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Coupling of Extratropical Mesoscale Eddies in the Ocean to Westerly Winds in the Atmospheric Boundary Layer

WARREN B. WHITE AND JEFFREY L. ANNIS Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California

(Manuscript received 19 December 2001, in ®nal form 8 November 2002)

ABSTRACT The sea surface temperature (SST) signature in mesoscale eddies in the western boundary current extensions around the globe and in the Antarctic Circumpolar Current are found to alter the surface stress associated with background westerly winds, producing wind stress curl (WSC) residuals of eddy scale that are capable of modifying the eddy dynamics. This is revealed by examining satellite-derived mesoscale sea level height (SLH), SST, and neutrally stable zonal surface wind (ZSW) residuals together for 18 months. In the presence of background westerly winds on basin scales, warm mesoscale eddies reduce the stability of the marine atmospheric boundary layer, increasing the zonal air±sea momentum ¯ux measured by satellite scatterometry. Warm SST residuals of ϳ0.8ЊC are capable of producing westerly ZSW residuals of ϳ1.2 m sϪ1 under background westerly winds of ϳ6msϪ1. Alternatively, this means increasing the otherwise neutrally stable drag coef®cient by ϳ40%, consistent with in situ measurements. The resulting feedback from atmosphere to ocean through the resulting mesoscale WSC residuals (ϳ5.0 ϫ 10Ϫ7 NmϪ3) produces residual Ekman pumping that can be on the same order as the residual SLH tendency in the eddy ®eld. Moreover, the spatial phasing of the mesoscale WSC residuals acts, on average, to displace the mesoscale eddies equatorward with meridional coupling phase speeds of ϳ0.01 m sϪ1 while suppressing their amplitude.

1. Introduction radiometry (Hughes 1996; Hughes et al. 1998; Hill et Mesoscale eddies achieve their largest magnitude in al. 2000). the western boundary current extensions in each ocean These mesoscale eddies are associated with sea sur- basin and in the Antarctic Circumpolar Current (ACC) face temperature (SST) signatures that alter the stability in the Southern Ocean (Fig. 1). Initially, the mesoscale of the marine atmospheric boundary layer above them eddy ®eld in the midlatitude North Paci®c Ocean was and have the potential for affecting the surface wind studied extensively using in situ upper-ocean tempera- stresses associated with the background winds. Work on ture measurements collected from volunteer observing this subject began in the eastern Paci®c Ocean, when ships (White and Bernstein 1979). The mesoscale eddy Greenhut (1982) found SST discontinuities altering the ®eld in the Kuroshio±Oyashio current extension was surface ¯uxes and associated drag coef®cients accom- found to be dominated by zonal wavelengths ranging panying the background wind. From aircraft observa- from 400 to 1200 km and by periods ranging from 6 to tions taken during the Joint Air±Sea Interaction (JASIN) 18 months (Bernstein and White 1977; Talley and White experiment, Guymer et al. (1983) found mesoscale var- 1987). The source of mesoscale eddy activity in the iability in SST producing similar results. From aircraft Kuroshio±Oyashio extension was observed to be mixed measurements taken during the Frontal Air±Sea Inter- baroclinic±barotropic shear instability (Bernstein and action Experiment (FASINEX), Friehe et al. (1991) fo- White 1982; Bennett and White 1986; Tai and White cused on background winds directed across an SST 1990), with mesoscale eddy activity governed largely front, ®nding larger surface wind stresses and lesser by Rossby wave dynamics, propagating eastward or buoyant stability in the marine atmospheric boundary westward depending on the background absolute vor- layer over the warm side of the front than on the cold ticity gradient (Mizuno and White 1983; Qiu et al. side, with surface wind stress magnitudes increasing by 1991). Similar results have been obtained for the me- a factor of ϳ2. From these measurements, they com- soscale eddies in the ACC using satellite altimetry and puted 10-m drag coef®cients on both sides of the front, ®nding nearly a 100% increase when passing from the cold side to the warm side. Friehe et al. (1991) proposed Corresponding author address: Dr. Warren B. White, Scripps In- stitution of Oceanography, University of California, San Diego, La that this rather extraordinary increase in surface wind Jolla, CA 92093-0230. stress across the front could produce a feedback to the E-mail: [email protected] ocean, thereby altering the dynamics of the front.

᭧ 2003 American Meteorological Society

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FIG. 1. (a) The Northern Hemisphere geographical distribution of the rms of mesoscale SLH, SST, and ZSW residuals in extratropical mesoscale eddy ®eld, ®ltered for zonal wavelengths of 400±1200 km and for periods Ͼ1 month, observed over the 18-month record for Jul 1999±Dec 2000. (b) As in (a) but for the Southern Hemisphere. Contour levels are 0.02 m, 0.08ЊC, and 0.05 m sϪ1. Shading is for effect. Black boxes indicate regions where the mesoscale eddy ®elds are examined in detail.

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In the present study, we ®nd mesoscale eddies in the MSW, and WSC residuals, while the spatial ®lter allows western boundary current extensions and in the ACC us to isolate the dominant signal in the mesoscale eddy associated with SST residuals that alter the surface wind spectrum (e.g., Bernstein and White 1977; Hughes stresses accompanying the background westerly wind 1995). ®eld. We ®nd warm (cool) SST residuals in these me- Since we observe only two to three cycles of me- soscale eddies associated with positive (negative) neu- soscale variability at each grid point over the 18-month trally stable zonal surface wind (ZSW) residuals at 10- record, we obtain statistical con®dence in the results by m height derived from satellite scatterometry by the conducting comparisons over latitude±longitude do- SeaWinds QuikScat project (Weiss 2000). These ZSW mains (15Њ lat by 60Њ lon) that encompass 12±16 me- residuals are an artifact of the method of producing soscale eddies. We do this for four independent domains, neutrally stable winds for a marine atmospheric bound- focusing on mesoscale eddies in the Kuroshio±Oyashio ary layer that is not neutrally stable. Their presence in current extension, the Gulf Stream current extension, these satellite winds indicates that the background west- the Brazil current extension, and the ACC south of Af- erly wind imparts greater stress to the ocean over warm rica. This allows us to examine the association between eddies than over cold eddies. So, we estimate the cor- mesoscale SLH, SST, ZSW, and WSC residuals over responding change in the drag coef®cient for the back- 48±64 independent eddies, yielding equivalent numbers ground westerly winds of basin scale that are directed of effective degrees of freedom (Emery and Thomson over warm and cold mesoscale eddies. This yields re- 2001). sults consistent with those of Friehe et al.; that is, a background westerly wind of basin scale is associated 3. Geographical distribution of the rms of with a surface wind stress ®eld of mesoscale. Here, we mesoscale SLH, SST, and ZSW residuals ®nd the corresponding wind stress curl (WSC) residuals exerting a signi®cant feedback on the dynamics of these The geographical distributions of the root-mean- mesoscale eddies, producing an Ekman pumping that square (rms) of mesoscale SLH, SST, and ZSW residuals signi®cantly alters the observed sea level height (SLH) in the extratropical Northern Hemisphere (Fig. 1a) and tendency in the eddy ®eld, capable of modifying not Southern Hemisphere (Fig. 1b) display largest estimates only their propagation characteristics but their stability in the western boundary current extensions in each as well. ocean basin and along the ACC in the Southern Ocean, achieving maximum rms estimates of 0.10 m, 0.6ЊC, and 0.3 m sϪ1. Mesoscale eddy activity in SLH, SST, 2. Data and methods and ZSW residuals can also be seen to be relatively We examine SLH from TOPEX/Poseidon and ERS- intense off the east and west coasts of southern Australia 1/2 altimetry (Ducet et al. 2000; Le Traon et al. 2001), and along the South Paci®c convergence zone (SPCZ). SST from multichannel advanced very-high resolution In the ACC, the mesoscale eddy activity is most intense radiometry (Smith et al. 1996), and neutrally stable zon- south and east of Africa, and south of Tasmania and al and meridional winds at 10-m height zonal surface New Zealand, with weaker eddy activity over the rest winds from the SeaWinds QuikScat project (Weiss of the ACC. The nominal correspondence in global dis- 2000) for 18 months from July 1999 through December tributions of the rms of mesoscale SLH, SST, and ZSW 2000. The ZSW and meridional surface winds (MSW) residuals in the strong current regions (delineated by are available on a daily basis, while SLH and SST are rectangular boxes, Fig. 1), where mesoscale eddies arise available every 10 days. We compute the surface wind principally from shear instability in the mean ¯ow, in- stresses and wind stress curl estimates from daily ZSW dicates that covarying SLH and SST residuals in¯uence and MSW estimates using a bulk formula with a wind- ZSW variability in the overlying marine atmospheric dependent drag coef®cient under neutrally stable con- boundary layer. ditions, increasing with wind speed but independent of air±sea temperature differences (Large and Pond 1981). 4. Phase relationships among mesoscale SLH, SST, We average the daily ZSW and MSW estimates, and the ZSW, and WSC residuals corresponding WSC estimates, onto the same 10-day grid as SLH and SST estimates, interpolating all ®ve To demonstrate that covarying SLH and SST resid- variables onto a common 0.25Њ latitude±longitude grid. uals in the extratropical mesoscale eddy ®eld in¯uence Then we form residuals by subtracting the 10-day es- the ZSW residuals along the western boundary current timates from an annual long-term mean from January extensions and in the ACC south of Africa, we overlay to December 2000. We temporally low-pass ®lter and SST residuals onto SLH residuals, ZSW residuals onto spatially bandpass ®lter the residuals for periods Ͼ1 SST residuals, and WSC residuals onto SLH residuals month and for zonal wavelengths of 400±1200 km, re- over the four domains delineated by the rectangular box- spectively (Kaylor 1977). The temporal ®ltering oper- es in Fig. 1. These four domains are in the Kuroshio± ation suppresses atmospheric synoptic-scale variability Oyashio current extension (Fig. 2a), the Gulf Stream in all the residuals, most particularly that in the ZSW, current extension (Fig. 2b), the Brazil current extension

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FIG. 2a. The distribution of mesoscale eddies in the Kuroshio±Oyashio current extension, revealed in mesoscale SLH, SST, ZSW, and WSC residuals mapped from 30Њ to 45ЊN and 140ЊEto160ЊW for the 10-day period bracketing 14 Mar 2000. This occurs during late winter when the depth of the near-surface mixed layer extends to the top of the main pycnocline and when the background westerly winds overlie the current (see Fig. 3a). The overlay of contours of mesoscale (a) SST residuals on colors of mesoscale SLH residuals, (b) ZSW residuals on colors of mesoscale SST residuals, and (c) WSC residuals on colors of mesoscale SLH residuals. Color levels for SLH residuals are 0.04 m; color levels and contour intervals for SST residuals are 0.2ЊC; contour intervals for ZSW residuals are 0.2 m sϪ1; and contour intervals for WSC residuals are 1.5 ϫ 10Ϫ7 NmϪ3. Positive contours are solid and negative contours are dashed. Zonal- and meridional-lag cross correlation between (d) mesoscale SST and SLH residuals, (e) mesoscale SST and ZSW residuals, and (f) mesoscale SLH and WSC residuals, with contours of 0.2, computed over the entire domain. Correlations Ͼ0.32 are signi®cant at the 95% con®dence level for ϳ40 effective spatial degrees of freedom over the domain (Snedecor and Cochran 1980).

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FIG. 2b. As in Fig. 2a but for the Gulf Stream current extension, mapped from 35Њ to 50ЊN and 80Њ to 20ЊW for the 10-day period bracketing 29 Nov 2000, during the middle of winter when the westerly winds are strong near the current (see Fig. 3b).

(Fig. 2c), and the ACC south of Africa (Fig. 2d). These overlying ZSW residuals through the boundary layer regions are chosen for two reasons: ®rst, they display stability mechanism, as we shall see below. In all four robust mesoscale eddy activity and, second, they are regions, the spatial phase relationships between these subject to signi®cant background westerly wind during four variables are quanti®ed by examining zonal- and most seasons of the year (Peixoto and Oort 1992), which meridional-lag cross-correlation matrices computed is a necessary requirement for SST residuals to force over each domain. These cross correlations are com-

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FIG. 2c. As in Fig. 2a but for the Brazil current extension, mapped from 30Њ to 45ЊS and 70Њ to 10ЊW for the 10- day period bracketing 12 Jun 2000, during the middle of winter when the westerly winds are strong near the current (see Fig. 3d). puted for residuals whose magnitudes exceed half the colors of warm SST residuals [(b) in Figs. 2a±d] as standard deviation. This allows the cross-correlation con®rmed by zonal- and meridional-lag cross-correla- matrices to focus on eddies that are ``robust.'' tion matrices [(e) in Figs. 2a±d]. Contours of cyclonic In each region, contours of warm SST residuals en- WSC residuals (positive in the Northern Hemisphere circle yellow-to-red colors of high SLH residuals [(a) and negative in the Southern Hemisphere) both overlay in Figs. 2a±d] as con®rmed by zonal- and meridional- and are displaced poleward of yellow-to-red colors of lag cross-correlation matrices [(d) in Figs. 2a±d]. Con- high SLH residuals [(c) in Figs. 2a±d] as con®rmed by tours of westward ZSW residuals encircle yellow-to-red zonal- and meridional-lag cross-correlation matrices [(f)

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FIG. 2d. As in Fig. 2a but for the ACC as it traverses south of Africa from 30Њ to 45ЊS and 10Њ to 70ЊE for the 10-day period bracketing 10 Sep 2000, during late winter when the westerly winds are strong near the current (see Fig. 3c).

in Figs. 2a±d]. The mesoscale SLH, SST, ZSW, and of ϳ600 km, g is gravity (9.8 m sϪ2), and f is the WSC residuals in these four regions range over Ϯ0.2 Coriolis parameter (ϳ1.0 ϫ 10Ϫ4 sϪ1)]. m, Ϯ0.8ЊC, Ϯ1.2 m sϪ1, and Ϯ1.5 ϫ 10Ϫ7 NmϪ3, The ZSW response to covarying mesoscale SLH and associated with residual meridional geostrophic ¯ows SST residuals in these eddy ®elds [(b) in Figs. 2a±d] is ranging over Ϯ0.2 m sϪ1 [i.e., k(g/ f ) SLH, where k is fundamentally different from the deep diabatic heating the zonal wavenumber corresponding to a wavelength scenario (White and Chen 2002) observed on basin

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Unauthenticated | Downloaded 10/02/21 05:00 AM UTC MAY 2003 WHITE AND ANNIS 1103 scales (e.g., White et al. 1998; White 2000a,b, 2001) 5. Response of ZSW residuals to SST residuals that yields an MSW response to SST residuals over the over the extratropical global ocean extratropical ocean. Here, the ZSW and MSW residuals are artifacts of the scatterometer source. Yet, they are To determine where over the extratropical global indicative of the in¯uence that mesoscale SST residuals ocean the SST residuals in the mesoscale eddy ®eld exert on the stability of the planetary boundary layer in drive ZSW residuals directly overhead, we display the the atmosphere and on the resulting air±sea turbulent distribution of pattern correlation between the two var- exchange of momentum. The satellite scatterometer iables over both Northern and Southern Hemispheres winds are derived from radar backscattering cross sec- (Fig. 3). The pattern correlations are computed over a tions (Li et al. 1989), directly measuring the friction 10Њ latitude±longitude box centered at each grid point velocity, which is proportional to the square root of wind for the 10-day period of 14 March 2000 in the Northern stress. These friction velocities have been transformed Hemisphere (Figs. 3a,b) and for the 10-day period of into 10-m winds using the neutral-stability criterion 10 September 2000 in the eastern Atlantic and Indian (Weiss 2000). Moreover, since the scatterometer cross ocean sectors of the Southern Ocean and of 12 June section is measured relative to the surface current, the 2000 for the South Paci®c Ocean (Figs. 3c,d). We also ZSW is measured relative to the surface current (Liu et display corresponding distributions of pattern correla- al. 1979). Thus, the SST/ZSW phase relationship in tions between SLH and SST residuals, together with these mesoscale eddy ®elds [(b) in Figs. 2a±d] can be those for average zonal wind for each 10-day period. explained by warm (cool) SST residuals destabilizing This reveals signi®cant positive pattern correlation be- (stabilizing) the marine atmospheric boundary layer in tween SLH and SST residuals, and between SST and the presence of background westerly winds, instigating ZSW residuals, where the background westerly winds 1 more (less) stress at the air±sea interface compared to exceed ϳ4msϪ . These signi®cant correlations tran- that expected under the neutral stability criterion (Friehe scend the western boundary current extensions and the et al. 1991). ACC examined in Fig. 2, extending into the interior Already, a similar mechanism has been observed op- ocean and the eastern boundary region. Thus, SST re- erating in instability waves along the equatorial front in siduals in the mesoscale eddies drive ZSW residuals the eastern Paci®c Ocean (Xie et al. 1998; Chelton et nearly everywhere over the extratropical oceans where al. 2001). The magnitude of the corresponding increase robust westerly winds and mesoscale eddies can be in wind stress is signi®cant, with warm SST residuals found. Thus, even though the 10-day background west- of ϳ0.8ЊC producing westerly ZSW residuals of ϳ1.2 erly wind ®eld is basin scale, the mesoscale SLH/SST msϪ1 under westerly winds of ϳ6msϪ1, or alternatively residuals produces a wind stress ®eld that has a signif- increasing the otherwise neutrally stable drag coef®cient icant mesoscale component. by ϳ40%, that is, nearly doubling the drag coef®cient We also can see easterly trade winds encroaching when transiting from a cold eddy to a warm eddy, con- into the extratropics in Fig. 3. Where this occurs, sig- sistent with the direct measurements of drag coef®cient ni®cant negative pattern correlations can be seen be- across a front by Friehe et al. This latter estimate of tween SST and ZSW residuals (Fig. 3), indicating that drag coef®cient is derived by equating the bulk formula negative (positive) ZSW residuals overlie warm for stress under neutrally stable conditions, where the (cool) SST residuals for mesoscale eddies in the trade background wind is augmented by the neutrally stable wind belt. wind, to the bulk formula for stress under stable or unstable conditions, where the background wind re- 6. Magnitude of the residual WSC feedback on mains unchanged but the drag coef®cient is altered. extratropical mesoscale eddies Thus, a neutrally stable drag coef®cient of ϳ1.4 ϫ 10Ϫ3 increases (decreases) to ϳ2.0 ϫ 10Ϫ3 (ϳ1.0 ϫ 10Ϫ3) The baroclinic Rossby wave equation, which nom- when a background wind of ϳ6msϪ1 is directed over inally governs the propagation of extratropical meso- a warm (cool) mesoscale eddy that is ϳ0.8ЊC warmer scale eddies (e.g., Mizuno and White 1983), can be (cooler) than the surrounding ocean. written as

←

FIG. 3. (a) The distributions of pattern correlations over the Kuroshio±Oyashio current extension between mesoscale SLH and SST residuals, and SST and ZSW residuals, together with the average zonal wind for 10 days bracketing 14 Mar 2000. (b) As in (a) but for the Gulf Stream current extension, (c) as in (a) but for the Antarctic Circumpolar Current for 10 days bracketing 10 Sep 2000, and (d) as in (a) but for the Brazil current extension for 10 days bracketing 12 Jun 2000. Pattern correlations are computed over a 10Њ lat±lon box at each grid point. Correlations of 0.6 are signi®cant at 90% con®dence level for ϳ8 effective degrees of freedom in the box. Additional con®dence is gained by adjacent correlations of similar sign and magnitude. Contour levels for the pattern correlations are 0.2, with positive correlations shaded and negative correlations unshaded. Correlations greater than 0.6 are shaded darker. Contour levels for average zonal wind are 2 m s Ϫ1, with mean westerly wind unshaded and mean easterly wind shaded.

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x back by the residual mesoscale WSC residuals to theץ/␩Јץt ϩ K␩ЈϩCRץ/␩Јץ mesoscale eddy ®eld is to produce equatorward cou-

ϭϪ(␳Ј/␳ 00)[WSCЈ/(␳ f )], (6.1) pling phase speeds throughout. Conducting a scaling analysis by relating the residual zonal surface wind where ␩Ј represents the SLH residual; CR is the baro- stress to the residual ZSW through the bulk formula, clinic Rossby wave phase speed, which includes the and relating the residual ZSW to residual SST and then meridional gradient of the mean potential vorticity (e.g., to residual SLH through their observed statistical as- Yang 2000); ␳Ј is the density difference between the sociations, ®nds the meridional coupling phase speed to upper ocean above the main pycnocline and the deep be on the same order (i.e., 0.01 m sϪ1) as the zonal ocean below, here computed over the upper 3000 m of Rossby wave phase speed [i.e., CR is Eq. (6.1)]. More- ocean based on the Levitus climatology (Levitus et al. over, since this displacement of the WSC residuals from 1998); ␳ 0 is the mean density in the ocean; f is the the SLH residuals is generally less than 90Њ of phase Coriolis parameter, where f k residual relative vortic- [(f) in Figs. 2a±d], they can also be expected to suppress ity; and K Ϫ1 is the e-folding dissipation timescale. The the intensity of the eddies, enhancing their intrinsic dis- divergence of the residual Ekman ¯ow [i.e., (␳Ј/ sipation. ␳ 0)WSCЈ/(␳ 0 f ) in Eq. (6.1)] produces Ekman pumping, which contributes to the residual SLH tendency (i.e., t). It competes with both the Rossby wave prop- 7. Discussion and conclusionsץ/␩Јץ x) and the dissipation (i.e., K␩Ј). We examine mesoscale eddies over the extratropicalץ/␩Јץagation (i.e., CR Normally, the background westerly winds do not pro- global ocean on period scales of 6±18 months and wave- duce WSC residuals large enough to in¯uence the me- length scales 400±1200 km in satellite SLH, SST, and soscale vorticity balance in Eq. (6.1) due to the mis- ZSW residuals for 18 months from July 1999 to De- match in spatial scale between the two ®elds. The ques- cember 2000. We ®nd covarying SLH and SST residuals tion now is whether the magnitude of mesoscale WSC in these eddies associated with ZSW residuals when and residuals in Figs. 2a±d are large enough to affect the where the eddies underlie background westerly winds residual SLH tendency in any signi®cant way. of basin scale. We focus on the eddies located in the To answer this question directly, we plot the zonal Kuroshio±Oyashio current extension, the Gulf Stream pro®le of the observed residual SLH tendency along a current extension, the Brazil current extension, and the constant latitude near the core of the western boundary ACC south of Africa. Since mesoscale eddies there are current extensions and the ACC south of Africa, to- driven primarily by shear instability in these currents, gether with that expected from residual Ekman pumping the link between SST and ZSW residuals there is in- [i.e., (␳Ј/␳ 0)WSCЈ/(␳ 0 f )], both displayed in units of m terpreted to be a response of the latter to the in¯uence sϪ1 (Fig. 4). This ®nds the rms of residual Ekman pump- by the former on the stability of the marine atmospheric ing to be ϳ66%, ϳ20%, ϳ29%, and ϳ19% of that in boundary layer, as observed by Friehe et al. (1991) and the observed residual SLH tendency in the Kuroshio± others. This alters the surface wind stress associated Oyashio current extension, the Gulf Stream current ex- with background westerly winds of basin scale, giving tension, the Brazil current extension, and the ACC south it a mesoscale component. of Africa, respectively. The effect can be much larger, Since the ZSW residuals derive from scatterometer or smaller, for individual mesoscale eddies in the dif- frictional velocities, eastward (westward) ZSW resid- ferent current regimes. Note as well, this favorable com- uals arise over warm (cool) SST residuals in the pres- parison extends into the interior ocean near the eastern ence of background westerly winds, with the opposite edge of the pro®le (Fig. 4) in the Kuroshio±Oyashio occurring over background easterly trade winds. A sim- current extension, the Gulf Stream current extension, ilar result has been observed with scatterometer-derived and the Brazil current extension. winds in instability waves observed in the eastern equa- Looking back at the meridional- and zonal-lag cross- torial Paci®c Ocean (Xie et al. 1998; Chelton et al. correlation matrices of SLH and WSC residuals in Figs. 2001). In the western boundary current extensions and 2a±d, we can see mesoscale cyclonic (anticyclonic) in the ACC south of Africa, we ®nd warm SST residuals WSC residuals consistently displaced poleward (equa- of ϳ0.8ЊC producing westerly ZSW residuals of ϳ1.2 torward) of positive SLH residuals in both Northern and msϪ1 under background westerly winds of ϳ6msϪ1, Southern Hemispheres, as indicated in the accompa- or alternatively increasing the otherwise neutrally stable nying cartoon (Fig. 5). Since the mesoscale WSC re- drag coef®cient by ϳ40%. This amounts to increasing siduals can be expected to produce residual Ekman the neutrally stable drag coef®cient of ϳ1.4 ϫ 10Ϫ3 to pumping via Eq. (6.1), this north±south alignment of ϳ2.0 ϫ 10Ϫ3 over warm eddies and decreasing it to the WSC residuals with respect to the warm mesoscale ϳ1.0 ϫ 10Ϫ3 over cold eddies. Since the evolution of eddies (Fig. 5) will tend to displace the mesoscale eddies the mesoscale eddies are governed largely by baroclinic equatorward, that is, pump them up (down) on the pole- Rossby wave dynamics, stable or not, the obvious ques- ward (equatorward) side of a typical anticyclonic eddy. tion is whether these residual ZSW responses produce The same goes for the cyclonic eddies. Thus, the feed- mesoscale WSC residuals large enough to effect a sig-

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t) and Ekman pumping [(␳Ј/␳)WSC/(␳0f )] at representative latitudes fromץ/SLHץ) FIG. 4. Zonal pro®les of SLH tendency each of the four regions depicted in Figs. 2a±d, each pro®le spanning the domain. (a) Zonal section from Fig. 2a at 40.1ЊN. (b) Zonal section from Fig. 2b at 42.6ЊN. (c) Zonal section from Fig. 2c at 34.9ЊS. (d) Zonal section from Fig. 2d at 37.4ЊN. The rms of the residuals for each zonal pro®le is given in the inset. ni®cant feedback to the mesoscale eddies themselves, been observed responding to SST anomalies according as observed by Chelton et al. (2001) in tropical insta- to the deep diabatic heating scenario (e.g., White and bility waves in the eastern equatorial Paci®c Ocean. Chen 2002), wherein anomalous SST-induced convec- Coupled Rossby waves have been observed operating tion extends upward into the mid- to upper-level tro- on annual, biennial, and interannual period scales of posphere, yielding a deep troposphere response. How- basin-scale dimensions (White et al. 1998; White ever, on mesoscales, the residual SST-induced convec- 2000a,b, 2001). Therein, surface wind anomalies have tion appears con®ned to the marine atmosphere bound-

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FIG. 5. Schematic diagram of an anticyclonic (warm) mesoscale eddy in a background westerly wind ®eld, displaying the increase in scatterometer-derived (neutrally stable) ZSW over the warm SST residual in the eddy. This yields corresponding WSC residuals, cyclonic north of the eddy and anticyclonic south of the eddy, which drive residual SLH tendencies that tend to displace the eddy toward the equator. In reality, the warm eddy decreases the stability of the planetary boundary layer in the atmosphere, thereby increasing the surface frictional velocity associated with the background westerly winds. This enhanced frictional velocity measured by the satellite scatterometer yields greater neutrally stable ZSW at 10-m height. ary layer (e.g., Friehe et al. 1991), where it alters the we ®nd this mesoscale boundary layer coupling ex- ef®ciency of the air±sea momentum transfer exerted by tending into the interior ocean as well, it remains to the background wind. Thus, any feedback from atmo- determine the in¯uence that it has on the evolution of sphere to ocean, driven by the associated mesoscale mesoscale eddies over the entire globe. WSC residuals, would be expected to generate coupled Rossby wave activity among the mesoscale eddies very Acknowledgments. This research was supported by different from that observed in the basin-scale Rossby the National Aeronautics and Space Administration waves (e.g., White et al. 1998). (NASA) under Contract JPL 1205106 and by the Na- The resulting feedback from atmosphere to ocean acts tional Science Foundation (OCE-9920730). Warren through the mesoscale WSC residuals to produce an White is also supported by the Scripps Institution of Ekman pumping, which can alter the baroclinic Rossby Oceanography. Our thanks extend to Andrea Fincham wave equation that governs the eddy activity. We ®nd who developed the ®nal ®gures. the rms of residual Ekman pumping to range from 19% to 66% of that in the residual SLH tendency for me- soscale eddies in the western boundary current exten- REFERENCES sions and in the ACC south of Africa. The effect can be much larger, or smaller, for individual mesoscale ed- Bennett, A. F., and W. B. White, 1986: Eddy heat ¯ux in the sub- tropical North Paci®c. J. Phys. Oceanogr., 16, 728±740. dies in these current regimes. The feedback is greatest Bernstein, R. L., and W. B. White, 1977: Zonal variability in the in the Kuroshio±Oyashio current extension and smallest distribution of eddy energy in the mid-latitude North Paci®c in the ACC south of Africa. Moreover, we ®nd the spa- Ocean. J. Phys. Oceanogr., 7, 123±126. tial phasing of this Ekman pumping feedback acting, on ÐÐ, and ÐÐ, 1982: Meridional eddy heat ¯ux in the Kuroshio average, to displace the eddies equatorward, and to sup- extension current. J. Phys. Oceanogr., 12, 154±159. Chelton, D. B., and Coauthors, 2001: Observations of coupling be- press their amplitude. A simple scaling argument ®nds tween surface wind stress and sea surface temperature in the the meridional coupling phase speed (i.e., 0.01 m sϪ1) eastern tropical Paci®c. J. Climate, 14, 1479±1498. to be on the same order as the Rossby wave phase speed. Ducet, N., P.Y. Le Traon, and G. Reverdin, 2000: Global high resolution It remains to construct a coupled Rossby wave model mapping of ocean circulation from the combination of TOPEX/ POSEIDON and ERS-1/2. J. Geophys. Res., 105, 19 477±19 498. for this brand of mesoscale coupling, operating it under Emery, W. J., and R. E. Thomson, 2001: Data Analysis Methods in realistic conditions over an entire year to determine the Physical Oceanography. Elsevier Science, 638 pp. cumulative effect that it has on individual eddies. Since Friehe, C. A., and Coauthors, 1991: Air±sea ¯uxes and surface layer

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