air–sea interaction and the seasonal cycle of the subtropical … · anticyclones arise...

1948 VOLUME 16J O U R N A L O F C L I M A T E

q 2003 American Meteorological Society

Air–Sea Interaction and the Seasonal Cycle of the Subtropical Anticyclones*

RICHARD SEAGER

Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York

RAGU MURTUGUDDE

ESSIC, University of Maryland, College Park, Maryland

NAOMI NAIK


AMY CLEMENT

Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida

NEIL GORDON1 AND JENNIFER MILLER


(Manuscript received 8 May 2002, in final form 17 November 2002)

ABSTRACT

The causes of the seasonal cycles of the subtropical anticyclones, and the associated zonal asymmetries of seasurface temperature (SST) across the subtropical oceans, are examined. In all basins the cool waters in the east andwarm waters in the west are sustained by a mix of atmosphere and ocean processes. When the anticyclones are bestdeveloped, during local summer, subsidence and equatorward advection on the eastern flanks of the anticyclones coolSSTs, while poleward flow on the western flanks warms SSTs. During local winter the SST asymmetry across thesubtropical North Atlantic and North Pacific is maintained by warm water advection in the western boundary currentsthat offsets the large extraction of heat by advection of cold, dry air of the continents and by transient eddies. In theSouthern Hemisphere ocean processes are equally important in cooling the eastern oceans by upwelling and advectionduring local winter. Ocean dynamics are important in amplifying the SST asymmetry, as experiments with generalcirculation models show. This amplification has little impact on the seasonal cycle of the anticyclones in the NorthernHemisphere, strengthens the anticyclones in the Southern Hemisphere, and helps position the anticyclones over theeastern basins in both hemispheres. Experiments with an idealized model are used to suggest that the subtropicalanticyclones arise fundamentally as a response to monsoonal heating over land but need further amplification to bringthem up to observed strength. The amplification is provided by local air–sea interaction. The SST asymmetry, generatedthrough local air–sea interaction by the weak anticyclones forced by heating over land, stabilizes the atmosphere todeep convection in the east and destabilizes it in the west. Convection spreads from the land regions to the adjacentregions of the western subtropical oceans, and the enhanced zonal asymmetry of atmospheric heating strengthens thesubtropical anticyclones.

1. Introduction

In passing from Northern Hemisphere winter to sum-mer the low-level atmospheric circulation undergoes adramatic transition. In Fig. 1 we show seasonal maps

* Lamont-Doherty Earth Observatory Contribution Number 6414.1 Current affiliation: Scripps Institute for Oceanography, Univer-

sity of California, San Diego, La Jolla, California.

Corresponding author address: Dr. Richard Seager, Lamont-Do-herty Earth Observatory, 61 Route 9W, Palisades, NY 10964.E-mail: [email protected]

of the sea level pressure and surface winds taken fromNational Centers For Environmental Prediction–Nation-al Center for Atmospheric Research (NCEP–NCAR) re-analyses (Kalnay et al. 1996; Kistler et al. 2001). InFig. 2 we show seasonal maps of the precipitation (Xieand Arkin 1996) and NCEP–NCAR reanalysis subsi-dence at 700 mb. In the winter the Northern Hemispherelow-level circulation is dominated by lows over the mid-latitude oceans while the subtropical anticyclones areweak and zonally elongated features. By early summer,precipitation is extensive over southeastern Asia andsubtropical North America, the lows have almost dis-appeared, and the subtropical anticyclones now occupy

15 JUNE 2003 1949S E A G E R E T A L .

FIG. 1. Sea level pressure (mb), surface winds, and the departurefrom the zonal mean of sea surface temperature (8C) for (a) Dec–Feb, (b) Mar–May, (c) Jun–Aug, and (d) Sep–Nov, from NCEP–NCAR reanalysis.

almost all of the Northern Hemisphere oceans. Strongmeridional, equatorward, flow occurs on the easternflank of the anticyclones and underlies strong subsi-dence. There is more diffuse southerly flow on the west-ern flank and weak ascent. These arrangements are inaccord with Sverdrup balance by 5 fvp where y is themeridional flow, v is the vertical velocity in pressure pcoordinates and b and f have their usual meaning (Rod-

well and Hoskins 2001, hereafter RH). Northeasterlytrade wind flow on the southern flanks, and weak mid-latitude westerlies complete the circulation cells. As theseasonal transition occurs, the difference between thesea surface temperatures (SSTs) of the eastern and west-ern subtropical oceans strengthens. This is also shownin Fig. 1. In winter the eastern oceans are cooler thanthe west by only about 38C but in summer this differencegrows to as much as 68C. The cool waters in the eastare reflected in lower surface air temperature and spe-cific humidity resulting in strong moist static stability(Fig. 3) ensuring that convection will be shallow.

The seasonal cycle in the Southern Hemisphere hasinteresting similarities and differences with that in thenorth. In the Southern Hemisphere, which is mostlyocean, the subtropical surface pressure over the oceanmaximizes at the end of winter when the Hadley celland midlatitude westerlies are at their strongest. In theNorthern Hemisphere maximum sea level pressure overthe oceans occurs in summer due to the seasonal ex-change of mass between the Asian continent and theoceans caused by the massive Asian monsoon. However,as in the Northern Hemisphere, the Southern Hemi-sphere subtropical anticyclones are best developed ascirculation cells (that is the departure from zonal sym-metry is most pronounced) during the local spring andearly summer season (September–January), similar tothe Northern Hemisphere timing. At this time there isextensive precipitation over South America and Africasouth of the equator (Fig. 2). In contrast, the zonal asym-metry of SST in the Southern Hemisphere is strongestduring spring and actually weakens from spring to sum-mer. These different seasonal cycles of SST and thesubtropical anticyclones suggest that the balance of pro-cesses linking the oceans and the subtropical anticy-clones must be different in the two hemispheres.

The seasonal cycle of SST asymmetry can be influ-enced by a number of processes. When the anticyclonesare most developed, the eastern subtropical oceans ex-perience equatorward flowing winds and subsidence, thewestern oceans poleward flowing winds and ascent.There is general equatorward flow in the ocean, andstrong coastal upwelling in the east and a poleward,westward-intensified, boundary current in the west.There is also extensive boundary layer cloud cover(closely linked to the strong moist static stability shownin Fig. 3), which has a net cooling effect on climate(Hartman et al. 1992; Klein and Hartmann 1993), in theeast but not in the west. Each of the processes couldcreate the SST asymmetry but their relative contribu-tions are not known. Further, each of them will tend toincrease the asymmetry year-round; so what processweakens the asymmetry (i.e., cools the west relative tothe east) as the anticyclones weaken?

The mechanisms that underlie the development andmaintenance of the subtropical anticyclones are still indispute. First of all, the mountains of the Americas cansplit the subtropical high pressure belt into cells over


FIG. 2. Precipitation (mm day21) and subsidence (Pa s21) for (a)Dec–Feb, (b) Mar–May, (c) Jun–Aug, and (d) Sep–Nov. The precip-itation is from Xie and Arkin (1996) and the subsidence is fromNCEP–NCAR reanalysis.

the ocean as shown in RH. The orographically forcedcells are weak, and RH suggest that the Pacific anti-cyclones develop strength as a remote response to sum-mer convection over the Americas. They also show thatthe Kelvin wave forced by heating in the Asian monsoonis important in forcing the equatorial, trade wind, partof both the North Pacific and North Atlantic anticy-clones. Ting (1994) and Chen et al. (2001) have alsoargued that the northern summer anticyclones are forcedby the Asian monsoon heating, while heating in theregion of the Americas (e.g., see Fig. 2) simply breaksthe high into two cells.

However, as RH show, the summer anticyclones forcedremotely by monsoonal heating are weak compared toobservations. To bring them up to observed strength theyappeal to ‘‘diabatic amplification.’’ Forced descent createsa warm anomaly, enhanced radiative cooling and, there-fore, enhanced descent that inhibits convection and latentheat release. However, NCEP–NCAR reanalysis data showthat the atmosphere is actually slightly cooler in the regionsof descent than in the regions of ascent. The low levelsof atmospheric moisture in the descent regions should alsoweaken the radiative cooling rates. Bergman and Hendon(2000) have shown that clouds reduce radiative coolingin regions of deep convection, enhance radiative coolingin regions of shallow convection, and amplify the circu-lation forced by condensational heating—providing an al-ternative mechanism for diabatic amplification. Liu et al.(2001) emphasize the role of surface sensible heating ofthe atmosphere in summer over the Tibetan Plateau forthe enhancement of the North Pacific anticyclone. In thecurrent paper we examine whether local air–sea interac-tions and ocean dynamics influence the development andmaintenance of the subtropical anticyclones.

We will address the following questions:

• What atmospheric mechanisms influence the devel-opment of the zonal asymmetries of SST across thesubtropical oceans and their seasonal cycle? Con-tenders are horizontal advection and subsidence bythe mean flow, transient eddy fluxes, and cloud ra-diative feedbacks, but what are the relative importanceof these processes?

• How important is ocean heat transport in explainingthe zonal asymmetry of SST and its seasonal cycle?Would the seasonal cycles of the subtropical anticy-clones and their strength be different if the ocean heattransport was absent?

• To what extent can the subtropical anticyclones beunderstood as the remote response to monsoonal heat-ing over land with the SST asymmetry arising as aresponse, or does the SST asymmetry subsequentlyimpact the atmospheric circulation?

• Can air–sea interaction provide a mechanism wherebythe atmospheric circulation, initially forced remotely byconvective heating over land, creates zonal asymmetricsof SST that favor precipitation over the western sub-tropical oceans that amplifies the initial circulation?

15 JUNE 2003 1951S E A G E R E T A L .

FIG. 3. The moist static instability, defined as the moist static energyat 1000 mb minus the saturation moist static energy at 700 mb for(a) Dec–Feb, (b) Mar–May, (c) Jun–Aug, and (d) Sep–Nov. The dataare from NCEP–NCAR reanalysis. Units are 103 J kg21.

In section 2 we describe the datasets and models used.Then, in section 3, we present an observational analysisof the atmospheric processes that can create the zonalasymmetries of SST. The ocean processes that influencethe SST—advection and mixing—are derived from

wind-forced simulations with ocean GCMs. This anal-ysis will reveal the relative importance of mean hori-zontal advection of heat and moisture, mean subsidencewarming and drying, transient eddy fluxes, cloud–ra-diation interactions, and ocean heat transport in estab-lishing the asymmetries of SST.

In section 4, we examine simulations conducted withatmospheric general circulation models (GCMs) cou-pled to mixed layer oceans to argue that the anticyclonesare only modestly influenced by the ocean heat transportbut that the SST asymmetry is strongly influenced. Insection 5 we use an idealized atmosphere–ocean modelto test this hypothesis. In section 6 we propose a theoryfor the seasonal cycle of the subtropical anticyclonesand associated zonal asymmetries of SST. Conclusionsare offered in section 7.

2. Observational datasets and models

a. Atmospheric datasets

The NCEP–NCAR reanalysis data from 1949 to 2001(Kalnay et al. 1996; Kistler et al. 2001) are used toderive estimates of the mean horizontal and vertical ad-vection and the transient eddy divergence of moist staticenergy. Values of temperature, specific humidity, geo-potential, and zonal and meridional winds are used atlevels from 700 to 1000 mb. The reanalysis is also usedfor the sea level pressure and the SST. The reanalysiscloud cover is poor, when compared to ship or satelliteobservations, over the eastern subtropical oceans. Thisprimarily affects the surface solar radiation, so for thatwe turn to satellite data but use NCEP–NCAR data forthe surface longwave radiation.

b. Surface solar radiation

The surface solar radiation is taken from Earth Ra-diation Budget Experiment (ERBE) data (1985–1989)derived from satellite observations (Li and Leighton1993). Estimates of the surface solar radiation fromERBE data are very similar to those from the indepen-dent International Satellite Cloud Climatology Project(ISCCP) data (Bishop and Rossow 1991) except for asimple offset (Seager and Blumenthal 1994). Here wewill only show results using the ERBE data.

c. Ocean GCMs

To derive estimates of the changes in SST due toocean advection and mixing two wind-driven oceanGCMs are used. For the Northern Hemisphere we usethe Lamont Ocean–Atmospheric Mixed Layer Model(LOAM). The model has been described by Visbeck etal. (1998). For the Pacific the configuration is describedby Seager et al. (2001) and for the Atlantic by Seageret al. (2000). The model extends to 408S with meridionalresolution varying from 0.58 near the equator to 28 in


midlatitudes and a fixed 28 resolution in longitude. Ver-tical resolution is concentrated in the upper 300 m withfewer than ten levels below. The model includes basingeometry and bathymetry consistent with the resolution.Temperature and salinity are restored to climatology atthe southern boundary. The model includes a bulk wind-driven mixed layer model, convective adjustment andisopycnal thickness diffusion. In this model the mixedlayer can contain several model levels.

LOAM performs poorly in the southeastern SouthPacific. Instead, for both the South Pacific and the SouthAtlantic, we use the layer model of Murtugudde et al.(1996). This ocean GCM is a reduced-gravity, primitiveequation, sigma-coordinate model (Gent and Cane1989) with an embedded hybrid mixing scheme (Chenet al. 1994). The model has a mixed layer and 19 layersbelow with the highest vertical resolution immediatelybelow the mixed layer. The motionless deep layer is at68C and 35 psu for the Atlantic and at 48C and 35 psufor the Pacific. The model domain for the Atlantic covers408S–408N and 458S–658N for the Pacific. For bothbasins the meridional resolution is 0.58 but 0.338 in the108S–108N band and 0.668 in longitude.

Both ocean GCMs are coupled to the advective at-mospheric mixed layer (AML) model of Seager et al.(1995). The AML model computes the air temperatureand air humidity together with the surface fluxes oflatent and sensible heat and longwave radiation, usingthe modeled SST and the observed wind speed and di-rection. This methodology takes proper account of thethermodynamic coupling between the atmosphere andocean that underlies how the SST is determined.

The ocean GCMs are forced by observed wind stresses,wind speeds and directions. For the North Pacific theseare taken from daSilva et al. (1994), but elsewhere theyare taken from NCEP–NCAR reanalyses. The oceanGCMs also need the surface solar radiation, which is takenfrom ERBE or ISCCP, corrected to remove the mean offsetbetween them (see Seager and Blumenthal 1994).

d. Idealized atmosphere–ocean model

An idealized atmospheric GCM is used to model theatmospheric circulation variously forced by imposedheating over land and sea. The dynamical core of themodel is based on the Geophysical Fluid Dynamics Lab-oratory (GFDL) GCM. The model has rhomboidal trun-cation at zonal wavenumber 20 and 11 vertical layersand contains orography. The circulation is forced byrelaxation of the model temperature to the referencestate of Held and Suarez (1994) plus specified heating.The vertical integral of the specified heating is equal tothe latent heat released by condensation as estimatedfrom the precipitation data of Xie and Arkin (1996).Dissipation is provided by a ¹4 diffusion, which dampsthe highest resolved wavenumber on a 2-day timescale.There is also a vertical momentum diffusion and frictionin the lowest layer that damps with a 0.5-day timescale.

The specific humidity, which does not feed back ontothe specified condensational heating, is computed ac-counting for advection and using simple parameteriza-tions of evaporation and rainout.

The atmosphere model forces a 50-m-deep mixed lay-er ocean model. The zonal mean SST in the mixed layerremains specified. Zonal deviations are computed interms of the zonal deviation of the latent plus sensibleheat flux (surface radiation is assumed to be zonallysymmetric). The sensible and latent heat fluxes are com-puted by bulk formulas using the air temperature andhumidity modeled by the atmospheric GCM. Deviationsof SST from zonal symmetry in this model can onlyarise due to advection within the atmospheric circulation(mean plus transient) forced by imposed heating.

e. Climate models

Two different climate models were used. Each is anatmospheric GCM (AGCM) coupled to a mixed layerocean. One AGCM is the Goddard Institute for SpaceStudies (GISS) model (Hansen et al. 1984; Del Genioet al. 1996), a gridpoint model with 48 3 58 resolutionand nine vertical levels. The other model is the NCARCommunity Climate Model 3 (CCM3; Kiehl et al. 1998)run with triangular truncation at zonal wavenumber 42and 18 vertical levels. The GISS model is coupled to amixed layer ocean whose depth varies in a specifiedway both seasonally and spatially (Russell et al. 1985),while the CCM3 is coupled to a mixed layer of uniform50-m depth. In each climate model the mixed layer con-tains a specified ‘‘q-flux’’ constructed so that, when cou-pled, the model reproduces the observed SST with rea-sonable fidelity. The q-flux accounts for the movementof heat by the ocean as well as for errors in the model’ssurface heat fluxes. In the case of CCM3, with a fixedmixed layer depth, the q-flux also implicitly accountsfor the exchange of heat between the mixed layer andwater below but this is explicitly computed within theGISS model where the mixed layer depth varies. Incompanion experiments the q-flux was set to zero sothat the SST was determined by a local balance of sur-face fluxes and SST tendency. The runs with a zero q-flux allow us to see the impact of the ocean heat trans-port on the seasonal cycle of subtropical SSTs and thesubtropical anticyclones.

Both GCMs seriously underestimate boundary layercloud cover over the eastern oceans. Their radiative im-pact on the SSTs is therefore included with the oceanheat transport (OHT) in the specified q-flux. Setting theq-flux to zero therefore also removes the radiative im-pact of boundary layer clouds.

3. Observational analysis of subtropical air–seainteraction

a. Basic concepts and equations

The SST is determined by a balance of horizontal andvertical ocean advection, vertical ocean mixing, and the

15 JUNE 2003 1953S E A G E R E T A L .

net surface heat flux. The surface fluxes of heat, radi-ation, and moisture are balanced within the atmosphereby advection, subsidence, vertical mixing (or turbu-lence), and radiation. It is useful to combine the oceanand atmospheric boundary layers, by eliminating thesurface fluxes, so we can examine the atmospheric pro-cesses that create variations in SST.1

The SST equation can be written as

]T0 0 0 0rc H 5 OP 2 F 2 F 2 F 2 F . (1)pw solar LW SH LH]t

Here r is the density of water; cpw is its specific heat;H is the mixed layer depth, which varies in space and

time; T is the mixed layer temperature, which equalsthe SST; OP is the SST tendency due to advection andmixing (see appendix), and the F terms refer to thesurface solar ( ) and longwave ( ) radiative flux-0 0F Fsolar LW

es, and the sensible ( ) and latent ( ) heat fluxes.0 0F FSH LH

The 0 superscript indicates a surface value. The surfaceflux terms are defined as positive upward, but OP isdefined as positive if it leads to an increase of SST.

The equation for the moist static energy, h 5 cpTa 1gz 1 Lq, of the lower levels of the atmosphere, whereTa is the air temperature, z is the geopotential, q is thespecific humidity, and L is the latent heat of conden-sation, is given by

p01 u ]h y ]h ]h 1 ](u9h9) ](y9h9) cosf ](v9h9)1 1 v 1 1 1 dpE 51 2 1 2 6[ ]g a cosf ]l a ]f ]p a cosf ]l ]f ]ppt

p0 ]F ]FRAD h p0 0 0 0 pt t5 1 dp 5 F 1 F 1 F 1 F 2 F 2 F . (2)E solar LW SH LH RAD h1 2]p ]ppt

Here cp is the specific heat for air, l is longitude, f islatitude, FRAD is the radiative flux, and Fh is the turbulentflux of moist static energy; the overbars denote monthlymeans and primes denote departures from monthlymean; is the turbulent flux of moist static energy atptFh

the top of the layer where the pressure is pt and ptFRAD

is the net radiative flux at this same level. Since the topof the layer is at 700 mb, the boundary layer mixingpart of will be small and this term will instead beptFh

dominated by convective mixing between the surfacelayers and the mid and deep troposphere. The first brack-eted term on the left within the square brackets is theadvection and subsidence of moist static energy by themean flow and the second term is the transient eddyflux divergence of moist static energy.

By combining these two equations we derive an equa-tion for the SST in terms of the ocean, atmosphere, andradiative processes that alter it:

p0]T 1 u ]h y ]h ]hrc H 5 OP 2 1 1 vpw E 51 2]t g a cosf ]l a ]f ]ppt

1 ](u9h9) ](y9h9) cosf ](v9h9)p0 t1 1 1 dp 2 F 2 DN 2 F , (3)RAD T h1 2 6[ ]a cosf ]l ]f ]p

where DNT 5 2 is the radiative flux diver-p 0tF FRAD RAD

gence across the atmospheric layer and is typically pos-itive.

Of the terms on the right-hand side, the SST tendencydue to ocean processes (advection plus mixing), OP, will

1 Looking at the surface fluxes alone can be misleading. For ex-ample the cool eastern subtropical oceans are places where the surfaceheat loss from the ocean is no larger than at other longitudes at thesame latitude. It might therefore be thought that atmospheric pro-cesses are not creating the cold SSTs, but instead, as atmosphericadvection and subsidence cool the SST, and cloud cover reduces theincoming solar radiation, the latent heat loss becomes less to restorea balance with close to zero net surface heat flux (Blumenthal 1990).

be evaluated from wind-forced ocean GCMs (see ap-pendix); the advection by the mean flow and the tran-sient eddy flux divergence will be evaluated fromNCEP–NCAR reanalyses; and the net radiation at theocean surface, , will be taken from ERBE for the0FRAD

solar component and NCEP–NCAR reanalyses for thelongwave component. The SST is taken from NCEP–NCAR reanalyses. Neither the radiative cooling of thelower levels of the atmosphere, DNT, nor the convectivemixing at the top of the level, , can be directly eval-ptF h

uated from the data. Here, they are instead evaluated asa residual from Eq. (3). The residual term also includeswithin it errors in the observational data and the model


diagnosis of OP. Although it includes all the error, thespatial distribution of the residual is much as expected.It is a cooling term for the SST essentially everywhere,increasing from about 50 W m22 in the extratropics tonear 100 W m22 in the deep Tropics [consistent withthe expected values of boundary layer radiative fluxdivergence (Betts and Ridgway 1989)], and can reach200 W m22 in the intertropical convergence zone whereconvective turbulent mixing of moist static energy islarge.

b. Climatological air–sea interaction during summerand winter

Figure 4 shows the SST tendencies in December–February (DJF), in W m22, due to each atmospheric oroceanic process. Figure 5 shows the same quantities forJJA.

In the subtropical Nothern Hemisphere during winterthe SST tendency due to mean advection and subsidence(Fig. 4a) cools the SST across the subtropical oceans.This reflects the zonally symmetric wintertime Hadleycell. Outflow of cold, dry air from Asia and North Amer-ica further cools the western oceans. In contrast, duringsummer (Fig. 5a) the mean circulation warms the west-ern subtropical oceans and cools the eastern oceans. Thisreflects both the direction of horizontal flow around thesubtropical anticyclones and the localization of the de-scent in the east. In the east the vertical term is dominantand the subsidence warming part (;70 W m22) is close-ly balanced by radiative cooling (Betts and Ridgway1989). The subsidence drying part is larger (;130 Wm22) and is balanced by evaporation, which cools theSST. The transient eddy moist static energy divergence(Figs. 4b and 5b) cools the subtropical SSTs throughoutthe year but by more in winter than summer and bymore in the west than in the east.

The SST tendencies due to mean circulation and tran-sients in the Southern Hemisphere have similar rela-tionships to local season as in the Northern Hemisphere.The creation of the zonal asymmetry of SSTs by themean flow is most obvious in local summer in the SouthPacific (Fig. 4a). In the South Atlantic the mean cir-culation cools most in the central ocean. Transients coolboth subtropical oceans throughout the year and by morein winter than summer, but the east to west differencein their effect is much smaller than in the north due tothe lack of marked continentality.

In the northern subtropics ocean processes (Figs. 4cand 5c)2 warm the west during winter and cool the eastduring summer. Both features are more confined to thecoast than either the SST tendencies due to the atmo-spheric circulation or the basin-scale features of the SSTasymmetry itself. However, in the Southern Hemi-sphere, cooling by ocean processes extends considerably

2 Note that we have not simulated the Indian Ocean and its areais left blank in these figures.

farther west from the coast. The warming in the westis less strong than in the Northern Hemisphere, presum-ably due to the weaker western boundary currents.

The net solar radiation absorbed by the ocean is quitezonally uniform during Northern Hemisphere winter. Atother times, and in the Southern Hemisphere throughoutthe year, the eastern subtropical oceans get less radiationthan areas to the west due to local maxima of low-levelcloud cover (Klein and Hartmann 1993), but these fea-tures are confined near the coast.

c. Causes of the seasonal cycle of SST asymmetryacross the subtropical oceans

In Figs. 6 through 9 we plot the seasonal evolutionof the SST and the terms on the right-hand side of Eq.(3), that is, the terms determining the SST, as a functionof longitude. All quantities have been averaged over158–358N in the Northern Hemisphere and 258–58S inthe Southern Hemisphere. The southern latitude band ischosen to be closer to the equator than the northern bandto reflect the fact that the latitude of maximum SSTasymmetry is also nearer the equator. All of the termsare plotted as the departure from the zonal mean of theappropriate ocean basin. This allows us to look directlyat the zonal asymmetry. Positive values indicates awarming tendency. Figure 6 is for the North Pacific,Fig. 7 for the North Atlantic, Fig. 8 for the South Pacific,and Fig. 9 for the South Atlantic.

1) THE NORTHERN HEMISPHERE

In the North Pacific and North Atlantic the SST asym-metry (Figs. 6a and 7a) has a maximum in August anda minimum in February. The seasonal cycle of the meanadvection of moist static energy is the most obviouscause (Figs. 6b and 7b): strong cooling in the east andwarming in the west during April–July precedes themaximum of the SST asymmetry. Once the SST asym-metry has reached its full value, the generation of theasymmetry by the mean advection has weakened. Short-ly afterward, in the fall, the mean advection begins tocool the west of the Pacific Ocean, but this effect isweak in the Atlantic. The winter mean advective coolingof the west is caused by the strengthening and southwardshift of the midlatitude westerlies and the advection ofcold dry air off the continents. Transient eddies (Figs.6c and 7c) assist the winter cooling of the westernoceans relative to the east. Consequently the atmospher-ic circulation, mean plus transient, creates the funda-mentals of the seasonal cycle of the asymmetry of SST:mean advection strengthens the asymmetry from springto summer and both the mean advection and transientsweaken the asymmetry from fall through the winter. Thewintertime weakening is almost strong enough that itwould eliminate the SST asymmetry if it was unop-posed.

The net surface radiation is shown in Figs. 6d and

15 JUNE 2003 1955S E A G E R E T A L .

FIG. 4. (a) The mean advection of moist static energy, (b) transientmoist static energy flux divergence, (c) the sum of ocean advectionand mixing (equatorial region where the two ocean models meet, andwhere values are very large, is masked out), and (d) the net surfacesolar radiation for Dec–Feb. The advection and transient flux diver-gence are evaluated from NCEP–NCAR reanalysis, the ocean pro-cesses tendency from the ocean GCM, and the net surface solar ra-diation from ERBE. All terms are plotted as positive if they tend toincrease the SST and are in W m22.

7d. Cloud–radiation interactions only have a modest im-pact on the seasonal cycle of the asymmetry and areparticularly weak over the Atlantic Ocean where sum-mertime cloud cover is less than over the North Pacific(Klein and Hartmann 1993). The timing is consistentwith the change in boundary layer clouds being a pos-itive feedback: the atmospheric circulation creates coolSSTs in the east and increases the static stability (seeFig. 3), which in turn increases the boundary layer cloudcover causing further cooling of the SST. This amplifiesthe SST asymmetry but only immediately west of theAmerican and African coasts.

Cooling of the eastern oceans by advection and mix-ing in the ocean is also quite confined to the easterncoast (Figs. 6e and 7e). More important is the largewarming in the west during winter. This is primarily aresult of the ocean mixed layer deepening, as it is cooledby strong surface heat loss sustained by cold, dry ad-vection and atmospheric transient eddy fluxes, and en-training warmer waters from below. The temperaturesof the waters below are themselves sustained by hori-zontal advection in the western boundary current. Theseocean processes oppose the cooling by the mean andtransient atmospheric circulation allowing the SSTasymmetry to persist year-round. In its absence wewould expect the SST asymmetry to be much weaker.

The residual, the sum of radiative cooling of theboundary layer and the turbulent flux of moist staticenergy at its top (plus error), is shown in Figs. 6f and7f. We expect the boundary layer radiative cooling tobe larger (on the order of 20 W m22) in the west thanin the east because of the greater water vapor content(Betts and Ridgway 1989). This term contains the pos-itive water vapor greenhouse feedback on the SST andwill enhance the SST asymmetry by increasing thedownward longwave flux in the west. However, theasymmetry of the turbulent flux at the top of the layeris probably larger. As the western oceans warm in sum-mer deep convection occurs and fluxes moist static en-ergy upward. In the east, convection is suppressed bythe stable boundary layer. Hence the turbulent fluxdamps the SST asymmetry in the late summer season.3

2) THE SOUTHERN HEMISPHERE

In the Southern Hemisphere the SST asymmetry isstrongest in October–December and weakest in March(Figs. 8a and 9a). Throughout the Southern Hemispherewinter the SST asymmetry is strengthening, in contrastto the Northern Hemisphere during winter when theasymmetry weakens. This difference seems to be a result

3 The apparent maintenance of the SST asymmetry by the residualterm in the North Pacific (and to a lesser extent in the North Atlantic)during winter almost certainly indicates error. Most likely it is at-tempting to account for underestimation of the warming in the westby advection in the western boundary currents caused by the modesthorizontal resolution in the ocean GCM.


FIG. 5. As in Fig. 4 but for Jun–Jul–Aug.

of ocean processes playing a more important role in theSouthern Hemisphere than the Northern Hemisphere. Inboth the South Pacific and the South Atlantic coolingby ocean processes (Figs. 8e and 9e) in the east is stron-ger than in the north and drives the strengthening of theSST asymmetry from March to October. Vertical en-trainment of cool water is the dominant term, but thecool water is spread west by horizontal advection. Oceandynamics are more important in explaining the SSTasymmetry in the Southern Hemisphere than in theNorthern Hemisphere for two reasons. First the windsblow parallel to the shore creating an ideal situation forcoastal upwelling year-round that is not matched in theNorth Pacific south of California (Philander et al. 1996).Second, the waters below the mixed layer are muchcooler in the South Pacific and South Atlantic than inthe North Pacific or North Atlantic (not shown), which,in turn, may in part be due to raising of the thermoclineby the strong alongshore flow but is also influenced bythe proximity of the cold waters of the Southern Ocean.

In the South Pacific the mean advection of moist staticenergy (Fig. 8b) strengthens the SST asymmetry mostduring local summer, as in the Northern Hemisphere. Inthe South Atlantic (Fig. 9b) it tends to cool the centralocean relative to the areas to both the east and the westand with little variation through the year. This confirmsocean dynamics to be dominant in creating the observedSST asymmetry and its seasonal cycle in the SouthernHemisphere.

The impact of the transient eddy fluxes in both basins(Figs. 8c and 9c) is weak because the Southern Hemi-sphere storm track is far more zonally symmetric thanits northern counterpart (which splits into two distinctcenters east of the Asian and American coasts). Similarto the Northern Hemisphere subtropics, cloud–radiativefeedbacks (Figs. 8d and 9d) provide an additional cool-ing of the SST in the east after the strong SST asym-metry has already been established. As in the NorthernHemisphere, the sum of radiative cooling of the bound-ary layer and the turbulent flux of moist static energyat its top (Figs. 8f and 9f) damps the SST asymmetry.

4. Simulation of subtropical air–sea interactionwith atmospheric GCMs coupled to mixed layeroceans

The previous analyses indicated that, while advectionby the mean and transient atmospheric circulation iscrucial to the generation of the zonal asymmetries ofSST, ocean heat flux convergence is also important. Inthe Northern Hemisphere ocean heat transport maintainsthe asymmetry during winter and in the Southern Hemi-sphere cooling by ocean heat flux divergence in the eastis the dominant process creating the SST asymmetry.

Next we turn to a comparison of the subtropical an-ticyclones and asymmetry of SSTs as simulated by twopairs of atmospheric GCMs coupled to mixed layeroceans. In each pair one experiment accounts for the

15 JUNE 2003 1957S E A G E R E T A L .

FIG. 6. Time–longitude plots of the (a) SST, (b) mean moist static energy advection, (c) transient eddy flux divergence of moist staticenergy, (d) net surface radiation, (e) sum of ocean advection and mixing, and (f ) sum of boundary layer radiative cooling and turbulent fluxof moist static energy at 700 mb for the North Pacific. Data are averaged from 158–358N and are plotted as the departure from the zonalaverage across the basin. Positive values indicate a warming. Data are from NCEP–NCAR reanalyses except the surface solar radiation,which is from ISCCP and the ocean advection and mixing, which are from the ocean GCM. Units are 8C for (a) and W m22 otherwise.

ocean heat transport in the mixed layer and the otherexperiment does not. On the basis of the observationalanalysis we expect that the SST asymmetry will beweaker than observed in the runs that do not accountfor ocean heat transport. However, the observationalanalysis cannot tell us what forces the seasonal cycleof the subtropical anticyclones. Here we examine wheth-er weakening the zonal asymmetry of SSTs when theocean heat transport is removed impacts the strengthand seasonal cycle of the anticyclones.

The sea level pressure and deviation of SST from

zonal symmetry for the two GISS models, for Decem-ber–February and June–August, are shown in Fig. 10and the same results from the CCM3 model are shownin Fig. 11. The models with the q-flux produce reason-able simulations of the seasonal cycle, strength, andlocation of the subtropical anticyclones as can be seenby comparing Figs. 10a,c and Figs. 11a,c with Figs.1a,c.

Looking at the asymmetry of SST, it is clear that thebasic patterns of warmer and colder SST than the zonalmean are reproduced in both models (and in midlatitudes


FIG. 7. As in Fig. 6 but for the North Atlantic (158–358N).

as well as in the Tropics) but the asymmetry across thesubtropical oceans is much weaker when the ocean heattransport is removed. When the ocean heat transport isremoved the maximum SST asymmetry in the NorthernHemisphere still occurs in local summer but now it alsooccurs in local summer in the Southern Hemisphere.This supports the suggestion that ocean dynamics arethe reason why the observed SST asymmetry in theSouthern Hemisphere maximizes during local spring.During Northern Hemisphere summer the area of theIndo-Pacific warm pool remains the warmest at its lat-itude, even when ocean heat transport is removed.

The Northern Hemisphere subtropical anticyclones

are not appreciably weaker when the ocean heat trans-port is removed. In contrast, the Southern Hemisphereanticyclones weaken when the ocean heat transport isnot accounted for. In both hemispheres the anticyclonesare best defined in local summer whether or not theocean heat transport is accounted for. Also, in bothhemispheres during local summer, high pressure extendsunrealistically far eastward in the western basins whenthe ocean heat transport is removed. The ocean heattransport, by causing warming in the west, thereforehelps locate the summer anticyclones over the easternparts of the oceans.

The GCMs have much less low-level cloud cover over

15 JUNE 2003 1959S E A G E R E T A L .

FIG. 8. As in Fig. 6 but for the South Pacific (58–258S).

the eastern subtropical oceans than is observed and con-sequently underrepresent their cooling effect. This mod-el deficiency helps explain why the SSTs are too warmin the far eastern subtropical basins, but it is also clearthat the clouds have little direct or indirect impact onthe atmospheric circulation within these models.

The GCM results support the idea that ocean dynamicsare required to achieve the strength of the observed sub-tropical SST asymmetry. Yet they also indicate that inthe Northern Hemisphere the full asymmetry of the SSTis not required for the subtropical anticyclones to achievefull strength while in the Southern Hemisphere it is. Sincethe precipitation over the tropical and subtropical oceans

is closely related to SST, there are two possible expla-nations for these results. First, in the Northern Hemi-sphere, the subtropical anticyclones are forced largely byorography and heating over land and these forcings arehardly altered when ocean heat transport is removed.Alternatively the subtropical anticyclones are forced byheating over both ocean and land, but the weak SSTasymmetry and warm Indian and west Pacific Oceans,still present when ocean heat transport is removed, isenough to place the ocean precipitation in the correctlocation. In the Southern Hemisphere ocean heat trans-port is required to get both the SST asymmetry and thefull strength of the subtropical anticyclones.


FIG. 9. As in Fig. 6 but for the South Atlantic (58–258S).

5. An idealized atmosphere–ocean model ofsubtropical air–sea interaction with imposedheating

The experiments with the atmospheric GCMs coupledto the ocean mixed layer have made it clear that thesubtropical anticyclones retain much of their characterwhen ocean heat transport is removed. However localair–sea interactions may still influence the pattern ofatmospheric heating that provides the forcing for theanticyclones. In this section we will address the relativeimportance of the heating over the land and the sea. Asin RH we assume that the monsoon is ‘‘triggered byland–sea sensible heating contrasts and is taken as giv-

en.’’ Here we use an idealized model to examine thecirculation and associated zonal asymmetry of SSTs,forced first by monsoon heating over land alone andthen by heating over land and ocean. Then, by removingthe ocean precipitation in the off-equatorial monsoonregions, we will demonstrate how the atmospheric cir-culation forced by heating over land creates asymme-tries of SST that favor convection over the westernoceans, which reinforces the original circulation.

The simple model of air–sea interaction contains anocean mixed layer in which the SST is determined bysurface fluxes of sensible heat and moisture alone, andan atmosphere model forced by imposed heating.

15 JUNE 2003 1961S E A G E R E T A L .

FIG. 10. The deviation of SST (8C) from zonal symmetry and thesea level pressure (mb) from the experiment with the GISS atmosphericGCM coupled to a mixed layer ocean: (a) DJF with q-flux, (b) DJFwithout q-flux, (c) JJA with q-flux, and (d) JJA without q-flux.

a. Atmosphere model

As described in section 2 we use the dynamical coreof the GFDL atmospheric GCM with an R20 truncationand 11 vertical layers. The model includes orography.The temperature equation of the model is given by

]T ]T RT1 u · =T 1 v 21 2]t ]p c pp

[T 2 T (y, p) 1 T*]R s5 Q 2 1 D , (4)Tt

where Q is the imposed heating due to condensation,TR(y, p), is a relaxation temperature that depends onlyon latitude and pressure derived using the observed zon-al mean surface air temperature (from NCEP–NCARreanalysis) and the vertical profile from. Held and Sua-rez (1994), is the modeled deviation of the SST fromT*szonal mean, t is a timescale that varies between 4 daysnear the surface and 60 days at height, and DT is ahorizontal and vertical diffusion. Hence the temperatureis forced by the imposed heating and is relaxed to zonalmean temperature adjusted by the modeled deviation ofthe surface temperature from the zonal mean. Here

is zero over land. In practice this feedback is veryT*sweak and the atmospheric circulation is essentially de-termined by the imposed heating. The vertical integralof the specified heating is equal to the latent heat re-leased by condensation as estimated from the precipi-tation data of Xie and Arkin (1996). The heating isspecified to peak in the midtroposphere near the equatorbut at gradually lower levels with increasing distancefrom the equator. The imposed heating and TR progressthrough a seasonal cycle by linearly interpolating be-tween monthly mean values.

The specific humidity, q, is computed from

]q ]q M(q 2 q )max1 u · =q 1 v 5 E 2 1 D . (5)q]t ]p t p

Here E 5 cEU(qs 2 q)/h, where cE is an exchangecoefficient taken to be 0.0015, is related to the surfaceevaporation and relaxes the specific humidity of the low-est model level (with depth h) to the surface saturationhumidity, qs, on a half-day timescale; E is zero abovethe lowest model level; Dq is a very weak diffusion;qmax is a maximum specific humidity above which mois-ture is removed on a quarter-day timescale; and qmax istaken to be 0.85 of its saturation value. To accomplishthis,

q 2 q , q 2 q $ 0max maxM(q 2 q ) 5 (6)max 50, q 2 q , 0.max

The specific humidity is therefore primarily influencedby horizontal advection and subsidence and impacts theevaporation and the modeled SST. The modeled specifichumidity is quite realistic, being close to 80% of sat-uration in the lowest level, moist in regions of ascent,

and very dry in free-troposphere regions of descent. Itis important to remember that the condensational heat-ing in the model remains specified and is not coupledto the modeled humidity distribution since our purposeis simply to examine how imposed heating can influence


FIG. 11. As in Fig. 10 but from the CCM3 atmospheric GCMcoupled to a mixed layer ocean.

the temperature and humidity distribution over the oceanand, therefore, the SST.

b. Ocean mixed layer model

The seasonally varying zonal mean SST, ^Ts&, isspecified from observations (NCEP–NCAR reanaly-

sis). The SST departure from zonal mean, , is de-T*stermined from

]T* r cs a E5 2 [U(c DT 1 LDq)p]t r c Hw p

2 ^U(c DT 1 LDq)&], (7)p

where U is the wind speed and H is the model oceanmixed layer depth (50 m); DT and Dq are the air–seatemperature and humidity differences given by

DT 5 T* 1 ^T & 2 T, (8)s s

Dq 5 q* 1 ^q & 2 q, (9)s s

where and ^qs& are the zonally asymmetric and zon-q*sally symmetric surface saturation humidity and T andq are the lowest-level air temperature and specific hu-midity from the atmosphere model. Consequently, T*sis determined by the departure from the zonal mean ofthe surface latent plus sensible heat flux. We ignore theimpact on the SST asymmetry of zonal asymmetries inthe net surface radiation. As shown in section 3c, theseare of lesser importance relative to the three-dimen-sional advection of moist static energy, but their neglectwill cause an underestimation of the modelled SSTasymmetry. Asymmetries of SST are driven by atmo-spheric motions that alter the air temperature and airhumidity, modifying the surface fluxes and altering theSST until the departure from zonal mean of the latentplus sensible surface flux returns to zero, satisfying Eq.(7).

c. Model results

The model is integrated for 10 yr, by which time thegross features are well established and steady. In Fig.12 we show the sea level pressure and SST asymmetryduring DJF forced by imposed heating over all land only(Fig. 12a, contour interval of 2 mb), by imposed heatingover all land plus the equatorial oceans between 158Sand 158N (Fig. 12b, contour interval of 2 mb); and byimposed heating over all land plus all ocean (Fig. 12c,contour interval of 5 mb). The June–August simulationsare shown in Fig. 13. In the case with forcing over landonly, during winter (Fig. 12a) the flow is quite zonaland in summer (Fig. 13a) the subtropical anticyclonesare clearly present but weak (note the small contourinterval). The asymmetries of the summer flow areenough to create cold water within the eastern basinsand warmer water within the western basins. The op-posite sign asymmetry in the mid- and high-latitudeNorth Atlantic is not recreated because of the lack of adeveloped Icelandic low. When the essentially zonallysymmetric (ITCZ) forcing in the equatorial region isadded (Figs. 12b and 13b) the equatorial trough deepensbut the flow becomes more zonal and the NorthernHemisphere subtropical anticyclones and SST asym-metry weaken.

15 JUNE 2003 1963S E A G E R E T A L .

The simulation of the sea level pressure and the SSTasymmetry is quite realistic when all the forcing overboth land and sea is included (Figs. 12c and 13c). Inthis case the North Pacific summer subtropical anticy-clones achieve full strength but the Atlantic counterpartis displaced. The quadropole pattern of the SST asym-metry, with opposite sign east–west asymmetric at sub-tropical and mid to high latitudes, is well recreated.4 Bycomparing to the case that includes forcing over theocean in only the equatorial region (Figs. 11b and 12b),it is clear that the extensive condensational heating thatoccurs over the western subtropical oceans greatlystrengthens the subtropical anticyclones.

In the Southern Hemisphere neither the identity ofthe anticyclones nor the SST asymmetry are well re-produced. The ocean GCM demonstrated the importanceof ocean heat transport, so the poor simulation of theSST asymmetry is expected. However the simulation ofthe anticyclones is worse than in the atmospheric GCMsthat also ignored the ocean heat transport, indicatingthat the pattern of diabatic heating in the idealized mod-el, imposed plus computed, is likely wrong. It is note-worthy that RH were also unable to model the southernanticylones unless they forced their model with the dia-batic cooling (deduced from reanalyses) within the col-umn immediately above.

6. A theory for the seasonal cycle of thesubtropical anticyclones and associated zonalasymmetries of SST

Based on the analysis of observations and modelingso far reported, the following schematic can be pre-sented for the seasonal cycle in the subtropics. Duringwinter the subtropical latitudes in each hemisphere areoccupied by a zonal band of high pressure underlyingthe subsiding branch of the Hadley cell. During northernwinter the subtropical ocean is cooled in the west byoutflow of cold, dry air from the Asian and Americancontinents. This cooling is opposed by poleward warmwater advection within the western boundary currentsensuring maintenance of the SST asymmetry throughoutthe winter. As the transition to early summer occurs,convection over subtropical land areas begins andbreaks the zonal symmetry of the atmospheric circu-lation. Vorticity balance requires poleward low-levelflow into the regions of deep convection, while the partof the heating that is symmetric about the equator con-tinues to force equatorial low pressure to the east via aKelvin wave. Over the eastern oceans, where there isno deep convection, vorticity balance requires that ra-diatively driven descent be balanced by equatorwardlow-level flow. In combination with weak midlatitude

4 The mid- to high-latitude asymmetry arises as the winter Aleutianand Icelandic lows achieve full development in response to orographicforcing and diabatic heating over the midlatitude oceans [see Seageret al. (2002)].

westerlies this completes the formation of weak closedsubtropical anticyclones. Air–sea interaction then comesinto play to strengthen the anticyclones.

Descent in the east causes subsidence drying andcools the SST while, in the west, poleward flow warmsby horizontal advection. As a result, as the transition tosummer occurs, the SST warms more in the west thanin the east. The atmosphere becomes very stable to deepconvection in the east and unstable in the west, allowingmonsoonal convection over land to spread to adjacentocean regions. This is assisted by warming of the west-ern oceans by ocean heat transport. The extensive con-vection over the western subtropical oceans, and overthe Indian Ocean, greatly enhances the forcing of thesubtropical anticyclones and positions them over theeastern basins. The SST asymmetry is then strengthenedby the enhanced atmospheric circulation, and is furtherenhanced by increased low cloud cover within the stableboundary layer in the east and by coastal upwelling.The vertical convective flux of moist static energydamps the SST asymmetry. Ultimately the subtropicalanticyclones and the SST asymmetry weaken as summerends and the precipitation, first over the subtropical landareas and later over the subtropcal oceans, weakens andfinally shifts to the other hemisphere.

Ocean dynamical cooling of the eastern oceans is veryimportant in the Southern Hemisphere but is auxillaryin the north. During Southern Hemisphere winter, whenthe subtropical anticyclones are weak, strong south-easterly flow occurs along the coasts of South Americanand southern Africa as part of the winter Hadley cell,causing strong upwelling and westward advection in theocean and cooling. Ocean heat transport is important inexplaining the Southern Hemisphere SST asymmetryand causes a noticeable strengthening of the southernsubtropical anticyclones.

7. Conclusions

We have attempted to explain the seasonal cycle ofthe atmospheric circulation and the surface temperatureof the ocean in the subtropics and assess the degree towhich they arise as a coupled phenomena. The conclu-sion are as follows:

• In agreement with Rodwell and Hoskins (2001) thesubtropical anticyclones owe their existence to mon-soonal heating during summer over land. However theanticyclones thus forced are weak.

• Enhancement of the subtropical anticyclones requiresair–sea interaction. Once the subtropical anticycloneshave been forced by heating over land, subsidence ofdry air over the eastern subtropical oceans lowers theSST while poleward lower tropospheric advectionover the western subtropical oceans raises the SST.The atmosphere is stabilized to deep convection in theeast but becomes unstable in the west. It is suggestedthat this allows the spread of deep convection from


FIG. 12. (a) The sea level pressure (mb) and SST asymmetry (8C)in the idealized model forced by observed condensational heatingover (a) land (contour interval of 2 mb), (b) land plus the equatorialocean (contour interval of 2 mb), and (c) all land and all ocean(contour interval of 5 mb) for Dec–Feb.

FIG. 13. As in Fig. 12 except for Jun–Aug.

land areas to nearby areas of the subtropical oceans,amplifying the forcing of the atmosphere circulationand bringing the subtropical anticyclones up to fullstrength. Further experiments are needed to prove thatthe distribution of atmospheric convection is influ-enced in this way. If it is, then this is a positive feed-back.

• The positive feedback is held in check by the con-vective cooling of the SST. More fundamentally theseasonal cycle prevents further growth as winter ap-proaches and the western oceans are cooled by outflowof cold, dry air from the continents, and both landand ocean convection retreat equatorward.

• In the Northern Hemisphere the SST asymmetry isfundamentally forced by the atmospheric circulationbut its full strength requires poleward advection ofwarm water in the western boundary currents. How-ever the atmospheric circulation alone is enough to

create sufficient SST asymmetry that the deep con-vection over the ocean is placed in the west.

• Cloud–radiation interactions further cool the easternsubtropical oceans after other processes have alreadymade them cool, but have little subsequent impact onthe atmospheric circulation.

• In the Southern Hemisphere ocean upwelling and ad-vection drives the seasonal cycle of the SST asym-metry and causes a strengthening of the southern an-ticyclones while not greatly influencing their seasonalcycle. However, in the idealized model presented here,and in Rodwell and Hoskins (2001), these were notthe most difficult of the anticyclones to model, indi-cating that we too have not fully explained their origin.

These conclusions build on the earlier work of Rod-well and Hoskins (2001), Chen et al. (2001), and Ting(1994) by addressing the role of air–sea interaction inmaintaining the summer stationary waves. We haveshown how the atmospheric circulation forced by mon-soonal convection over land creates zonal asymmetriesof SST that favor convection over the western ocean

15 JUNE 2003 1965S E A G E R E T A L .

basins, providing a diabatic forcing for the atmospherethat enhances the subtropical anticyclones. The ocean,through a combination of mixed layer processes and heattransport, plays a crucial role in the seasonal cycle ofthe subtropical anticyclones and in positioning them inthe eastern basins. In the Southern Hemisphere in par-ticular, ocean heat transport is critical in generating theobserved strength of the SST asymmetry and causes anoticeable strengthening of the atmospheric circulation.

Acknowledgments. This work was supported byNOAA Grants UCSIO-CU-02165401-SCF andNA16GP1574 and NSF Grant ATM-9986072. Wewould like to thank Mark Cane for suggesting, in 1986,that the subtropical anticyclones would be a suitablesubject for study and for several subsequent discussions.

APPENDIX

Ocean Mixed Layer Heat Budget

The temperature of the mixed layer of the ocean mod-el is determined by horizontal and vertical advectionwithin the mixed layer and across the mixed layer base,by the net surface heat flux less the deep penetration ofsolar radiation below the mixed layer, and by verticalmixing. The temperature equation is given by

]T ]T ]T ]T ]Q ](w9T9)rc 1 u 1 y 1 w 5 2 rc .p p1 2]t ]x ]y ]z ]z ]z

(A1)

Here most terms have their usual meaning ]Q/]z is thevertical convergence of the net radiative and turbulentsurface fluxes and equals the net surface heat flux (de-fined positive downward) minus solar radiation trans-mitted out of the bottom of the mixed layer, and] /]z is the turbulent mixing.w9T9

Integration from the bottom of the mixed layer z 52H to the surface we get

]T ]T ]Trc H 5 2rc H u 1 yp p 1 2[]t ]x ]y

]H ]H2 (T 2 T ) u 1 yH 1 2]]x ]y

]H2 rc (T 2 T ) 1 w 1 (w9T9)p H H H1 2[ ]]t

pnet1 Q 2 Q , (A2)sfc H

where TH is the temperature immediately below themixed layer, wH is the vertical velocity at the base ofthe mixed layer, is the net surface heat flux, andnetQsfc

is the transmitted solar radiation. The first bracketedpQH

term is the SST tendency due to horizontal processesand the second bracketed term is the tendency due tovertical processes. The sum of these we call the SST

tendency due to ocean processes. In the Murtugudde etal. (1996) model used in the Southern Hemisphere thevertical integral is just over the bulk mixed layer, whilein the Lamont-Doherty ocean GCM we explicitly in-tegrate the advective terms over the levels within themixed layer and these terms in Eq. (A2) should bethought of in that way.

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