN= I PCw (u•T.)dz '•'•

(4)

o

whereZ is the mixed layer depthof 30 m [Sprintalland Tomczak,

SPRINTALL ANDTOMCZAK: EVIDENCE OFBARRIER LAYERINTHETROPICS

7313

1990],u is thecurrent speedalongthedirection x, andCwis the during MJJ(Figure 4b),ASO(Figure 4c),andNDJ(Figure 4d), specific heatof waterat4180W s/ kg øC.Recentestimates of net westto around 85øEandbounded by the25-misopleth. During annual heatfluxbyOberhuber [1988]in theequatorial northwest FMA(Figure da)there isauniform 10-mbarrier layerpresent over

Atlanticareof theorderof -5 W m-2,whiletheLevitus [1982] muchoftheIndianOcean northof 10ø$. This10-misopleth isalso mapsof surface temperature givea horizontal gradient of 1 x 10-6 present during otherseasons, extending northward intotheBayof øCm-1.Thisleads toanestimated westward current speed (u)of Bengalandsouthintotheequatorial IndianOceanto-10ø-20øS. approximately -4.0cms-1.Robinson andStornmel [1959]estimate According toOberhuber [1988]theregion lyingadjacent tothe

themeanwestward velocity of thelayercontaining thesalinity westcoastof Sumatrareceivesan annualfreshwaterflux of-1800 which is in generalagreementwith our results.Of coursethis

mmyr-1. This region oflocal maximum in(P-E) ispresent during

estimate mustbetreated withcaution duetoinsufficient knowledge allmonths, indicating thatrainfalllikelycontributes totheexistence concerningtypical heat fluxes and advectivevelocities. Still it ofthesalinity stratified surface layerofthisregion. appearslikely that the presenceof the shallowermixed than Tothenorth, intheBayofBengal, Oberhuber [1988]shows that isothermal layerin thenorthequatorial Arianticismaintained notso thefreshwater fluxwouldbesufficient toproduce a freshlensatthe muchbythedilution of salinity in thesurface layers(although the surfaceonly duringthe wettermonthsof the southwest monsoon outflow fromtheAmazon andOrinoco Rivers probably contribute (June-September), andeven then onlyintheeastern BayofBengal. to this)asby thepresence of a welldefinedsalinitymaximum Frequently, thewestern Bayof Bengal experiences a regime of subducted fromthesurface inthesubtropics belowthemixedlayer evaporative dominance inthefreshwater flux.It ishypothesized that andadvected bythemajorequatorial current system oftheregion. in thisregionthefreshwater fluxrequired to maintain thebarrier layeris obtainedfromthesubstantial riverrunoffintothenorthern

BayofBengal, andfurthermore theseasonality whichexists inthis

TheEasternEquatorialIndianOcean

runoff isreflected intheseasonal distribution ofthebarrier layer.

Thethirdregionwheretheisohaline layerisshallower thanthe TheGanges-Bramaputra-Irrawaddy contribute anannual average isothermal layerislocated inthesmallareaimmediately tothewest ranoffof 455-1070-1020 ram,respectively [Baumgartner and of Sumatra (Figure4), extending alongthewholewestern coastline Reichel, 1975],intothenorthandeastern sides of theBayof 30øN

30øN

(a)

(b)

20øN

20øN

10ON

10ON

...........

o

0ø

10øS

10øS

20øS

20øS

30øS 60øE

30oS 60øE

80øE

100øE

120øE

.,

12( )OE

Fig.4. Differences (meters) between isothermal and mixed layer depth fortheIndian Ocean during (a)Febmary-April, (b)MayJuly, (c)August-October, and (d)November-January. Positive differences (solid lines) indicate ashallower isohaline layer than isothermal layer,andnumbers givethethickness ofthebarrier layer.

7314

SPRINTALL ANDTOMCZAK: EVIDENCE OFBARRIER LAYER INTHETROPICS 30øN

30øN

(d)

(c)

20øN

20øN

10øN

10øN

o

o

•5 10

10øS

10øS

20øS

20øS

30øS

30øS

60øE

80øE

100øE

60øE

120øE Fig. 4.

----. .............

8( tOE

I..-............

I............

100øE

4 ...........

120øE

(continued)

from28øCattheequator to25øCupin theverynorth.This Bengal.Thisfreshwater flux is morethanadequate to sustain a decrease removes the mechanism for advection of the low-salinity water shallowisohaline layer.Mostof thisriverrunoffoccurs duringthe southwestmonsoonand resultsin a stronghorizontalsurface whichmaintainsthe halocline.Hence activemixing againtakes

salinitygradient southward across theBayof Bengalfroma lowof

placeuntilfinallythehalocline iseroded andthebarrier layeris

30 in the north to around 34 in the south (~5øN) [Pickard and

diminished(Figure4a).

Emery, 1990]. A strongpycnoclinealsoformsas a resultof this freshwaterdischarge,leadingto stablestratificationin the upper layersof thewatercolumn.Previously estimated mixedlayerdepths basedon temperature dataalone[e.g.,seeColburn,1975;Krishna et al., 1988] indicatehorizontalhomogeneity of temperature in the isothermallayer throughoutthe region duringthis period, and surfacetemperature mapsof PickardandEmery[ 1990]indicatethat theentireBay of Bengalregionsouthto theequatoris of thesame 28øC temperature.This enablesthe warm, low-salinitysurface

Anotherregionof theIndianOceanthatdisplaysa shallower isohalinethanisothermallayeris on the southeastern sideof the ArabianSea.Here, 10-m barrierlayersare presentduringFMA

waters from the fiver runoff to form a slab over the warm, more

(Figure 4a)andASO(Figure 4c)andhavethickened to25m during theintervening NDJ (Figure4d). The influence of themonsoonal

system is againevident; however, it seems tobea season outof phase withtheBayofBengal region. In thiscase, riverrunofffrom theIndusRiverprovidesthemostsubstantial contribution to the freshwater flux.

DuringMJJ(Figure4b) andASO(Figure4c) thethickbarrier

at~ 20øS,80øEisprobably associated withCentral salinewatersof the easternequatorialIndian Ocean,producing layercentered stronghalodinesandtheirresultingpycnoclines withinthewarm Water formationand subductioninducedby Ekmanpumpingas in thesubtropical region.This isothermal layer(Figure4c). This supports thepresenthypothesis shownby thenegativeisopleths that stratification due to the considerable freshwater flux from river

process is similarto thatfoundanddescribed in theequatorial

Atlantic;however,in theIndianOceanit is presentmainlyduring runoffplaysa dominant rolein thesurface layerdynamics. CentralWaterformation can Graduallyas the runoff decreases duringthe drier northeast theaustralwintermonths,asobviously only occur in the southern subtropics of the Indian Ocean basin due monsoonin February-April,the salinity in the Bay of Bengal of thenorthern Asianlandmass. increases and becomes more uniform, while the temperatures to theproximity

SPRINTALL ANDTOMCZAK: EVIDENCE OFBARRIER LAYER INTHETROPICS 4. SUMMARY

AND CONCLUSIONS

In this analysiswe determinethe thicknessof the barrier layer that separatesthe mixed layer from the thermoclinein the tropical regionsof the world oceans.The thermoclinedepthwas estimated asthedepthat whicha net temperature changeof 0.5øCoccursfrom the surface.This methodwas frequentlyemployedby previous studiesasbeingproxy to mixed layer depth.Here, the mixed layer depthis calculatedasthedepthwherethepotentialdensityis the sea surfacedensityplus an estimateof the densitychangerequiredto obtaina temperaturedifferencefrom the surfaceequalto 0.5øC if salinityis held constant.Henceany differencesin depthproduced by thetwo criteriamustbe causedby salinityeffects. Three equatorialregionsconsistentlydisplayeda significant positivedifferencebetweenthemixedlayerdepthandtheisothermal layer. Different forcing mechanisms were proposed for the existenceof the shallowhaloclinesin eachof the threeregions.In thewestemequatorialPacifica salinitybudgetconfmnedthatheavy localprecipitationmostlikely resultsin the formationof the layer which is isothermalbut salt stratified.This is expecteddue to the coincidence of thedistributionof thethickbarrierlayerwith a global minimumin E-P. Understanding the thermodynamics thatmaintain the barrier layer, and also its role in the E1 Nifio-Southern Oscillationinstabilitywill necessarilybe of high priority in the imminent Tropical Ocean Global Atmosphere-CoupledOcean Atmosphere Response Experiment(TOGA-COARE) in thewestern Pacificwarmpoolregion. In theGuianaBasinof thenorthequatorialAtlantic,theregionis characterized by net water loss to the atmosphere;hence precipitationcannotbe responsible for thedistributionof thebarrier layer here.It appearsthat the high salinitiesfoundat the surfacein the subtropicsare subductedfrom both hemispherestoward the equatorduringthe respectivewinterseason,at the upperendof the temperature/salinityrange of the Central Water. This creates a salinitymaximumabovetheCentralWaterin the tropics.Thismay thenbe advectedwestwardinto regionsof uniform temperaturein theequatorialcurrentsystem,forminga barrierlayerin thetropical Atlantic Ocean.

7315

REFERENCES

Bamett,B., The principaltime andspacescalesof the Pacifictradewind fields, J. Atmos. Sci., 34, 221-236, 1977. Baumgartner,A., and E. Reichel, The World Water Balance:Mean Annual Global, Continental and Maritime Precipitation, Evaporation and

Runoff, 179 pp., Elsevier,New York, 1975. Bunker,A.F., Computations of surfaceenergyflux and annualair-sea interactioncyclesof theNorth AtlanticOcean,Mon. WeatherRev.,104, 1122-1140, 1976.

Colburn,J.G., The ThermalStructureof the Indian Ocean,173 pp., Universityof Hawaii Press,Honolulu, 1975.

Defant,A., PhysicalOceanography , Vol. 1, 729 pp., Pergamon, New York, 1961.

Delcroix,T., G. Eldin, and C. Henin, Upper oceanwatermassesand transports in the westerntropicalPacific,J. Phys.Oceanogr.,17, 22482262, 1987.

Dorman,C.E., and R.H. Bourke,Precipitationover the AtlanticOcean, 30øS-70øN,Mon. WeatherRev., 109, 554-563, 1981.

Elliott, G.W., Precipitationsignatures in the seasurfacelayer conditions duringBOMEX, J. Phys.Oceanogr.,4, 498-501, 1974. Enfield, D.B., Zonal and seasonalvariations of near surfaceheat balanceof

the equatorialPacificOcean,J. Phys.Oceanogr.,16, 1038-1054, 1986.

Gill, A.E., Modelsof the equatorialcurrents,in NumericalModelsof Ocean Circulation, pp. 181-203, National Academyof Sciences, WashingtonD.C., 1975.

Godfrey,J.S., and E.J. Lindstrom,The heatbudgetof the equatorial westernPacificsurfacemixedlayer,J. Geophys.Res.,94, 8007-8017, 1989.

Goldenburg, S., andJ.J.O'Brien,Time andspacevariabilityof tropical Pacific wind stress,Mon. WeatherRev., 109, 1190-1207, 1981.

Krishna,V.V.G., Y. Sadhuram, andV. Rameshbabu, Variabilityof mixed layerdepthin thenorthernIndianOceanduring1977and 1979 summer monsoonseasons,Indian J. Mari. Sci., 17, 258-264, 1988.

Landis,R.C., Early BOMEX Resultsof seasurfacesalinityandAmazon river water,J. Phys. Oceanogr.,1,278-281, 1971. Levitus,S., Climatological Atlasof theWord Ocean,NOAAProf.Pap., 13, 173pp.,Natl. OceanicandAtmos.Admin.,Rockville,Md., 1982. Lewis, M.R., M.E. Carr, G.C. Feldman, W. Esaias, and C. McClain, Influenceof penetratingsolarradiationon the heat budgetof the equatorialPacificOcean,Nature, 347, 543-545, 1990.

Lindstrom,E., R. Lukas,R. F/ne, E. Firing,J.S. Godfrey,G. Meyers, andM. Tsuchiya,The WesternEquatorialPacificOceanCirculation Study,Nature, 330, 533-537, 1987. Lukas, R., On the role of the western Pacific air-sea interaction in the E1

Nino / SouthernOscillationphenomenon, Proceedings of the U.S. In theeasternequatorialIndian Ocean,monsoonalrelatedrainfall TOGA WesternPacificAir-SeaInteractionWorkshop, editedby R. and river runoff producesalinitystratificationin the upperlayers. Lukasand P. Webster,pp. 43-69, UCAR Tech.Rep.,US TOGA 8, The Ganges-Bramaputra-Irrawaddy river systems emptyinginto the Univ. Corp.for Atmos.Res., Boulder,Colo., 1988. Bay of Bengalduringthe southwestmonsoonseasoncontributeto Lukas,R., andE. Lindstrom, The mixedlayerof the westernequatorial PacificOcean,J. Geophys.Res.,96 (Suppl.),3343-3357,1991. thefreshwaterflux, maintainingtheshallowerisohalinelayer,while in the easternArabian Sea this task is undertakenby the Indus Luyten,J.R., J. Pedlosky,and H. Stommel,The ventilatedthermocline,J. River.

The abovefindingsstresstheimportance of includingtheeffects of salinitystratification whendetermining mixedlayerdepthof the world oceans.Previouslyit was thoughtthat the significanceof haloclinesin the surfacelayerneedonly be considered for higher latitudes.However, the resultspresentedhere indicatethat the inclusionof salinitydynamics arealsonecessary in themixedlayers of the tropicalregions.Often a depthproducedby temperature criteria alone does not truly representthe depth of convective overturnwhichphysicallydefinesthe mixed layer. The variablec•t criterionformixedlayerdepthusedhereallowsfor a comparison of mixed layer depthson a world oceanbasis, as it accountsfor the

Phys. Oceanogr.,13(2), 292-309, 1983. McPhaden, M.J., and J. Picaut, E1 Nifio-Southern Oscillation

displacements of the westernequatorialPacificwarmpool,Science, 250, 1385-1388, 1990.

Neumann, G., Seasonal salinityvariations in theupperstrataof thewestern tropicalAtlanticOcean,1. Sea surfacesalinities,Deep SeaRes.,16, 165-177, 1969.

Oberhuber, J.M., An atlasbasedon theCOADS dataset:The budgets of heat,buoyancy andturbulent kineticenergyat thesurface of theglobal ocean,Rep. 15, Max-Planck-Institutffir Meteorologie, Hamburg, 1988.

Peters,A., Peter'sAtlasof theWorld,231 pp., LongmanPress,Harlow, 1989.

Pickard,G.L., andW.J.Emery,Descriptive PhysicalOceanography, 5th ed., Pergamon,New York, 1990.

differences in surfacesalinitiesandtemperatures with theireffecton Robinson, W., and H. Stommel, The oceanic thermocline and the associated thermohalinecirculation,Tellus,11(3), 295-308, 1959. thecoefficientof expansion of seawater. In regionswheresalinity Schmitt, R.W., P.S. Bogdena,and C.E. Dorman, Evaporationminus stratifications arenot of consequence, mixed layer depthsof the precipitation and density fluxes for the north Atlantic, J. Phys. sameorder as derivedfrom a temperature-based criterionwill be Oceanogr.,19, 1208-1221, 1989. found.However,wheretheydiffer, truly representative measures Shea, D.J., Climatological Atlas: 1950-1979, National Center for of mixedlayerdepthsmaybe quantified. AtmosphericResearch,Boulder,Colo., 1986.

7316

SPRINTALL AND TOMCZAK: EVIDENCE OF BARRIER LAYER IN THE TROPICS

Sprintall, J., and M. Tomczak, Salinity considerations in the oceanic Wyrtki, K., and G. Meyers, The tradewind field over the PacificOcean, Part 1, The meanfield andthe meanannualvariation,Publ. HIG-75-1, surfacemixed layer, Ocean SciencesInstitute Rep. 36, 170 pp., Universityof Sydney, 1990. 26 pp., Univ. of Hawaii, Honolulu,1975. Taylor, R.C., An atlasof Pacificislandsrainfall,Data Rep.25, HIG-73-9, 83 pp., Hawaii Inst. of Geophys.,Honolulu,1973. J. Sprintall,NOAA/PMEL, 7600 SandPointWay, Seattle,WA 98115. Tomczak, M., and G. Krause, The Peters projection as a tool for M. Tomczak, Schoolof Earth Sciences,FlindersUniversity,GPO Box oceanographers duringWOCE, WOCENewslett.,8, 9-11, 1989. U.S. Navy Oceanographic Office, U.S. Navy Oceanographic Atlas of the 2100, Adelaide, SA 5001, Australia. North Atlantic, Washington,D.C., 1965. Weare, B.C., P.T. Strubb, and M.D. Samuel, Annual mean surface heat

fluxesin the tropicalPacificOcean,J. Phys.Oceanogr.,11,705-717, 1981.

Worthington,L.V., On the North Atlantic Circulation, 110 pp., John HopkinsUniversityPress,Baltimore,Md., 1976.

(ReceivedMay 2, 1991; revisedJanuary17, 1992; accepted January17, 1992.)