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

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Coupling of Extratropical Mesoscale Eddies in the Ocean to Westerly Winds in the Atmospheric Boundary Layer MAY 2003 WHITE AND ANNIS 1095 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 ;0.88C are capable of producing westerly ZSW residuals of ;1.2 m s21 under background westerly winds of ;6ms21. Alternatively, this means increasing the otherwise neutrally stable drag coef®cient by ;40%, consistent with in situ measurements. The resulting feedback from atmosphere to ocean through the resulting mesoscale WSC residuals (;5.0 3 1027 Nm23) 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 ;0.01 m s21 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 ;2. 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. q 2003 American Meteorological Society Unauthenticated | Downloaded 10/02/21 05:00 AM UTC 1096 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33 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 .1 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, 0.088C, and 0.05 m s21. Shading is for effect. Black boxes indicate regions where the mesoscale eddy ®elds are examined in detail. Unauthenticated | Downloaded 10/02/21 05:00 AM UTC MAY 2003 WHITE AND ANNIS 1097 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 (158 lat by 608 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, 0.68C, and 0.3 m s21. 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 0.258 latitude±longitude grid.
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