Dynamics of the surface wind eld over the equatorial Indian Ocean

Q. J. R. Meteorol. Soc. (24), 13, pp. 53 517 doi: 1.1256/qj.3.79 Dynamics of the surface wind eld over the equatorial Indian Ocean By STEFAN HASTENRATH and DIERK POLZIN Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, USA (Received 12 May 23; revised 4 September 23) SUMMARY The annual cycle and interannual variability of the equatorial westerlies over the Indian Ocean are examined in the context of the equations of motion, using long-term surface ship observations. Eastward pressure gradient along the equator may accelerate equatorial westerlies, but the process is more intricate. The south Indian Ocean trade winds can recurve in the southern hemisphere only with an eastward pressure gradient. This is counteracted by the Coriolis acceleration directed at right angles to the left of the motion. With weak trades and strong eastward pressure gradient, the ow can recurve at relatively high southern latitudes, thus allowing a broad equatorial zone in which westerlies can develop. In the annual cycle, the southern Indian Ocean high and trade winds are closest to the equator and strongest in austral winter when, however, the eastward pressure gradient in the equatorial zone is also steepest. Accordingly, the latitude of ow recurvature in the southern trade winds stays farthest away from the equator in boreal spring and autumn. Consequently, the equatorial westerlies peak in these short transition seasons. The boreal autumn equatorial westerlies are the surface manifestation of a powerful zonal circulation cell along the Indian Ocean equator. The interannual variability of boreal autumn is characterized by weak westerlies accompanying a slack zonal pressure gradient and strong southern trade winds with recurvature near the equator. Such an ensemble of circulation departures is characteristic of abundant rainfall in East Africa and de cient precipitation in Indonesia. KEYWORDS: Boreal autumn Equatorial westerlies Rainfall anomalies Zonal gradients 1. INTRODUCTION The surface ow eld over the tropical oceans is dominated by the trade wind airstreams from the two hemispheres meeting in a con uence in the equatorial zone. Based on long-term ship observations (Hastenrath and Lamb 1977, 1979), the dynamics and climatology of surface ow over the equatorial oceans were diagnosed in a study a quarter of a century ago (Hastenrath and Lamb 1978; Hastenrath 1985). The con uence is embedded in a near-equatorial trough of low pressure, which over the Atlantic and eastern and central Paci c is located in the northern hemisphere all the year round. With a trough position near the equator, over the western Atlantic and central Paci c and particularly in boreal winter, the cross-equatorial airstream from the southern hemisphere continues to be south-easterly all the way to the con uence. By contrast, over the eastern Atlantic and Paci c and especially in boreal summer, the near-equatorial trough is displaced far poleward; the cross-equatorial ow from the southern hemisphere, in response to the Coriolis acceleration, recurves from south-easterly to south-westerly, and this to the north of the equator, for lack of a very strong zonal pressure gradient. Between equator and recurvature latitude a band of divergence develops, the Intertropical Divergence Zone (ITDZ, Hastenrath 22); further downstream beyond the recurvature latitude this gives way to a band of convergence straddling the wind con uence, the Intertropical Convergence Zone (ITCZ). Remarkably different conditions are found over the Indian Ocean. During boreal summer, the south Indian Ocean trade winds sweep across the equator to continue as the south-west monsoon into southern Asia. In boreal winter, surface ow emanating from south Asia crosses the equator to meet the south Indian Ocean trades along a con uence located in the southern hemisphere. Only in the two short transition periods between the summer and winter monsoons do strong westerlies sweep the equatorial Corresponding author: Department of Atmospheric and Oceanic Sciences, University of Wisconsin, 1225 West Dayton Street, Madison, WI 5376, USA. e-mail: slhasten@facstaff.wisc.edu c Royal Meteorological Society, 24. 53

54 S. HASTENRATH and D. POLZIN 4 6 E 9 1 N SPW, SVW, STW UEQ, V PN PE SJB RON 1 S PW SIW SIP PS SPE, SVE, STE Figure 1. Orientation map showing domains of indices. Solid lines delineate: the indices of equatorial westerlies UEQ and meridional wind component v (4 B N 4 B S, 6 9 B E), of total wind speed in the downstream portion of the south Indian Ocean trades SIW (4 12 B S, 6 9 B E); and the pressure indices PW (8 B N 8 B S, 4 5 B E) and PE (8 B N 9 B S, 9 1 B E). PWE D PW PE represents the zonal pressure gradient along the equator. LUZ indicates the latitude of zero zonal wind, that is the recurvature from south-easterly to south-westerly ow; similarly, LVZ is the latitude of zero meridional wind, that is the wind con uence, both are compiled for the longitude domain 6 9 B E. Dashed lines delineate: the index of pressure on the equatorward side of the south Indian Ocean high SIP (12 2 B S, 6 9 B E); indices of pressure to the north PN (4 1 B N, 6 9 B E) and south of the equator PS (4 1 B S, 6 9 B E). PNS D PN PS represents the meridional pressure gradient across the equator. Dash-dotted lines indicate: the pressure, wind, and SST indices SPW, SVW, STW, respectively, in a western domain (1 B N 5 B S, 45 55 B E); and the corresponding indices SPE, SVE, STE in an eastern domain (5 15 B S, 9 1 B E). Dotted lines denote the rainfall indices RON for the East African coast and SBJ for Indonesia (5 B N 1 B S, 1 12 B E). Indian Ocean. Throughout the year, recurvature of ow from an easterly to a westerly component takes place in the hemisphere from where the airstream originates, unlike the equatorial eastern Atlantic and Paci c. The boreal autumn equatorial westerlies are the surface manifestation of a powerful zonal circulation cell (Hastenrath 2); they drive a jet in the upper hydrosphere (Wyrtki 1973; Hastenrath and Greischar 1991), they are tightly correlated with East African rainfall (Hastenrath et al. 1993), and only in very recent years have they received renewed attention (Behera et al. 1999; Birkett et al. 1999; Saji et al. 1999; Webster et al. 1999; Murtugudde et al. 2; Baquero- Bernal et al. 21; Philippon et al. 22; Hastenrath and Polzin 23; Black et al. 23; Clarke et al. 23;Lau and Nath 24). Accordingly, the dynamics of surface ow over the equatorial Indian Ocean merit further exploration. This is the objective of the present study. Section 2 describes the data, section 3 summarizes essential background, sections 4 and 5 diagnose the annual cycle and the interannual variability during the boreal autumn season of equatorial westerlies, and a synthesis is offered in the closing section 6. 2. DATA Long-term surface ship observations stemming from the same source as used in our atlases (Hastenrath and Lamb 1977, 1979) are available in the Comprehensive Ocean Atmosphere Data Set (COADS) collection, with spatial resolutions of 2 and 1 degree latitude longitude squares (Woodruff et al. 1987, 1993). Of interest here are wind, pressure and sea surface temperature (SST) for the period 1958 97. As in earlier work (Hastenrath et al. 1993), various indices were compiled for the domains indicated in Fig. 1: UEQ (4 B N 4 B S, 6 9 B E) represents the zonal wind along the Indian Ocean equator and v the meridional wind component in the same domain; SIW (4 12 B S, 6 9 B E) represents the total wind speed in the downstream part of the south Indian Ocean trades. For the same longitude domain, LUZ is an index of the latitude of zero

SURFACE WIND DYNAMICS OVER THE INDIAN OCEAN 55 zonal wind, that is the recurvature from south-easterly to south-westerly ow. Similarly, LVZ is an index of the latitude of zero meridional wind, that is the wind con uence. PW (8 B N 8 B S, 4 5 B E) describes the pressure at the western, and PE (8 B N 8 B S, 9 1 B E) at the eastern extremity of the basin, and PWE D PW PE represents the zonal pressure gradient along the equator. For the same western and eastern domains TW and TE are the respective indices of SST. The index PN (4 1 B N, 6 9 B E) denotes the pressure to the north and PS (4 1 B S, 6 9 B E) to the south of the equator; PNS D PN PS is the meridional pressure gradient across the equator. SIP (12 2 B S, 6 9 B E) captures the pressure on the equatorward ank of the south Indian Ocean high. The indices of pressure, SPW, of total wind speed, SVW, and of SST, STW, were compiled for a domain in the west (1 B N 5 B S, 45 55 B E), and the corresponding indices SPE, SVE, STE for a domain in the east (5 15 B S, 9 11 B E). October November rainfall is captured by the index RON for East Africa and SJB (Sumatra, Java, Borneo) for Indonesia (Hastenrath and Polzin 23). 3. BACKGROUND The annual cycle of the surface wind eld over the Indian Ocean is dominated by the alternation between the boreal winter (Fig. 2(a)) and summer monsoons (Fig. 2(c)). Strong westerlies along the equator are limited to the short transition seasons between the monsoons (Figs. 2(b) and (d)). In boreal winter (Fig. 2(a)) the surface ship observations show ow from southern Asia recurving near the equator and a con uence in the southern hemisphere; in boreal spring (Fig. 2(b)) recurvature to the south of the equator and con uence in the equatorial region; in boreal summer (Fig. 2(c)) southern hemispheric trade winds recurving near the equator and not reaching any con uence within the map domain; and in boreal autumn (Fig. 2(d)) recurvature of southern trade winds near the equator and con uence in the equatorial zone. For the dynamics of the time-averaged ow, the equations of motion detailed before (Hastenrath and Lamb 1978; Hastenrath 1985) can be simpli ed. Two equations are pertinent here. The vector equation of motion can be written dv dt D f k V rp C F: (1) The rst component equation of motion simpli es at the latitude of recurvature (u D ) to v @u @y D Cf v @p @x : (2) Following conventional notation, V is the horizontal wind vector, u and v its eastward (x) and northward (y) components, k is a vertical unit vector, F is the frictional force per unit mass; is speci c volume, p is pressure, f is the Coriolis parameter, and r is the horizontal r operator. 4. ANNUAL CYCLE With the focus on the central equatorial Indian Ocean and with the background of the maps for the cardinal months in Fig. 2, Fig. 3 highlights the characteristics of the average annual cycle of circulation by indicative indices (Fig. 1). Regarding the meridional component, the diagram illustrates the alternation between the boreal winter northerly and summer southerly wind component(v, Fig. 3(a);

56 S. HASTENRATH and D. POLZIN Figure 2. Surface wind elds for: (a) January, (b) April, (c) July, (d) October. Isotach spacing is 2 m s 1. Shading highlights the domain of index UEQ (see Fig. 1).

SURFACE WIND DYNAMICS OVER THE INDIAN OCEAN 57 +2 m s -1 1 a J F M A M J J A S O N D J v b V 5 m s -1 +3 m s -1 c UEQ +1 mb 2 112 d PNS 118 mb e SIP +5 f SIW +3 +1 N S 1 S 5 +4 mb g LVZ h LUZ i PWE J F M A M J J A S O N D J Figure 3. Annual cycle of indicative elements: (a) meridional wind component v in the domain of index UEQ (4 B N 4 B S, 6 9 B E) in m s 1 ; (b) wind vector V in the domain of UEQ, scaled by reference to the given 5 m s 1 vector; (c) zonal wind component in the domain of UEQ in m s 1 ; (d) pressure difference across the equator, PNS, in mb; (e) pressure on the equatorward side of the south Indian Ocean high, SIP (12 2 B S, 6 9 B E), in mb; (f) total wind speed in the downstream portion of the southern Indian Ocean trades, domain SIW (4 12 B S, 6 8 B E), in m s 1 ; (g) latitude of surface wind con uence (v D zero), LVZ; (h) latitude of the recurvature of cross-equatorial ow (u D zero), LUZ; (i) zonal pressure difference PWE, in mb. See Fig. 1 for details of indices.

58 S. HASTENRATH and D. POLZIN V, Fig. 3(b)), along with the seasonal reversal of PNS (Fig. 3(d)). Broadly opposite to this runs the annual cycle of SIP (Fig. 3(e)). This, as well as the SIW (Fig. 3(f)), are strongest in the austral winter. Consistent with the reversal in PNS (Fig. 3(d)) and wind direction (v, Fig. 3(a); V Fig. 3(b)), the surface wind con uence between the airstreams from the two hemispheres sits to the south of the equator in boreal winter and far to the north in summer (LVZ, Fig. 3(g)). More intriguing are the dynamics of the zonal component. The annual cycle of PWE (Fig. 3(i)) results from the approximately inverse variations of PW and PE (not shown here), with PW having its maximum and PE its minimum in boreal summer, so that PWE becomes strongest in that season of the year (Fig. 3(i)). UEQ is also weaker in boreal winter and stronger in the summer half-year, but it reaches maxima around April May and October November (Fig. 3(c)). This discrepancy between the zonal wind and zonal pressure gradient along the Indian Ocean equator raises the question: what controls the strength of the equatorial westerlies? Consider the latitude at which ow recurves from an easterly to a westerly component (LUZ, Fig. 3(h)). The more poleward such recurvature takes place, the broader the band within which westerlies can develop in the equatorial belt. The weak PWE during boreal winter (Fig. 3(i)) is in any case unfavourable for westerly ow, but why do the strongest westerlies occur in the transition seasons rather than at the core of boreal summer (u, Fig. 3(c)), when the PWE is strongest (Fig. 3(i))? For the downstream portion of the trade wind south-easterlies in the southern hemisphere consider the imbalance of the Coriolis and pressure gradient terms in Eq. (1). In boreal winter the eastward PWE acceleration is weak (Fig. 3(i)), matching the Coriolis term and thus accomplishing recurvature only at very low latitude. At the height of boreal summer the PWE acceleration is strong (Fig. 3(i)), but then the southern trade winds are also strong (SIW, Fig. 3(f)) so that the Coriolis term allows recurvature only at very low latitude. In the transition seasons, the PWE (Fig. 3(i)) is not at its strongest, but the SIW is also weaker than at the peak of austral winter (Fig. 3(f)). Consequently, in these seasons LUZ can occur at relatively high southern latitudes, as compared to the boreal winter and the core of the summer half-year (Fig. 3(h)). Equation (2) shows that recurvature before reaching the equator can take place only with suf ciently strong eastward pressure gradient: at the recurvature latitude the left-hand term is zero, the rst righthand term is negative, so the second right-hand term must be suf ciently large positive. Compare the timing of extrema (LUZ, PWE, u, Figs. 3(h), (i), and (c)): the maxima of the westerly wind (May, October) are timed between the favourable highest southern latitude of recurvature (April, November) and the favourable strongest eastward pressure gradient (July). The equatorial westerlies are somewhat better developed in boreal autumn than in spring (Fig. 3(c)), so it is appropriate to compare the underlying factors. Regarding spring, April has as favourable factors: weak PNS, weak SIP, and weak SIW; but the weak PWE is unfavourable (Figs. 3(d), (e), (f) and (i)). In May, the favourable PWE has strengthened, but this is now counteracted by the unfavourable accelerated SIW which accompany the rising SIP, the steepened PNS, and concomitant southerly wind component (v/ in the equatorial zone (Figs. 3(i), (f), (e), (d) and (a)). Consistent with this, LUZ draws closer to the equator (Fig. 3(h)). By comparison, in boreal autumn conditions remain favourable through October and November (Figs. 3(i), (f), (e), (d), (a) and (h)): PWE weakens somewhat, but this is paralleled by the favourable weakening of the SIW, which accompany the decrease in SIP, PNS and v; in conjunction, these allow for a more southerly location of LUZ and equatorial westerlies little diminished from October to November.

SURFACE WIND DYNAMICS OVER THE INDIAN OCEAN 59 TABLE 1. MATRIX OF CORRELATION COEFFICIENTS UEQ PWE LUZ SIW PWE C86** LUZ 94** 78** SIW 68** 45** C68** SIP 54** 46** C46** C4* Values are given in hundredths, with one and two asterisks denoting signi cance at the 5% and 1% levels, respectively. The period is 1958 97 and time series are of October November values. See Fig. 1 for details of the circulation indices. These considerations for the average annual cycle are also relevant to the study of interannual variability, which is of particular interest for the boreal autumn circulation. 5. BOREAL AUTUMN The equatorial westerlies of boreal autumn are the surface manifestation of a powerful zonal circulation cell along the Indian Ocean equator, which plays an important role in the climate dynamics of the region: the westerlies are correlated at.85 with East African rainfall, arguably the highest such correlation on the planet, and positively with precipitation over Indonesia (Hastenrath et al. 1993; Hastenrath 2; Hastenrath and Polzin 23). Accordingly, the circulation mechanisms related to the origin and maintenance of the equatorial westerlies, and diagnosed from Eqs. (1) and (2), merit attention regarding the annual cycle and, more particularly, concerning the interannual variability of boreal autumn. In this spirit, Table 1 presents the relation between pertinent circulation components, and the time series plots in Fig. 4 illustrate their interannual variability. Table 1 shows that years with strong UEQ are favoured by strong eastward PWE, which according to Eq. (2) allows LUZ at relatively high southern latitudes, which in turn favours the westerlies in the UEQ. Weak SIP is accompanied by weak SIW, and these according to Eqs. (1) and (2) allow a relatively high southern latitude LUZ, which again favours the high UEQ index. SIP and SIW, plausibly, vary somewhat in the same sense. They both tend to be weaker with steeper PWE, and thus cooperate with it in favouring the UEQ. The correlation between SIP and PWE results from the spatial coherence with the pressure in the east, the correlation between SIP and PE being C.7. The time series plots in Fig. 4 exhibit the concurrent variability in the pertinent elements over four decades and, more particularly, serve to highlight years with extreme departures in UEQ. Various earlier studies (Thompson and Morth 1965; Lamb 1966; Hastenrath 1984, 21; Reverdin et al. 1986; Kapala et al. 1994; review in Hastenrath and Polzin 23) called attention to the years 1961, 1994 and 1997. These featured weak equatorial westerlies (Fig. 4(a)), abundant boreal autumn rainfall in East Africa (Fig. 5(a)), and concurrently de cient precipitation in Indonesia (Fig. 5(b)). In the same vein, over the period 1958 96 the UEQ indices have correlations of.84 with RON values, of C.79 with SJB values, and RON and SJB are correlated at.62, all signi cant at better than the 1% level (Hastenrath and Polzin 23). These extremely tight teleconnections between the climate of the western and eastern extremities of the equatorial Indian Ocean basin must be seen in the context of the boreal autumn equatorial zonal circulation cell and concomitant zonal contrasts in vertical motion (Hastenrath 2: Hastenrath and Polzin 23); such cells require well developed zonal ow in the lower troposphere (Hastenrath et al. 22).

51 S. HASTENRATH and D. POLZIN +5 m s -1 a UEQ 2 +4 mb +2 b PWE 6 7 8 9 +1 c LUZ N +5 6 7 8 9 S 5 +7 m s -1 +5 d SIW 6 7 8 9 17 mb 15 e SIP 6 7 8 9 13 Figure 4. Time series plots of selected indices (see Fig. 1 for locations): (a) UEQ, in m s 1 ; (b) PWE, in mb; (c) LUZ, in degrees of latitude; (d) SIW, in m s 1 ; (e) SIP, in mb 1. Open circles highlight the extreme years 1961, 1994, and 1997.

SURFACE WIND DYNAMICS OVER THE INDIAN OCEAN 511 +3 a RON -1 +3 196 7 8 9 b SJB -1 Figure 5. Time series plots of October November rainfall indices: (a) RON for East Africa, and (b) SJB for Indonesia. Open circles highlight the extreme years 1961, 1994, and 1997. See Fig. 1 for details of indices. It is noted from the comparison of time series in Fig. 4 that especially 1961 and 1997 also featured slack eastward PWE, far north LUZ, as well as strong SIW, and to a lesser extent SIP, all consistent with Table 1 in the sense of Eqs. (1) and (2). The maps of correlation with UEQ in Fig. 6 illustrate the larger spatial context. These re ect earlier results (Hastenrath et al. 1993) supported in later papers (Saji et al. 1999; Webster et al. 1999; Black et al. 23; Clark et al. 23; Hastenrath and Polzin 23). In particular, Fig. 6(a) shows for strong UEQ low pressure in the east as well as in a band extending from there westward to the south of the equator, consistent with the positive correlations of SIP with PWE (Table 1) and PE; and Figs. 6(b) and (c) feature weak winds in the downstream portion of the south Indian Ocean trades, the realm of SIW (contrasting with the trivially high positive correlations in the domain of UEQ). Complementing Figs. 6(a), (b) and (c), Fig. 6(d) maps the corresponding correlations with SST. It shows contrasting extrema at locations in the western and eastern extremities of the basin, similar to those for pressure in Fig. 6(a), but with signs reversed. Information on these domains is compacted in the indices SPW, SVW, STW, SPE, SVE. STE (see section 2 and Fig. 1). Table 2 details the associations among the indices and with UEQ. At the outset it should be noted that the concurrent correlations between SPW and SPE and between STW and STE are negligible, showing no evidence of temperature or pressure see-saws between the western and eastern domains. This notwithstanding, a strong positive association is apparent between STW and SPE, consistent in sign and magnitude with the strong negative correlations of UEQ versus STW and SPE. Regarding the shared variance between the three indices, the partial correlation coef cient for STW/SPE (with UEQ kept constant) drops to C.3, that for UEQ/STW to.3, while that for UEQ/SPE is much larger at.55; thus, of the 41% variance in the apparent STW/SPE correlation, only 9% is due to the STW/SPE associations as such and 32% to the variance shared through UEQ. Similarly, concerning the small negative STW/STE correlation in Table 2, the partial correlation for STW/STE (with UEQ kept constant) becomes larger positive (even less indicative of SST see-saw). Further, not included

512 S. HASTENRATH and D. POLZIN Figure 6. Maps of October November 1958 97 correlations with the October November equatorial westerlies index UEQ: (a) sea level pressure; (b) wind vector; (c) wind speed; (d) sea surface temperature (SST). Isoline spacing is.2, dashed lines indicate negative values, shading represents signi cance at the 5% level, and rectangles delineate the domain of index UEQ.

SURFACE WIND DYNAMICS OVER THE INDIAN OCEAN 513 TABLE 2. MATRIX OF CORRELATION COEFFICIENTS UEQ SPW SVW STW SPE SVE SPW C53** SVW C26 C16 STW 65** 22 17 SPE 74** 8 19 C64** SVE 59** 27 6 C6** C55** STE C64** C16 C43** 23 4* 37* Values are given in hundredths, with one and two asterisks denoting signi cance at the 5% and 1% levels, respectively. The period is 1958 97 and time series are of October November values. See Fig. 1 for details of the circulation indices. in Table 2, there is much spatial coherence: correlations range from C.7 to C.9 for SPW/TW, SPE/PE, SIP/PE, STE/TE (Fig. 1). Regarding the eastern domain in Table 2, the negative correlations of UEQ with SPE and SVE are consistent with the above discussion and Table 1, and the negative correlation between SVE and STE is plausible from the wind effect (evaporation, stirring, Ekman transport) on SST. Figure 7 illustrates the seasonal evolution of correlations with UEQ, and shows an increase until October for SPE and STE, and to November for SVE. For the western domain matters appear more intriguing. Table 2 and Fig. 6 show correlations for UEQ with SPW are strongly positive, with SVW are positive, and with STW are strongly negative, all consistent with earlier ndings (Hastenrath et al. 1993). The seasonal evolution of correlations with UEQ of October November in Fig. 7 shows a strengthening through November for SPW, but only through October for SVW and STW. High pressure in the west through November is functionally favourable for strong equatorial westerlies; by contrast, the south-westerlies of the boreal summer monsoon, conducive to cold waters in the west (through evaporation, stirring, and Ekman-induced coastal upwelling), decay by October (Figs. 2(c) and (d)). With reference to Fig. 2, the maps of Figs. 6(b) and (c) indeed re ect for strong UEQ some lingering on of the summer south-west monsoon and delay of the winter north-east monsoon. Cold water anomalies in the west (Fig. 6(d)) are known to be characteristic of the wake of good Indian summer monsoons (Hastenrath and Polzin 23; Hastenrath et al. 1993). In synthesis from Table 2 and Figs. 6 and 7, UEQ may have departures concurrent with those in the east (SPE, SVE, STE; Figs. 6(a), (c) and (d)) in some years, and anomalies simultaneous with those in the west (SPW, SVW, STW; Figs. 6(a), (c) and (d)) in other years. Thus, the interannual variability of the boreal autumn equatorial westerlies is on the one hand associated with strong anomalies of pressure, wind and SST in an eastern domain, and on the other hand also with signi cant anomalies in a western domain, although there is little common variance between SSTs in the west and east, nor between pressure in the west and east. The characteristics of the basin-wide surface wind eld identi ed in Figs. 6(a) and (b) are also relevant to climatic change on longer time-scales. Thus, in the era of abundant rainfall, high lake stands and extensive mountain glaciers in East Africa before the 188 s, the boreal autumn equatorial westerlies and concomitant upper-hydrospheric Wyrtki Jet were slack, and the south Indian Ocean trades strong; this contrasts with fast westerlies and weak trade winds in the drier East African climate of the 2th century (Hastenrath 1975, 1984, 1994, 1997, 21; Hastenrath and Larson 1993). Consistent with this long historical background, a negative association between variability in the south Indian Ocean pressure and the equatorial westerlies appeared in a recent numerical modelling experiment (Lau and Nath 24).

514 S. HASTENRATH and D. POLZIN +1 1 J A S O N ON J A S +1 O N ON J A S O N ON 1 +1 J A S O N ON 1 Figure 7. Correlations between UEQ in October November 1958 97 and selected indices in July, August, September, October, November, and October November. Horizontal dashed lines denote signi cance at the 5% and 1% levels, respectively. See Fig. 1 for de nitions and domains of indices. A comparative evaluation for boreal spring, not detailed here, showed similar, albeit weaker, relationships regarding ow recurvature and pressure eld in the east; however, consistent with earlier ndings (Hastenrath et al. 1993), during spring pressure in the west and the SST eld exhibit no association with the equatorial westerlies. 6. CONCLUSIONS The characteristics of surface ow over the Indian Ocean differ markedly from the tropical Atlantic and Paci c, where the con uence between the airstreams from the two hemispheres lies to the north of the equator all the year round and zonal pressure gradients are weak. Over the western to central part of these oceans, the southern trade winds continue as south-easterlies right into the con uence; over the eastern part of the basins, and especially in boreal summer, the cross-equatorial ow from the southern hemisphere recurves, with the ITDZ extending from the equator to the recurvature, and the ITCZ from the recurvature northward to beyond the wind con uence. Drastically different circulation conditions are found over the great monsoon ocean. Most impressive and widely noted is the complete reversal of interhemispheric pressure gradient and cross-equatorial airstreams between the winter and summer monsoons. By comparison, only very recently has renewed attention been given to the short transition seasons between the monsoons, when surface westerlies sweep the central equatorial Indian Ocean. Building in particular on a study a quarter of a century ago

SURFACE WIND DYNAMICS OVER THE INDIAN OCEAN 515 (Hastenrath and Lamb 1978), the dynamics of the equatorial westerlies were examined here, drawing on the equations of motion and observational evidence. It is tempting to ascribe the equatorial westerlies of the spring and autumn transition seasons to the perennial eastward pressure gradient, but the details of the dynamics are essential. The eastward pressure gradient allows the southern trade winds to recurve before reaching the equator. The eastward pressure gradient is opposed by the Coriolis acceleration directed to the left of the wind. The latitude of recurvature depends on the imbalance between these two accelerations: with steep eastward pressure gradient and slow trade winds the ow can recurve relatively far south; with slack pressure gradient and fast trades the recurvature occurs only nearer to the equator. The farther south the recurvature takes place, the broader the equatorial band in which westerlies can develop. These elementary dynamics are pertinent to both annual cycle and interannual variability. Considering the annual cycle, recall that subtropical highs and trade winds tend to be closest to the equator and strongest in their respective winter. However, it so happens that the eastward pressure gradient in the equatorial zone of the Indian Ocean is also steepest during boreal summer. Consequently, the conditions most favourable for recurvature of the southern trade winds at relatively high southern latitudes are not found at the middle of the boreal summer half-year, but rather in spring and autumn, when the zonal pressure gradient is not yet at its peak and nor are the southern trade winds. It is during these short transition seasons that the recurvature is farthest south, allowing a broad equatorial band for the westerlies to develop. Of particular interest are the equatorial westerlies of boreal autumn which are part of a zonal circulation cell along the Indian Ocean equator, which in turn plays an important role in the climate dynamics of the region. The interannual variability of the boreal autumn equatorial westerlies must likewise be seen in the context of the equations of motion: weak westerlies accompany not only slack zonal pressure gradient but also fast ow in the downstream portion of the south Indian Ocean trade winds and a recurvature near the equator. Weak surface westerlies, in turn, entail a weak equatorial zonal circulation cell with weak vertical motion at the western and eastern extremities of the basin. Such circulation departures are typical of oods along the coast of East Africa and concurrent de cient rainfall in Indonesia. ACKNOWLEDGEMENTS This study is supported by NSF Grant ATM-1161. We thank the Editor and the anonymous reviewers for helpful comments. Baquero-Bernal, A., Latif, M. and Legutke, S. Behera, S. K., Krishnan, R. and Yamagata, T. Birkett, C., Murtugudde, R. and Allan, T. Black, E., Slingo, J. and Sperber, K. R. Clark, C. O., Webster, P. J. and Cole, J. E. REFERENCES 21 On dipole-like variability of sea surface temperature in the tropical Indian Ocean. J. Climate, 15, 1358 1368 1999 Unusual ocean atmosphere conditions in the tropical Indian Ocean during 1994. Geophys. Res. Lett., 26, 31 34 1999 Indian Ocean climate event brings oods to East Africa s lakes and the Sudd Marsh. Geophys. Res. Lett., 26, 131 134 23 An observational study of the relationship between excessively strong Short Rains in coastal East Africa and Indian Ocean SST. Mon. Weather Rev., 131, 74 94 23 Interdecadal variability of the relationship between the Indian Ocean zonal mode and East African coastal rainfall anomalies. J. Climate, 16, 548 554 Hastenrath, S. 1975 Glacier recession in East Africa. Pp. 135 142 in: Proceedings of the WMO/IAMAP symposium on long-term climatic variations, Norwich, England, August l975. WMO No. 421, Geneva, Switzerland

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