the zonal asymmetry of the southern hemisphere … journal of climate volume 17 q 2004 american...

4882 VOLUME 17J O U R N A L O F C L I M A T E

q 2004 American Meteorological Society

The Zonal Asymmetry of the Southern Hemisphere Winter Storm Track

MASARU INATSU

Center for Climate System Research, University of Tokyo, Tokyo, Japan

BRIAN J. HOSKINS

Department of Meteorology, University of Reading, Reading, United Kingdom

(Manuscript received 24 October 2003, in final form 25 May 2004)

ABSTRACT

Atmospheric general circulation model experiments have been performed to investigate how the significantzonal asymmetry in the Southern Hemisphere (SH) winter storm track is forced by sea surface temperature(SST) and orography. An experiment with zonally symmetric tropical SSTs expands the SH upper-troposphericstorm track poleward and eastward and destroys its spiral structure. Diagnosis suggests that these aspects of theobserved storm track result from Rossby wave propagation from a wave source in the Indian Ocean regionassociated with the monsoon there. The lower-tropospheric storm track is not sensitive to this forcing. However,an experiment with zonally symmetric midlatitude SSTs exhibits a marked reduction in the magnitude of themaximum intensity of the lower-tropospheric storm track associated with reduced SST gradients in the westernIndian Ocean. Experiments without the elevation of the South African Plateau or the Andes show reductions inthe intensity of the major storm track downstream of them due to reduced cyclogenesis associated with thetopography. These results suggest that the zonal asymmetry of the SH winter storm track is mainly establishedby stationary waves excited by zonal asymmetry in tropical SST in the upper troposphere and by local SSTgradients in the lower troposphere, and that it is modified through cyclogenesis associated with the topographyof South Africa and South America.

1. Introduction

The wintertime storm tracks, the regions where syn-optic-scale eddy kinetic energy (EKE) and polewardtemperature flux are maxima, have significant zonalasymmetries in both hemispheres. There are many dis-cussions of the Northern Hemisphere (NH) winter stormtracks, centered in the oceanic basins (e.g., Blackmonet al. 1977; Chang et al. 2002). This zonal asymmetryin the NH storm tracks can be explained by a varietyof zonally asymmetric terrestrial conditions such astropical and midlatitude sea surface temperature (SST)distributions, the Himalayas, and the Rockies. In spiteof more zonally distributed SSTs and less topographyin the Southern Hemisphere (SH; Fig. 1), the winterstorm track there still exhibits marked zonal asymmetry.Figure 2a shows the 300-hPa EKE for synoptic, 2–8-day1

1 The synoptic-scale filtered data are obtained as (t) 5 5F Sk525

a | k | F(t 1 kday), where (a0, a1, . . . , a5) 5 (0.7, 20.25, 20.15,20.042, 0.041, 0.057).

Corresponding author address: Dr. Masaru Inatsu, Center for Cli-mate System Research, University of Tokyo, 4-6-1, Komaba, Meguro,Tokyo 1538904, Japan.E-mail: [email protected]

periods derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis(ERA-15) June–July–August (JJA) daily data for 1979–93. The zonal-mean synoptic-scale EKE has a maximumnear 458S. In two dimensions, the largest 300-hPa syn-optic-scale EKE region (hereafter the major storm track)stretches from the central Atlantic to the eastern IndianOcean in the latitudinal band 408–508S. The smallestEKE region in these latitudes extends from southeast ofAustralia to the Straits of Magellan (508S, 708W), witha minimum near New Zealand (NZ; 408S, 1708E). Tren-berth (1991) previously described such an overall pic-ture based upon ECMWF data.

To clarify the structure, cross marks have been addedin Fig. 2a to show the local maxima of synoptic-scaleEKE in each longitude. In Australian longitudes, themajor storm track seems to split into two minor stormtracks, one extending to the northeast of Australia (258S,1508E) and the other toward the Ross Sea (758S,1708W). Chang (1999) describes this feature in termsof the waveguides of baroclinic eddies detected by theapplication of the one-point lag correlation (Lim andWallace 1991). He finds that a well-defined waveguidesplits into two in the eastern Indian Ocean with a pri-mary branch joining up with the subtropical jet and asecondary one spiraling poleward. Farther to the east,

15 DECEMBER 2004 4883I N A T S U A N D H O S K I N S

FIG. 1. JJA climatological SSTs (contour interval is 28C) and theirzonal deviations (shading as grayscale at the bottom) shown in theoceanic regions and surface height above sea level with dashed andsolid lines for 500 and 1000 m, respectively, and black shading for.2000 m.

the subtropical branch continues along the equatorialedge of the small EKE region and eventually connectswith the major storm track after the abrupt break in theregion of the Andes (408S, 708W). Taking an overview,the shape of the SH storm track is that of a spiral thatstarts north of NZ, runs across the South Pacific, breaksover Chile, restarts over the Patagonian Plateau, peaksin the Indian Ocean, decays along the edge of Antarc-tica, and ends at the Ross Sea. Applying the state-of-the-art tracking technique, B. J. Hoskins and K. Hodges(2004, unpublished manuscript, hereafter HH04) findsuch a spiral in the density of the tracks of lower-tro-pospheric cyclonic systems.

The theoretical understanding of the zonal asymme-tries in the SH winter storm track is far from complete.The large-scale terrestrial forcing that could be respon-sible can be classified into tropical SSTs, Antarctica,and a variety of midlatitude forcings. Tropical SST isused to characterize the warm pool from the IndianOcean to the western Pacific and colder water in theeastern Pacific (Fig. 1). It is known that zonal variationsin tropical SST are effective through their influence onconvection in forming extratropical stationary eddies inboth hemispheres (e.g., Horel and Wallace 1981; Karoly1989). Recently Sinclair et al. (1997) and Solman andMenendez (2002) have suggested that during the ElNino events baroclinic eddies are reduced in the south-western Pacific and preferentially occur in the subpolarwaveguide. Tropical SST asymmetries may therefore be

an important agent for the SH storm track. ConcerningAntarctica, it appears possible that the zonal wavenum-ber-1 forcing associated with its off-polar location couldbe important. However, Quintanar and Mechoso (1995)stated that its effect on the SH atmosphere is confinednear the South Pole and is negligible away from Ant-arctica.

The forcing in midlatitudes contains a number of dif-ferent aspects; in particular, midlatitude SST, the SouthAfrican Plateau, and the Andes. First, midlatitude SSTforcing would be important if the meridional SST gra-dient directly controls the baroclinicity above it (Hos-kins and Valdes 1990). In SH winter, midlatitude SSTshave a clear zonal wavenumber-1 structure with the larg-est meridional gradient to the south of Madagascar(408S, 458E; Fig. 1). A close relationship between thesharp SST front and the SH storm track is suggested byNakamura and Shimpo (2004), who show that this re-lationship holds on both the seasonal and interannualtime scales. Inatsu et al. (2002, 2003) also showed thisrelationship in an aquaplanet experiment with zonalwavenumber-1 SST gradient forcing centered at thestorm track axis. Second, effects of the Andes havefrequently been investigated. Cyclogenesis (cyclolysis)is prominent down- (up-) stream of the Andes (Sinclair1995; Gan and Rao 1994; HH04). Another aspect thatmay be relevant to cyclogenesis is the moisture transportby northerly winds in Brazil caused by the blockingeffect of the Andes (James and Anderson 1984). Finally,an effect of the South African Plateau is hinted at bythe results of HH04, whose analysis of the genesis ofcyclone systems shows cyclogenesis on the west coastof South Africa possibly feeding the development ofbaroclinic eddies in the Indian Ocean.

The extent to which the various tropical, midlatitude,and polar forcings create zonal asymmetry in the SHwinter storm track is the subject investigated here. Fol-lowing the above discussion, we investigate four as-pects: tropical SSTs, midlatitude SSTs, the South Af-rican Plateau, and the Andes. Antarctica is not consid-ered separately. In order to measure the effect of onesurface condition, atmospheric general circulation mod-el (AGCM) experiments are performed without each ofthe conditions in turn and comparison is made with thecontrol. In this paper, we concentrate on austral winter,as the SH summer storm track has less zonal asymmetry.

Section 2 gives the experimental setup in this paper.In section 3, the upper-tropospheric EKE storm track inthe experiments is discussed. The mechanism by whichtropical SSTs are able to influence the storm track isinvestigated in section 4. Section 5 contains results forthe lower-tropospheric storm track. Finally a summaryand some discussion are presented in section 6.

2. Experiments

The AGCM used is the Hadley Centre AtmosphericModel, version 3 (HadAM3), with a 2.58 latitude by


FIG. 2. (a) The 300-hPa synoptic time-scale EKE (contour interval is 8 m2 s22) and itsdeviation from the zonal mean (shading as grayscale at bottom left) for JJA based uponERA-15 data. Crosses denote maximum EKE points in each longitude. (b) The zonallyasymmetric component of the 300-hPa geopotential height field in JJA based upon ERA-15 data (contour interval is 20 m with negative contours dashed). (c), (d) As in (a), (b),but for the CTR run, and (e), (f ) as in (a), (b), but for the ZTS run. In (f ), the shadingshows differences from the CTR run using the grayscale given at the bottom right.


TABLE 1. Experiment design.

Run Removed aspect

CTRZTSZMSNMTNSANAD

NoneZonal asymmetry in tropical SSTZonal asymmetry in midlatitude SSTAll SH mountains except AntarcticaMountains in South AfricaMountains in South America

FIG. 3. JJA stationary-eddy 300-hPa geopotential height for the (a)ZMS and (b) NMT runs. The contour interval is 20 m with negativecontours dashed. The grayscale shading indicates the differences fromthe CTR run.

3.758 longitude grid and 19 levels in the vertical. (SeePope et al. 2000 for more details.) In all of the exper-iments, the model was integrated for 9 model yr afterthe spinup (with the seasonal solar cycle included), andthen a daily mean dataset was archived.

The runs performed were the control (CTR) with cli-matological SSTs and full mountains (Fig. 1), and fiveexperiments named ZTS, ZMS, NMT, NSA, and NAD,to examine, respectively, the effect of removing tropicalSST asymmetries, midlatitude SST asymmetries, SH to-pography except Antarctica, the South African Plateau,and the Andes (Table 1). The effect of each surfacecondition could be assessed by comparison of the rel-evant experiment without this aspect with the CTR run.In the ZTS run, the SSTs within 208S–208N are replacedby the zonal average of their climatology, beyond 358they are climatological, and in the latitudinal range 208–358 there is a smooth transition. In contrast, the ZMSrun has zonally symmetric SST south of 358S and cli-matological SSTs north of 208S, with a smooth transitionbetween these latitudes. The NMT run has no mountains(zero topographic height) between 608 and 158S, witha smooth change in surface height from 158S to theequator. In this run, the SSTs are climatological, and theroughness is specified as zero where mountains havebeen removed. The NSA and NAD runs are the sameas the NMT run, except that only the mountains in theSouth African and South American continents, respec-tively, are removed.

3. Upper-tropospheric storm-track results

Figures 2c–f, 3, and 4 show the simulated synoptic-scale EKE and stationary eddies at 300 hPa in SH winter.The CTR run (Fig. 2c) has an EKE that is generallyabout 10% weaker but captures the large-scale and mostof the detailed features of the observed upper-tropo-spheric storm track (Fig. 2a), including the spiral struc-ture from Australia to the Ross Sea (cross marks in Figs.2a,c), and the largest EKE from the central Atlantic tothe western Indian Ocean within the band 408–508S. Thestationary eddies in the CTR run (Fig. 2d) are also quitesimilar to those observed (Fig. 2b). South of 408S, thelargest amplitude stationary cyclone is centered near508S, 608E and extends from the Atlantic to the IndianOceans. Two stationary anticyclones are centered south-west of NZ (508S, 1608E) and over the BellingshausenSea (608S, 1008W). The anticyclone near NZ is sand-

wiched between two stationary cyclones in the sub-tropical and polar regions, corresponding to a strongsubtropical jet over Australia and a modest subpolar jetnear Antarctica (not shown). These are downstream ofthe single jet in the Indian Ocean along the northernedge of the large stationary cyclone. Given its realism,the CTR run will be regarded as a suitable basis forcomparison with the other experiments.

The largest change in the asymmetric structure isfound in the ZTS run with zonally uniform tropical SST(Fig. 2e), and this will be discussed first. The majorstorm track has a broader latitudinal scale and startsfarther eastward. The reduction in the EKE near NZ isnow weaker, and the minima are now much farther east.Taking a broad view, the storm track in this experimenthas the appearance more of a loop rather than a spiral(cross marks in Fig. 2e). The changes in stationary ed-dies are consistent with the fact that they are largelyresponsible for the storm-track changes. The amplitudes


FIG. 4. JJA synoptic-scale 300-hPa EKE (contour interval is 8 m2 s22) and its deviation fromthe zonal mean (grayscale) for the (a) ZMS, (b) NMT, (c) NSA, and (d) NAD runs.

of zonal wavenumbers 1 and 2 are much smaller, andthe positions of the stationary cyclones and anticycloneshave changed drastically (Fig. 2f). The largest cyclonein the south Indian Ocean associated with the broadermajor storm track has disappeared. The strong anticy-clones near NZ and over the Bellingshausen Sea haveweakened and merged, consistent with the associatedweaker minimum in EKE. As a result, the subpolar jetintrudes into the central Pacific, and there is little dif-fluence of the jet in the eastern Indian Ocean.

In contrast, in the runs without aspects of the mid-latitude forcing, the stationary eddies are generally quitesimilar to those in the CTR run (Figs. 2d and 3; theNSA and NAD runs are not shown). The extended cy-clone in the Atlantic and Indian Oceans and the NZ andBellingshausen Sea anticyclones are still present in allthese experiments. As shown in Fig. 4, the spiral stormtrack structure is also maintained (cross marks in Fig.4). However, the storm track is significantly modified.First, considering the case with zero zonal asymmetryin midlatitude SST (the ZMS run), the EKE in the Indian

Ocean decreases by 10%–20%, and that off Chile in-creases by 10%–20% (Fig. 4a). The EKE minimum re-gion near NZ and the other features are retained in thisrun. Second, in the absence of all the SH mountainsexcept those in Antarctica (the NMT run), the zonalasymmetry in EKE is much reduced, but the maximumand minimum of EKE are not shifted (Fig. 4b). Thespiral storm-track structure is also somewhat reduced(cross marks in Fig. 4b). That this is probably a com-bined effect of the South African Plateau and the Andescan be seen from the other two experiments. Removingonly the South African Plateau (the NSA run), the majorstorm track starts in the central Atlantic as in the CTRrun, but it reaches a peak south of South Africa and isweaker in the Indian Ocean (Fig. 4c). This is consistentwith there being less cyclogenesis in the absence of theSouth African Plateau. The main impact of removingthe Andes instead (the NAD run) is that the start of themajor storm track in the Atlantic is much weaker (Fig.4d). However, it still attains a clear peak in the centralIndian Ocean, as in the CTR run. This is consistent with


FIG. 5. The 200-hPa diagnostics for the CTR run. (a) Stationary-eddy streamfunction (contour interval is 2.5 3 106 m2 s21 with neg-ative contours dashed) and wave activity flux vectors. (b) Divergence(contour interval is 0.1 day21 with negative contours dashed and zerocontours omitted) and divergent wind (scale on right). (c) Absolutevorticity (thick solid contour with 0.5 day21 interval), streamfunction(thin dotted contour with 2.0 3 106 m2 s21 interval), and divergentwind (vectors) at 200 hPa in the Indian Ocean sector (258S–08,33.758–101.258E).

there being less cyclogenesis downstream of SouthAmerica in the absence of its topography.

In summary, only the removal of zonal asymmetriesin tropical SST drastically changes the stationary eddies.These changes appear to give associated storm trackchanges, in particular removing the EKE minimum re-gion near NZ and broadening the EKE maximum in theIndian Ocean. In contrast, midlatitude forcing has onlya modest effect on the stationary eddies, but it is stillimportant in the buildup of the peak of the major stormtrack in the central Indian Ocean.

4. The tropical SST asymmetric forcingmechanism

The focus in this section is on how tropical SST asym-metric forcing affects the stationary eddies and thus thestorm track in the SH. Tropical SST zonal asymmetriesgive asymmetries in convective heating and the asso-ciated vertical circulations. The divergent flow due tothe convection can act as the source of Rossby wavesthat propagate into the extratropics. To investigate this,we will compare the ZTS and CTR runs using two di-agnostics: the wave activity flux (Takaya and Nakamura2001) and the Rossby wave source (RWS) ideas (Hos-kins and Sardeshmukh 1988). Definitions and meaningsof these diagnostics are given in the appendix, alongwith a discussion of their significance.

A variety of diagnostics for the CTR run are shownin Fig. 5. The stationary eddy 200-hPa streamfunctionis given in Fig. 5a. Comparing with Fig. 2b, the cycloneis again found in the southern Indian Ocean and theanticyclone south of NZ. Now, however, the strongwesterlies in the Indian Ocean are seen to be associatedmore with the strong anticyclone in the tropical regionand the weak westerlies in the NZ region also with acyclone to the northwest of it. The wave activity fluxvectors in Fig. 5a indicate that wave activity propagatesfrom the eastern equatorial Indian Ocean and the anti-cyclone there to the higher latitude cyclone, eastwardto the anticyclone south of NZ, and then equatorwardto the cyclone region. The enhanced westerlies in theIndian Ocean and the reduced westerlies in the NZ re-gion are then seen as being associated with a Rossbywave train that originates in the tropical Indian Oceanand follows an almost great circle path poleward, east-ward, and then equatorward.

To obtain a view of the tropical asymmetric circu-lation, Fig. 5b gives the divergence and divergent flowvectors at 200 hPa. Generally, there is divergence abovethe deep convection that is predominantly in the NH,with a divergent flow toward the southern winter hemi-sphere. However there is much enhanced divergenceabove the northern summer monsoon regions of Africa,Southern Asia, and the Americas. In particular, thesouthward divergent flow vectors in the southern trop-ical Indian Ocean are prominent. To investigate howthese could indicate the origin of the Rossby wave train,

Fig. 5c gives the divergent flow vectors and absolutevorticity contours for this region. It is clear that thereis a large divergent flux of negative absolute vorticityout of the region. As discussed in the appendix, this isthe integral of the RWS over the region and, in the SH,constitutes a source of anticyclonic vorticity, consistentwith the anticyclone there seen in Fig. 5a. It is also seenin the total streamfunction shown in Fig. 5c. In thefrictionless, steady balance described in the appendix,the rotational flow must flux negative vorticity into theregion on average. However, given the larger magnitudeof the rotational flow and its large cancellation betweenvorticity fluxes into and out of the region that is apparent


FIG. 6. Upper-tropospheric diagnostics as in Fig. 5, but for theZTS run.

from the full streamfunction contours in Fig. 5c, the netflux would require numerical calculation.

These ideas discussed with reference to a Rossbywave train forced by summer monsoonal asymmetriesnorth of the equator in the Indian Ocean region aresupported by the diagnostics of the ZTS run given inFig. 6. The stationary eddy streamfunction (Fig. 6a) ismuch weaker and has a totally different structure thanin the CTR run. The wave activity flux (Fig. 6a) in theeastern Indian Ocean sector has reversed direction, sug-gesting high-latitude forcing of the tropical pattern. Theupper tropospheric divergence field (Fig. 6b) indicatesmuch-reduced asymmetries in tropical convection andthe associated divergent wind predominantly reflects thezonally averaged Hadley cell structure. The small re-maining zonal asymmetries in convection are probablyproduced by the land–sea contrast in the NH. The dif-ference in the magnitude of the divergent winds in theIndian Ocean sector, particularly away from the equator,

is clear (Fig. 6c), and the integrated RWS as given bythe divergent flux of absolute vorticity out of the regionis much smaller.

From the diagnostics of these two runs presented here,it is apparent that the asymmetries in tropical heatinggenerate a Rossby wave pattern that leads to the dom-inant zonal asymmetries in the SH upper-troposphericstorm track through its production of elongated strongwesterlies in the Indian Ocean and weak westerlies nearNZ.

5. Lower-tropospheric storm track

Indications of the origin of the zonally asymmetricstructure of the lower-tropospheric storm track, as de-duced from the various experiments, will now be dis-cussed. The basic field used for this discussion is the850-hPa meridional flux of temperature by synoptictime-scale (2–8-day period) eddies (hereafter denotedVT850). For ERA-15 and the model control, CTR,VT850 fields are given in Figs. 7a,b. The latter alsoshows the regions of large meridional SST gradient inshading. The major storm track is again seen to start inthe Atlantic, reach its maximum in the western IndianOcean, and decrease in the eastern Indian Ocean. East-ward of this, latitudinal maxima spiral in toward Ant-arctica, with a local maximum occurring there near thedate line. In the upper troposphere, there is also a sec-ondary storm track in the eastern Pacific near 358–408S.These features are again quite well captured by the mod-el, CTR, though with maximum amplitudes reducedsome 15%. The exception is the eastern Pacific sec-ondary storm track, which is not represented wellenough for deductions to be made from the experiments.

Zonal tropical SSTs (ZTS shown in Fig. 7c) do notproduce such a marked change here as they did in theupper-tropospheric storm track (Figs. 2c,e). The largestchange is that the major storm track maximum is broadermeridionally. As was true in the upper troposphere, thisis consistent with the lack in this case of the upperanticyclone to the north and the cyclone to the south,which give the strong upper westerlies in between inCTR (Figs. 2d,f). There is an indication of a slightlyless abrupt end to the storm track south of Australia,but the behavior in the NZ sector is generally similarto that in CTR. The upper-tropospheric changes asso-ciated with the absence of the propagating Rossby wavealso have a signature in the lower-tropospheric stormtrack, but it is not as strong as in the upper troposphere.

Making the midlatitude SSTs zonal, ZMS makes thestorm track much more zonally uniform (Fig. 7d). Theelongated maximum in the Indian Ocean is lost and themaximum amplitude is considerably reduced. This isconsistent with the fact that the control has a meridionalSST gradient of more than 12 K/108 latitude south ofMadagascar (458E), which is about 3 times larger thanthat to the east of NZ. As in the upper troposphere, westof Chile the storm track is slightly enhanced in this case.


FIG. 7. High-pass-filtered meridional thermal flux at 850 hPa (VT850 in text; contour)and the meridional gradient of prescribed SST (shading) (a) based upon ERA-15 data andin the (b) CTR, (c) ZTS, (d) ZMS, (e) NSA, and (f ) NAD runs. The contour interval is 1K m s21, and the light and heavy shading denotes values .6 and .12 K/108 lat, respectively.

Despite the lack of the region of maximum SST gradientthat might have been thought to be associated with thestorm track spiraling in toward Antarctica, the ZMS runstill seems to show this feature and also the date linemaximum.

Removing the Andes results in little qualitativechange in the storm track VT850 (NAD; Fig. 7f). How-

ever, as in the upper troposphere, the intensity is some-what reduced downstream of the Andes, in the Atlanticthrough to the eastern Indian Ocean maximum. This isconsistent with the cyclogenesis regions found by HH04in the lee of the Andes. Also, as was the case in theupper troposphere, removing the South African topog-raphy, the NSA run (Fig. 7e) gives similar reductions


FIG. 8. The 850-hPa temperature (contour) and its meridional gra-dient (shading). The contour interval is 2.58C and the light and heavyshading is for values .7 and .9 K/108 lat, respectively.

FIG. 9. Schematic of the relationship among the aspects of theatmosphere and the surface conditions that are important for the SHstorm track.

FIG. 10. CTR zonal wavenumber-1 and -2 components of the ver-tical average zonal wind (contour interval is 1 m s21) and synoptic-scale E (vectors) and its divergence (light and heavy shading at values.0.2 and ,20.2 m s21 day21, respectively).

in magnitudes downstream of it and a maximum valuenow occurring some 308 farther east than in CTR. Alsoin support of this, the CTR 850-hPa temperature fieldgiven in Fig. 8 shows much-enhanced baroclinicity onthe western side of South Africa as well as near theAndes (the region below the topography should be ig-nored).

In summary, the most important ingredient in theasymmetry of the major lower-tropospheric storm trackis the midlatitude SST distribution. This is in agreementwith the previous studies of Inatsu et al. (2002, 2003)and Nakamura and Shimpo (2004). However, tropicalasymmetries influence the structure of the Indian Oceanmaximum in the major storm track and its abrupt end.The topography of South America and South Africaincreases the intensity of the storm track downstreamof each of them. The date line maximum near Antarcticais present in all of the experiments, which suggests itmay be associated with Antarctic topography.

6. Discussion and conclusions

From the experiments diagnosed in this paper, it hasbeen deduced that Indian Ocean Rossby wave forcingassociated with the zonal asymmetry in tropical SSTscreates the SH stationary eddies that are the primarycontrol of both the major minimum and maximum ofupper-tropospheric synoptic-scale EKE. Midlatitudeforcings give small changes to the stationary eddies butare important for the storm tracks. In particular, mid-latitude SST asymmetries are very important for thelower-tropospheric storm-track distribution as measuredby the meridional heat flux. The Andes contribute totheir downstream storm track intensity through cyclo-genesis in their lee. More surprisingly, cyclogenesis onthe western side of the South African Plateau is alsoimportant in this regard.

Referring to Fig. 9, the arguments given above havebeen in terms of the passive response of the storm trackto stationary eddies forced by tropical-SST-related con-vection (the left and top solid arrows) or directly toterrestrial forcings (the two right solid arrows in Fig.9). However, there are some important feedback issues(the dashed arrows in Fig. 9) that should be recognized.

First, the storm-track activity in general has a feed-back onto time-mean zonal wind (arrow A in Fig. 9).The thermal effect, as indicated by VT850, is to weakenthe lower-tropospheric mean baroclinicity, particularlythe largest values imposed by the SST distribution. Fol-lowing Hoskins et al. (1983), the mechanical forcing bythe storm-track eddies can be diagnosed using E 5( , 2 ), where the prime denotes synoptic-2 2y9 2 u9 u9y9scale variability. The divergence (convergence) of Eindicates rotational forcing consistent with westerly(easterly) wind acceleration by synoptic-scale eddies.Figure 10 shows the zonal wavenumber-1 and -2 com-ponents of the barotropic zonal wind and of the syn-optic-scale E and its divergence. It is evident that theeddy activity tends to accentuate the asymmetries in thebasic flow. As usual, the baroclinic damping and baro-tropic forcing of the mean flow give a mixed negativeand positive feedback on the coupled asymmetric meanflow and storm track.


The mean low-level zonal wind tendency due to syn-optic-scale eddies (Fig. 10) may be expected to affectthe ocean circulation in the fully coupled atmosphere–ocean system (arrow B in Fig. 9). This implies the feed-back loop suggested in some previous studies (arrowsA, B, and C in Fig. 9; e.g., Hoskins and Valdes 1990;Watanabe and Kimoto 2000). Enhanced zonal winddrives enhanced ocean gyres, and at the western bound-ary the stronger confluence of the ocean currents createslarger SST gradients, which in turn creates stronger syn-optic-scale eddies. In the SH, the surface zonal windleads to the large SST gradient off Argentina where theMalvinas and Brazil Currents are confluent. This canbe viewed as being the origin of the larger SST gradientin the Atlantic Ocean that has been shown to be im-portant particularly for the lower-tropospheric stormtrack.

Acknowledgments. We wish to thank Dr. H. Spencer,Dr. L. C. Shaffery, and Dr. L. Steenman-Clark for help-ing the first author to run HadAM3 under the appropriateboundary conditions. We also thank Dr. M. Watanabe,Dr. H. Mukougawa, Prof. S.-P. Xie, Dr. H. Nakamura,Dr. J. H. Yin, Prof. M. Kimoto, and Prof. F.-F. Jin forinsightful comments on this study. This research wasperformed and the manuscript was written when the firstauthor, who was funded by the Japan Society of Pro-moting Sciences, was visiting the University of Reading.The figures were produced using the GFD-DENNOULibrary.

APPENDIX

Wave Activity Flux and Rossby Wave Source

Rossby wave propagation can be diagnosed by thewave activity flux derived by Takaya and Nakamura(2001):

2 ]c* ]c* ] c*2 c*

2]x ]x ]x

2 p ]c* ]c* ] c*W 5 2 c* , (A1)

2 ]x ]y ]x]y

2 2f ]c* ]c* ] c*2 c*

21 2N ]x ]z ]x]z

where p is the pressure, c the streamfunction, f theCoriolis parameter, N the Brunt-Vaisala frequency, andthe asterisk is a derivation from the zonal average. Un-der the Wentzel–Kramers–Brillouin (WKB) approxi-mation, W is parallel to the group velocity of linearstationary Rossby waves and is independent of the phaseof the waves.

Following Hoskins and Sardeshmukh (1988), the fullvorticity equation can be written as

]z ]v1 = · (vz) 1 k · = 3 v 5 0, (A2)1 2]t ]p

where z is the vertical component of absolute vorticity,v is the horizontal velocity, and v the vertical velocityin p coordinates. Near the tropopause, v is small andthe last term in the equation becomes small. This impliesthat in a long-term average for which the last term isnegligible, the horizontal absolute vorticity flux mustbe nondivergent.

Dividing the wind into its rotational, vc, and diver-gent, vx, components, the approximated vorticity equa-tion can be written as

]z1 = · (v z) 5 2= · (v z). (A3)c x]t

Given that the left-hand side is the barotropic vorticityequation, Hoskins and Sardeshmukh (1988) have calledthe term in the right-hand side the Rossby wave source(RWS). The integral of RWS over a region bounded bya circuit G is equal to 26G zvx · n dl, where n is theoutward normal to G. In a long-term average, this mustbe balanced by a rotational vorticity flux out of theregion, 6G zvc · n dl. For a segment Gz along an absolutevorticity contour, the contribution to this integral is2zDc, where Dc is the difference in c between thetwo ends of the segment. If transients are negligible,then in a long-term mean, all variables can be replacedby their mean values.

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the zonal asymmetry of the southern hemisphere … journal of climate volume 17 q 2004 american...

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