1 INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 17, (1997) INTERANNUAL VARIABILITY OF SOUTH-EASTERN AFRICAN SUMMER RAINFALL. PART II. MODELLING THE IMPACT OF SEA-SURFACE TEMPERATURES ON RAINFALL AND CIRCULATION ALFREDO ROCHA Departamento de Física, Universidade de Aveiro, 3800 Aveiro, Portugal AND IAN SIMMONDS School of Earth Sciences, University of Melbourne, Parkville, Victoria, 3052, Australia Received 29 September 1995 Revised 28 June 1996 Accepted 4 July 1996 ABSTRACT This study is concerned with the possible physical link between global sea-surface temperatures (SSTs) and south-eastern African summer rainfall. We have performed a series of general circulation model (GCM) experiments, where the model atmosphere has been forced with certain SST anomaly patterns. These have been identified by Rocha and Simmonds (in part I) to be related to drought conditions over the subcontinent. Results show that anomalously warm SSTs in the tropical Pacific and Indian Oceans, typical of ENSO events, can generate dry conditions over much of south-eastern Africa. However, those in the central Indian Ocean, which are partially independent of ENSO, dominate the rainfall response. Sea-surface temperatures in the Atlantic Ocean have little or no effect on rainfall. In the model, warming of the central Indian Ocean generates low-level cyclonic atmospheric anomalies there, which weaken the predominantly eastern flow across the eastern coast of Africa. As a result, less moisture enters the continent and reduced precipitation takes place. Cool SSTs in the south Indian Ocean further enhance this scenario. Warm surface waters in the central and eastern Pacific Ocean generate upper-level westerly wind anomalies, which extend eastwards across into the Indian Ocean. Such upper-level wind changes have been related previously to ENSO and southern African drought. KEY WORDS: south-eastern Africa; general circulation model; sea surface temperature; atmospheric circulation; summer rainfall. 1. INTRODUCTION Year-to-year variability of south-eastern African summer rainfall and its possible association with the El Niño Southern Oscillation (ENSO) and circulation characteristics have been investigated previously by the authors (Rocha and Simmonds, 1997; hereafter referred to as RS). That study revealed that ENSO is only moderately related to rainfall over a small region in south-eastern parts of southern Africa. By contrast, much stronger links were found with a geostrophic index over the Indian Ocean (the Brandon Marion Index: BMI) suggesting that the atmospheric circulation over the western Indian Ocean is more important than that over the Pacific Ocean in influencing south-eastern African rainfall. A partial correlation analysis has revealed than when the influence of ENSO is removed the BMI rainfall relationship remains strong. It has also been shown in RS that large coherent areas of positive SST anomalies are present over the tropical Pacific and Indian Oceans, before and during dry summers. A partial correlation analysis has revealed that SST rainfall correlations remain high over the Indian Ocean when the influence of ENSO is removed, whereas those over the Pacific Ocean become insignificant. Well-defined changes in the regional atmospheric circulation were identified as occurring during dry summers. Mean sea-level pressure drops over the central Indian Ocean and low-level cyclonic circulation is present north-east of Madagascar. As a result, low-level westerly wind CCC /97/ $17.50 # 1997 by the Royal Meteorological Society
2 268 A. ROCHA AND I. SIMMONDS anomalies are observed along much of the eastern coast, thereby weakening the predominantly easterly flux of moisture inland. The aim of the present study is to test the hypothesis that the mechanism described above has a physical basis, and that SST anomalies can generate dry summers over south-eastern Africa by inducing atmospheric circulation changes. Here a series of experiments is carried out with the Melbourne University General Circulation Model (MUGCM) in which SST anomalies were prescribed as a boundary forcing. We should mention that, as in RS, this study is not concerned directly with the synoptic systems whereby the large-scale circulation changes generate rainfall anomalies. Rather, attention is focused upon the large scale circulation adjustment to SST forcing and consequent rainfall changes. 2. THE MODEL Relevant aspects of the MUGCM are described in Simmonds and Lynch (1992) and, therefore, only a general outline of its characteristics will be given here. The horizontal variance of most variables is represented in terms of spherical harmonic series which are rhomboidally truncated at wavenumber 21. The prognostic variables are represented at nine s vertical levels. The version of the MUGCM used here includes the seasonal but not the diurnal cycle. The model uses envelope topography spectrally analysed data from the data set of Gates and Nelson (1975). Soil moisture content is computed by a two-layer scheme developed by Deardorff (1977). Surface layer energy fluxes are calculated using the Monin Obukhov similarity theory described in Simmonds (1985) and Simmonds and Dix (1989). Precipitation can be generated by the large-scale circulation, whenever the relative humidity reaches 100 per cent, and by convective processes. In the latter, the model uses the moist convective adjustment scheme of Manabe et al. (1965), later modified by Weymouth (pers. comm.). In this study the model atmosphere is forced with prescribed SST anomalies and one hopes the model s atmospheric response to be an indication of how the real atmosphere would evolve under the same influences. Of course, due to many aspects, GCM solutions are approximations only of real atmospheric processes. Moreover, part of the climate system s variability is not present because, by prescribing SSTs, atmospheric ocean feedbacks are not allowed to take place. Nevertheless, meaningful interpretations of the SST sensitivity experiments can still be fruitful if the model is able to reproduce, with reasonable accuracy, the currently observed climate by prescribing observed climatological conditions. Changes in the mean state of the model are assessed against the natural variability of the model s climate. To estimate the statistical significance of a particular change as a result of imposed forcing, Student s t-test, as described by Chervin and Schneider (1976), is used in this study. Owing to the relatively low resolution of the model, atmospheric changes, particularly those of rainfall, generated by SST anomalies in the model should not be investigated over small regions and cannot be strictly compared with the results of the observational analysis performed in RS. Rather, this paper is concerned with large-scale signals. The model generates creditable simulations of the present climate (Simmonds et al., 1988). An intercomparison of the climates simulated by 14 GCMs (including the MUGCM) and the present climate is summarized by Boer et al. (1992). The climate generated by the MUGCM is comparable to that of the other GCMs and matches well with the observed. A detailed assessment of the performance is reported by Lynch (1994). A considerable number of sensitivity climate studies have been carried out with the MUGCM, in particular on the SST forcing in the tropics and related circulation and rainfall changes (e.g. Simmonds and Smith, 1986; Budd and Simmonds, 1990; Simmonds, 1990; Rocha, 1990, 1992). 3. DESIGN OF EXPERIMENTS In RS we identified a pattern of SST anomalies related to the most important spatial mode of rainfall interannual variability of south-eastern Africa (identified there as region 1). That pattern appears to be made up of two parts. The first of these and the most common is associated with ENSO. Positive SST anomalies appear in the central and eastern tropical Pacific and central Indian Oceans 6 months before dry summers (JJA). These anomalies strengthen 3 months later (SON), and decay during the peak of summer (DJF) and 3 months later (MAM). The second part is related to abnormal warmth only in the central Indian Ocean and is basically independent of ENSO. Also evident from the correlation analyses performed in RS is a tendency for SST anomalies in the Indian
3 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 269 Ocean north of about 20 S to be out of phase with those to the south before and during dry south-eastern African summers. That is, anomalously warm waters are observed north of 20 S, whereas to the south the ocean surface is abnormally cold. The net effect of these anomalies is a stronger SST meridional gradient in the Indian Ocean. To test the above-mentioned hypothesis, the SST anomaly patterns used to force the model s atmosphere were constructed in the following way. The SST rainfall correlation patterns shown in Figure 12 of RS, plus that for MAM, were translated to SST anomalies by weighting, at each grid-point, the correlation by the respective SST temporal standard deviation of the season in question. Subsequently, all values were multiplied by a constant (six) to bring them up to typically observed SST anomalies. Finally, the sign of the anomalies was reversed because here we are concerned with SST anomalies related to dry conditions. The resulting SST anomaly patterns were very similar to the SST composites obtained in RS for region 1. Figure 1 displays these SST anomaly fields for JJA, SON, DJF, and MAM, which correspond to SSTs at 6 and 3 months lead, zero lag, and at 3 months lag, respectively. It should be noted that in the peak of summer (DJF), when most precipitation occurs over south-eastern Africa, SST anomalies are rather weak and their magnitudes are comparable to their interannual variability. The four patterns were subsequently interpolated in time, at each grid-point, to daily patterns from the beginning of June to the end of March. The resultant SST anomaly maps were then added to the daily climatological SSTs, which had been used to perform the control experiment (the climate of the model). Three experiments were performed using the SSTs generated as lower boundary forcing of the model s atmosphere. The first of these used the full domain of the anomalies and hereafter will be referred to as the global experiment. This run enables us to test the hypothesis that dry south-eastern African summers can be caused by the SST anomaly patterns of Figure 1. The second experiment used only the part of the global SST anomaly pattern that lies between 20 E and 180 E and serves to assess the impact of the Indian and western Pacific Oceans on south-eastern African rainfall. Note that the large positive anomalies located in the central and eastern Pacific Ocean are not considered in this experiment. This run will be referred to as the Ind WPac experiment. The third experiment considers only the positive SST anomalies in the Indian Ocean, and tests the importance of the SST meridional gradients in the Indican Ocean (evident in Figure 1) in influencing rainfall over the subcontinent. This experiment is named PInd. It should be noted that the spatial patterns, magnitude, and evolution of the SST anomalies shown in Figure 1 are realistic because, as mentioned earlier, they broadly represent ENSO. In individual ENSO years, SST anomalies often reach values greater than those used here and, therefore, the anomalies of Figure 1 are best representative of a moderate ENSO event. Figure 2 displays the lag correlations between SST anomalies in June and December. Positive significant (5 per cent) correlations are observed over much of the tropical eastern and central Pacific and Indian Oceans, indicating the high temporal persistence of anomalies in those areas, and further supporting the time evolution of SSTs of Figure 1. Each of the three experiments were started on 1 June and ended on 31 March (a total of 10 months). Atmospheric changes as a result of SST anomalies were analysed for the November March period relative to the control. We want to be sure which model responses are above the noise level. For this reason, for each experiment, four integrations were carried out, each starting from different initial conditions, extracted randomly from the control. The statisical significance of the seasonal (November to March) response was then assessed in the manner described by Simmonds and Lynch (1992). However, it is felt that significance of many of the results would have been enhanced considerably if a greater number of integrations had been performed. The results to be analysed and shown below are the ensemble average of four runs for the summer period (November to March) Rainfall and moisture 4. RESULTS Figure 3 displays the precipitation anomalies for the (a) global, (b) Ind WPac and (c) PInd experiments. Stippling indicates differences significant at the 10 per cent significance level. It is evident that rainfall deficits dominate most of south-eastern Africa, particularly in the Global and Ind WPac runs, but significance is barely
4 270 A. ROCHA AND I. SIMMONDS Figure 1. SST anomaly patterns used in the GCM experiments for (a) JJA (6 months before the peak of summer), (b) SON (3 months before the peak of summer) (c) DJF (at zero lag), and (d) MAM (3 months after the peak of summer). The isotherm interval is 015 C. Stippling and hatching indicate values smaller than and ÿ1 C and greater than 1 C, respectively (see text for details)
5 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 271 Figure 2. Temporal persistence of SST anomalies from June to the following December as indicated by the 6 month lag correlations. The isopleth interval is 012. Stippling indicates correlations significant at a 5 per cent significance level achieved in the simulations. When averaged over region 1, these anomalies are ÿ017 mm day 71 (global), 110 mm day 71 (Ind WPac), and ÿ019 mm day 71 (PInd). Significance (10 per cent level) is achieved only in the Ind Wpac simulation. Over the Indican Ocean, where SSTs are anomalously warm, rainfall increases substantially in all experiments and significant differences cover large areas. A consequence of SST forcing is a shift of rainfall from the subcontinent towards the tropical Indian Ocean with anomalies orientated in a northwest south-east direction. In the PInd run, the magnitudes of the rainfall anomalies are similar to those of the other runs but their spatial extent is somewhat smaller over south-eastern Africa than in the global and Ind WPac experiments. The absence of the large positive SST anomalies east of the dateline in the Ind WPac experiment seems to have little impact on south-eastern African summer rainfall. Similarly, the abnormally cold ocean waters in the Indian Ocean south of about 20 S have little effect on the rainfall response over the subcontinent. However, a slightly stronger rainfall signal is obtained when these SST anomalies are included. These results suggest that the abnormally warm SSTs in the central Indian Ocean and small positive SST anomalies in the central and western Pacific are important controls of summer rainfall deficits over south-eastern Africa. Figure 4 displays for the global run the anomalies in the (a) soil moisture and (b) 850 hpa relative humidity fields. These plots, which support the modelled rainfall changes presented above, also show that the low-level moisture and integrated surface moisture availability are greatly diminished over south-eastern Africa. However, as with rainfall, significance is not achieved over large areas. Similar changes were obtained for the other two simulations (not shown). Evaporation changes are observed over the Indian Ocean in all experiments and, in general, positive and negative anomalies coincide with anomalously warm and cold surface waters, respectively. However, these evaporation anomalies never exceed 2 mm day 71 and do not, therefore, explain completely the rainfall changes. Reduced evaporation is observed along the Mozambique Channel in the global experiment and to a lesser extent in the Ind WPac and PInd simulations. These may be explained partly by the local SSTs, which are, on average, about 016 C cooler than in the control run. Weaker low-level easterlies (i.e. westerly anomalies) may also contribute to smaller evaporation rates there. Because SSTs in the Indian Ocean are the same in the global and Ind WPac runs, evaporation differences between these simulations originate in the way the low-level flow adjusts to the SST forcing. An alternative way of evaluating the moisture budget in the atmospheric column is through precipitation minus evaporation (P 7 E) which represents the moisture flux across the lower surface. Changes in P7E are shown in Figure 5 for the (a) global, (b) Ind WPac, and (c) PInd runs. As expected, P 7 E anomalies resemble those of rainfall (see Figure 3) and clearly imply that changes to moisture advection dominate most of the rainfall response. Moisture converges into the Indian Ocean from neighbouring areas, where divergence of moisture is observed. Note that in the PInd simulation, where only Indian Ocean positive anomalies are used, net moisture losses are simulated over Madagascar and to the east, and over the Indonesian region despite increased evaporation there. Further confirmation of the importance of moisture transport is given in Figure 6, which shows the 950 hpa stationary moisture flux anomalies for the (a) global, (b) Ind WPac, and (c) PInd simulations. We show here
6 272 A. ROCHA AND I. SIMMONDS Figure 3. Precipitation anomalies for the (a) global, (b) Ind WPac, and (c) PInd experiments. The isopleth interval is 1 mm day 71. Stippling indicates significant (10 per cent) anomalies
7 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 273 Figure 4. (a) Soil moisture and (b) 850 hpa relative humidity anomalies for the global experiment. The isopleth interval is 1 cm for (a) and 215 per cent for (b). Stippling indicates significant (5 per cent) anomalies only the stationary part of the moisture flux. (Chen (1985) has shown that the total moisture flux in the tropics is mostly explained by its stationary modes.) It is evident from the predominantly westerly moisture flux anomalies along the east coast of Africa that southern Africa loses moisture to the Indian Ocean, which, in turn, also receives moisture from the Indonesian region. The most noticeable aspect is that, in all three experiments, P 7 E anomalies are similar despite the non-inclusion of the central and eastern Pacific SST anomalies in the Ind WPac integration, and the usage of only positive Indian Ocean SST anomalies in the PInd run. As for precipitation, the magnitude of these changes are slightly greater in the Global and Ind WPac than in the PInd runs. One may conclude, thus, that the anomalously warm surface waters in the tropical Indian Ocean are the most important oceanic forcing causing south-eastern African dry summers, and that a slightly stronger signal occurs when either the Pacific and/or the south Indian (negative SST anomalies) Oceans are considered.
8 274 A. ROCHA AND I. SIMMONDS Figure 5. Precipitation minus evaporation (P 7 E) anomalies for the (a) global, (b) Ind WPac, and (c) PInd experiments. The isopleth interval is 1 mm day 71. Stippling and hatching indicate anomalies smaller than ÿ1 mm day 71 and greater than 1 mm day 71, respectively
9 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 275 Figure 6. As in Figure 5 but for the stationary moisture flux at 950 hpa. The longest vector corresponds to 0104 (kg kg 71 )ms 71
10 276 A. ROCHA AND I. SIMMONDS 4.2. Mean sea-level pressure (MSLP) The MSLP anomalies are shown in Figure 7 for the (a) global and (b) Ind WPac experiments. Pressure anomalies in the PInd experiment (not displayed) are almost identical to those in the Ind WPac run. As expected, the pressure changes when the full SST anomaly pattern is considered (in the global run) reflect ENSO, with positive anomalies over the Indonesian region and anomalously low pressures over the central and eastern Pacific Ocean. Over the tropical Atlantic and Indian Oceans, pressure also increases. Differences in the MSLP field are significant over large areas of the tropical oceans, with the exception of the Indian Ocean, where significance is achieved only over a relatively small area near Madagascar. Over south-eastern Africa pressures are higher than normal but these changes are not significant. In the Ind WPac integration, the MSLP response is confined mostly to the tropical Indian Ocean, where much of the basin experiences significantly lower pressures. Interestingly, anomalies over the Indian Ocean and over south-eastern Africa are very different in the global and Figure 7. MSLP anomalies for the (a) global and (b) Ind WPac experiments. The isobar interval is 1 hpa. Stippling indicates significant (5 per cent) anomalies
11 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 277 the other two runs, despite the presence of the same SST anomalies there (slightly different in the PInd run). The GCM experiments performed by Fenessy et al. (1985) and Cubasch (1985), which used anomalously warm SSTs in the central and eastern Pacific Ocean, showed that over the whole Indian Ocean, MSLP increases in a similar way to the global run performed here. This suggests that these differences may originate in some mechanism at higher levels triggered by the remote, ENSO-related, SST anomalies in the central and eastern Pacific Ocean (in the global run). It is clear that in the Ind WPac and PInd experiments, the Indian Ocean positive SST anomalies are responsible for the modelled lower than normal MSLP there. This reduced MSLP, which favours convection, is consistent with the modelled precipitation increase over the same area. The abnormally high MSLP over the subcontinent suggested by the observational work in RS as being typical of south-eastern African summers is not neatly reproduced by the model. However, in the Ind WPac and PInd experiments, significantly large MSLP reductions are simulated over Brandon Island, east of Madagascar. This is consistent with the strong positive relationship found between summer rainfall over most of south-eastern Africa and the BMI Low-level circulation At 850 hpa over the Indian Ocean, the atmosphere undergoes major changes in response to the anomalously warm waters there. Figure 8 displays the wind vector anomalies for the (a) global and (b) Ind WPac simulations. Stippling indicates significant (5 per cent) differences in only the zonal wind component. The plot for the PInd experiment is not shown, because it is very similar to that for the Ind WPac run. The low-level circulation responds to the SST forcing over the Indian Ocean, with westerly and easterly wind anomalies to the west and east of the area of forcing, respectively. Indeed in all three experiments, but more evidently in Ind WPac and PInd, the low-level reorganization of the atmosphere is reminiscent of the linear response of the tropical atmosphere to a diabatic heating anomaly described by Gill (1980) and Heckley and Gill (1984). Typical is the cyclonic circulation anomaly located east of Madagascar and poleward of the warm water. This low-type anomaly in the Ind WPac run introduces a northerly component in the south-east Trade winds near the southeast coast of Africa. As a result, moisture carried inland along the Trades is diverted north-eastwards towards the central Indian Ocean. To the north, the north-east monsoonal circulation, also an important source of moisture for southern Africa, is subject to a strong easterly anomaly in all experiments, which acts to reduce the influx of moisture into the subcontinent. Also, the boundary between easterly and westerly anomalies is located much further east in that run. It should be emphasised again that relatively small SST anomalies over the Indian Ocean in summer can generate quite large and spatial coherent low-level circulation changes. Two factors contribute to this. Firstly, during summer the ITCZ is located over the Indian Ocean at about 5 S, coinciding with the location of the largest SST anomalies in our experiments. Along the ITCZ, which is a zone of strong low-level convergence and weak horizontal circulation, positive SST anomalies can readily enhance the upward motion and convection, therefore amplifying the low-level convergence and rainfall. Secondly, SSTs over the central Indian Ocean in summer are, on average, about 28 C and, thus, slightly above the critical temperature of 27 C as reported by Graham and Barnett (1987) over which deep convection is dramatically enhanced. An increase of about 1 C (SST anomalies in SON over the central Indian Ocean) would further intensify the ITCZ. Comparing the global simulation with the other two simulations, it is worth noting that, although the modelled easterly wind anomalies centred on the Indonesian region are very similar to the west, westerly anomalies are stronger in the latter experiments. Also the boundary between easterly and westerly anomalies is located much further east in those runs. Again, because SST forcing is the same in the adjacent areas, an explanation for the relatively weaker westerly changes in the global run must be sought at higher levels. During DJF a strong ENSO event was underway, westerly 850 hpa wind anomalies were observed along the south-eastern coast of Africa and a cyclonic circulation anomaly was located east of Madagascar (Arkin et al., 1983). These features are present in all the experiments. Lindesay (1988) and Walker (1989) have also reported northerly and westerly wind anomalies along the south-east coast during dry conditions over the summer rainfall region of South Africa.
12 278 A. ROCHA AND I. SIMMONDS Figure 8. As in Figure 7 but for wind anomalies at 850 hpa. Stippling indicates significant (5 per cent) changes in the zonal wind component at the same level. The longest vector corresponds to 312 ms Upper level circulation Figure 9 shows, for the 200 hpa level, the wind vector anomalies for the (a) global and (b) Ind WPac experiments. Stippling indicates significant (5 per cent) differences in the zonal wind component. As was found for the 850 hpa, circulation changes at this level for the PInd are very similar to those in the Ind WPac experiment and are, therefore, not shown. In the Ind WPac and PInd simulations the upper atmosphere adjusts to the SST forcing, with circulation anomalies being roughly the reverse of those found at 850 hpa. Anomalously divergent flow is observed over the central Indian Ocean, where warm SSTs and low-level convergence anomalies are located. Note that the upper level anticyclonic anomaly east of Madagascar is placed above its cyclonic counterpart at 850 hpa. In the global run, westerly wind anomalies emanate from the eastern Pacific Ocean. These are associated with an anticyclonic
13 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 279 Figure 9. As in Figure 8 but for 200 hpa. The longest vector corresponds to 610 ms 71 couplet anomaly located in the central Pacific Ocean, evident in the 200 hpa streamfunction anomaly for the global simulation (not shown). This anomalous feature was observed during the ENSO event (although stronger than that modelled here possibly because SST anomalies in were also stronger than those used in this study) and has been modelled by many GCM experiments that were forced with ENSO like SSTs (e.g. Boer, 1985; Palmer, 1985; Tourre et al., 1985). This feature was not modelled in either the Ind WPac or PInd simulations, confirming that SSTs in the central and eastern Pacific, or possibly the full SST pattern in the Pacific Ocean (which imply a weaker SST zonal gradient), cause this feature. These 200 hpa westerly anomalies extend eastwards into the Atlantic Ocean and southern Africa, and link with the anomalies of the same sign over the eastern Indian Ocean. However, over the equatorial western Indian Ocean these anomalies are offset by the strong easterly anomalies observed in the Ind WPac and PInd runs and, as a result, insignificant circulation changes are observed there. Indeed, in the GCM experiments performed by Palmer (1985) and Fenessy et al. (1985), where only the Pacific portion of an El Niño-type SST anomaly field was used,
14 280 A. ROCHA AND I. SIMMONDS westerly circulation changes were modelled over the whole tropical Indian Ocean. The most important aspect relevant to southern Africa is the fact that westerly wind anomalies are present in all three simulations over the subcontinent, but somewhat stronger and occupying a larger area in the global experiment. The model is successful in simulating the anomalous winds that are known to be typical of ENSO events (Arkin, 1982) and related to dry conditions over the sub continent (Lindesay, 1988; RS). In all simulations 200 hpa wind anomalies were modelled over southern Africa, although stronger changes were observed in the global run Walker Circulation Figure 10 shows the 200 hpa velocity potential and the divergent wind component for the (a) control and the respective anomalies for the (b) global and (c) PInd experiments. The climatology of the model shows upper level divergent circulations over the Indonesian region that are associated with the low-level convergence, upward motion, and high precipitation characteristic of that region, particularly during summer. Over the Atlantic, and extending westwards into the eastern Pacific Ocean, a convergence centre is observed which implies sinking motion. In general, these east-west circulations are matched by reverse circulations in the lower troposphere in what constitutes the Walker Circulation. These two main cells of the Walker Circulation are interrupted by secondary centres over Africa and South America. As a result of SST forcing, large changes take place in the 200 hpa velocity potential field. In the global run a major anomalous convergence centre is situated over the Indonesian region, which has the net effect of weakening the rising branch of the Walker Circulation there. Weaker divergent anomalies are located over the Indian, and central and eastern Pacific Oceans. In the PInd and Ind WPac experiments the Indian and the western Pacific dipole of the global run is reproduced, but with relative intensities being reversed. Also the positive pole has shifted slightly eastwards. Interestingly, despite the absence of SST forcing over the western Pacific Ocean in the PInd simulation, changes are observed in the velocity potential field over there as in the Ind WPac simulation. The anomalies shown here are consistent with the precipitation and low and upper level wind responses analysed above. They imply a weaker Walker Circulation, and a shift of rainfall from southeastern Africa towards the central Indian Ocean, related to the strength and position of the Walker Circulation cells Vertical structure of the atmospheric responses Meridional circulation changes are investigated here for the E sector, which encompasses most of southern Africa east of 25 E. Figure 11 displays meridional cross-sections of the mass transport streamfunction of the model s climatology (control run) averaged over this domain. The ITCZ position can be inferred by the zero isopleth (which indicates null mass transport) adjacent to areas of upward motion. Over the land, the ITCZ mean summer position (which is determined primarily by the maximum low-level heating near the thermal low) is at about 15 S. Over southern Africa east of 25 E, the structure of the Hadley cells is complex, in the sense that to the south of the ITCZ equatorward low-level flow occurs over a limited latitudinal band that extends from 15 to 19 S. This band corresponds to the inland entrance region of the south-east Trades across the south-eastern coast, where the flow gains a northerly component (see also Figure 20(b) of RS). The vertical structure of the simulated Hadley cells over southern Africa east of 25 E agrees well with that described by Lindesay (1988) along the 30 E meridian for the JFM season (her Figure 9(d)). The climatologies and anomalies of the Hadley circulations over land for all three experiments are shown in Figure 12. Figures 12(a) to 12(c) show the modelled Hadley circulation whereas parts 12(d) to 12(f ) display the respective anomalies. The ITCZ position at the surface in the global simulation is unaffected, but has shifted equatorwards to 12 S and 13 S(a3 and 2 equatorward shift compared with the control run) in the Ind WPac and PInd experiments, respectively. It can, therefore, be concluded that dry south-eastern African summers are not related unambiguously to more equatorward ITCZ positions because in the global run, where the spatial coherence of the rainfall anomalies is the greatest, the ITCZ remains at 15 S. Weakening of the Hadley cells along the 30 E meridian have been reported by Lindesay (1988) to occur over
15 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 281 Figure 10. Velocity potential and divergent wind at 200 hpa for the (a) control and respective anomalies for the (b) global and (c) PInd experiments. The isopleth interval is m 2 s 71 for (a) and m 2 s 71 for (b) and (c). The longest vector corresponds to 2115 ms 71 in (a), 412 ms 71 in (b), and 517 ms 71 in (c)
16 282 A. ROCHA AND I. SIMMONDS Figure 11. Meridional cross-section of the mass-transport streamfunction averaged over southern Africa (25 45 E) for the control. The isopleth interval is Kg s 71. The data have been normalized (in this case multiplied by 18) so that the values give the mass-transport per unit circumference at each latitude southern Africa during ENSO. Shinoda (1990) has attributed the dry southern African summers that occurred between 1950 and 1970 to a weakening of the ITCZ, rather than changes in its position. Harrison (1986) has also reported that interannual changes of the ITCZ intensity, rather than latitudinal shifts in its position, are the main cause of rainfall anomalies over southern Africa. Important here is that SST anomalies are able to generate dry conditions over south-eastern Africa associated with both a weaker ITCZ (all experiments) and a more equatorward ITCZ position (in the Ind WPac and PInd simulations). Despite the same SST forcing over the Indian Ocean in the global and Ind WPac runs, the differences in the modelled ITCZ position may be ascribed to the stronger and larger spatial coverage of the 200 hpa westerly anomalies simulated in the global experiment as a result of SST anomalies in the central and eastern Pacific Ocean. Such wind anomalies have been related to dry conditions over southern Africa, due to a weaker vertical wind shear, which inhibits convection (Harrison, 1986). The vertical distribution of atmospheric temperature over the Indian Ocean reflects strongly the SST anomalies there. Figure 13 displays these anomalies averaged between 50 and 115 E for the global run (note that the latitudinal scale in these and subsequent cross sections is different from those shown above in Figures 11 and 12). North of about 20 S, SST warming propagates deep into the atmosphere and statistically significant anomalies are simulated up to the upper atmosphere. This warming is due to increased evaporation over the Indian Ocean and subsequent latent heat release at higher levels. To the south, significant cooling takes place over the anomalously cool waters. In the Ind WPac and PInd experiments temperature anomalies (not displayed) were similarly modelled, but the cooling between 20 and 35 S is weak. Thus, the strong warming throughout the atmosphere over the tropical Indian Ocean appears to be caused by the warm waters there. Over southern Africa east of about 25 E, similar atmospheric temperature changes take place in all simulations. These are shown in Figure 14 for the PInd experiment. Significant warming is observed only over the northern parts of southern Africa, and the cooling south of about 20 S is not present in this run but is evident in the global and Ind WPac experiments (not shown). Temperature changes over land, particularly for the global and Ind WPac runs (where cooling occurs south of about 20 S), imply a stronger Hadley circulation over southern Africa east of 25 E, contrary to what has been shown in Figure 12. This may be because the poleward Hadley cell (which is confined between about 12 and 20 S in the lower levels) is, in fact, located within the area
17 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 283 Figure 12. Meridional cross-sections of the mass-transport streamfunction averaged over southern Africa (25 45 E). The data have been normalized (in this case multiplied by 18) so that the values give the mass transport per unit circumferance at each latitude. (a) global, (b) Ind WPac and (c) PInd and the respective anomalies for the (d) global, (e) Ind WPac and (f) PInd experiments. The isopleth interval is Kg s 71 in (a), (b) and (c), and Kg s 71 in (d), (e) and (f) of warming. Therefore, little or no change in the meridional temperature gradient is observed at Hadley Circulation latitudes, between 25 and 45 E. Temperature changes over the subcontinent are at least partly caused by westward advection of those over the Indian Ocean. This factor clearly prevails over the cooling associated with the weaker condensational warming as a result of drier conditions (note that in the model cloudcover amounts are kept fixed and, therefore, changes in insolation caused by anomalous cloud cover do not take place).
18 284 A. ROCHA AND I. SIMMONDS Figure 13. Meridional cross-section of temperature anomalies averaged over the Indian Ocean ( E) for the global experiment. The isotherm interval is 0125 C. Stippling indicates significant (5 per cent) anomalies Figure 14. Meridional cross-sections of temperature anomalies averaged over southern Africa (25 45 E) for the PInd experiment. The isotherm interval is 0125 C. Stippling indicates significant (5 per cent) anomalies 5. DISCUSSION The results indicate that the MUGCM is capable of simulating the complicated and interactive physics associated with the SST-rainfall connection reported in RS. However, most of the rainfall and circulation changes modelled were similarly reproduced when only the positive SST anomalies in the Indian Ocean were used (i.e. PInd simulation). In fact, warming tends to occur over the central Indian Ocean during ENSO events, but it has also
19 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 285 Figure 15. Relative humidity anomalies at 500 hpa for the Ind WPac experiment. The isopleth interval is 215 per cent. Stippling indicates significant (5 per cent) anomalies occurred independently of ENSO in the past (e.g. in ). Slightly stronger atmospheric responses were obtained when the SST anomalies in the western Pacific Ocean were added (i.e. Ind WPac). Further small increases were modelled when the full SST anomaly pattern was considered (i.e. global run). In general, rainfall deficits over south-eastern Africa are generated by weaker than normal low-level horizontal fluxes of moisture being directed inland by the south-east Trades and north-east monsoon. This arises as a result of the SST warming in the central Indian Ocean, which sets up cyclonic low-level circulation anomalies located east of Madagascar. As a result, moisture is diverted from the Trades and monsoonal air masses towards the central Indian Ocean. Precipitation increases over the Indian Ocean at the expense of moisture losses over south-eastern Africa and Indonesian region. In the Ind WPac and PInd experiments the ITCZ over south-eastern Africa shifted 2 3 equatorwards, but in the global run, where similar rainfall and low-level circulation anomalies were obtained, the ITCZ remained at its normal position at about 15 S. In the global run, the Walker Circulation weakens in agreement with what has been reported in many observational (Rasmusson and Arkin, 1985; Lindesay, 1988) and modelling (Boer, 1985; Gordon and Hunt, 1991) studies of ENSO. Easterly upper-level wind anomalies are present over the equatorial Pacific Ocean. These are associated with an upper-level twin anticyclone, which also sets up westerly anomalies on its poleward sides. Such westerly changes propagate eastwards into the Atlantic and are evident over much of southern Africa. Harrison (1983, 1986) has shown that weaker 200 hpa easterlies are related to dry conditions over southern Africa. Changes in the 200 hpa velocity potential field and in the zonal equatorial mass transport imply anomalously rising motions over the central Indian Ocean and subsidence over Africa and the Indonesian region. The vertical profile of temperature anomalies over the Indian Ocean shows, for all three experiments (see Figure 13 for the global experiment), significant warming and cooling of the 200 hpa level north and south of about 20 S, respectively. These changes broadly coincide with the anomalously warm and cool surface oceanic waters there. The SST forcing in the equatorial Indian Ocean is, due to the weakness of the advective terms near the ITCZ, able to propagate vertically into the upper levels through surface evaporation, and latent heat release at higher levels. This warming (and cooling south of 20 S) is subsequently advected westwards towards the African continent, where similar but less significant temperature changes occur. The boundary separating warming and cooling shifts polewards to about 30 S. Indeed, the correlation analyses between summer rainfall in region 1 and temperature at 850 and 500 hpa undertaken in RS (their figure 18) also imply widespread, significant warming over much of the subcontinent during dry DJF seasons. The synoptic wave pattern in the 500 hpa relative humidity field reported in RS (see Figure 19(b) of RS) typical of dry region 1 summers is also reproduced by the model, as shown in Figure 15 by the respective anomalies for the Ind WPac simulation. As in Figure 19(b) of RS, strong negative anomalies are located over south-eastern Africa, whereas to the east, large increases are present over the Indian Ocean. A smaller maximum
20 286 A. ROCHA AND I. SIMMONDS Figure 16. Differences between the Ind WPac and PInd experiments for the (a) MSLP and stationary moisture flux at 950 hpa, and (b) velocity potential and divergent wind at 200 hpa. The isopleth interval is 015 hpa in (a) and m 2 s 71 in (b). The longest vector corresponds to 0103 kg kg 71 ms 71 in (a) and 3 m s 71 in (b). Stippling in (a) indicates values greater than 015 hpa is located over south-west Africa, extending south-eastwards and resembling the positive rainfall anomalies over the same area shown in Figure 3(a and b). In the global and PInd runs, changes in the relative humidity at 500 hpa are similar to those in the Ind WPac, but the significant negative differences over the land are not significant in the PInd simulation. Although it has been shown that anomalously warm SSTs in the central Indian Ocean dominate the atmospheric and rainfall response over south-eastern Africa and the Indian Ocean, some consideration must be given to the relative importance of anomalies in the west Pacific and the negative SST anomalies in the Indian Ocean (both not considered in the PInd experiment. Figure 16 displays differences between the Ind WPac and PInd runs for (a) 950 hpa stationary moisture flux and MSLP and (b) 200 hpa velocity potential and divergent wind. A cyclonic anomaly is present at 950 hpa over the central Indian Ocean with a weaker anticyclonic counterpart to the south. These anomalies, together with another anticyclonic change off the south-east coast of Africa, generate northerly moisture flux anomalies along most of the Mozambique Channel. All these low-level changes can be attributed to a stronger SST meridional gradient in the Ind WPac compared with the PInd run. Note that anticyclonic anomalies associated with increased MSLP prevail over the anomalously cold waters south
21 SOUTH-EAST AFRICAN SUMMER RAIN: PART II 287 of about 20 S. However, anomalously cyclonic flow is present to the north, despite no differences in the SST field in the two runs. This suggests that the low-level anomalies stem from the relatively steeper SST meridional gradient in the Ind WPac experiment. On the other hand, only weak low-level circulation changes are observed propogating westwards from the western Pacific Ocean, implying that SST anomalies there (not used in the PInd run) appear to have little relevance to the low-level circulation over the Indian Ocean and south-eastern Africa. Anomalous upper-level divergence occurs over the western Pacific, associated with the positive SST anomalies just west of the dateline, and over the equatorial Indian Ocean. Over eastern Africa, anomalous convergence takes place. This suggests that the inclusion of the western Pacific SSTs in the Ind WPac experiment shifts the rising branch of the Walker Circulation over Africa more to the east than in the PInd run. This scenario agrees well with the rainfall anomalies shown in Figure 3, which display not only smaller magnitudes over the Indian Ocean and south-eastern Africa, but also a relative westward shift of the rainfall maximum in the PInd simulation. It has been shown that, despite the dominance of abnormally warm surface waters in the equatorial Indian Ocean in generating dry south-eastern African summers, small SST anomalies over the western Pacific and anomalous cooling in the south Indian Ocean enhance rainfall and circulation changes over the region. It should be emphasized that the magnitude of the SST anomalies used in the experiments, particularly those over the tropical Indian and Pacific Oceans, are smaller than those observed in years of extreme warming there. This may partly explain why the modelled rainfall deficits over south-eastern Africa are not significant despite covering a large area. During the peak of summer, when most rainfall occurs, SST anomalies are relatively small, but at this time the ITCZ is well established over south-eastern Africa (at around 15 S), as well as over the equatorial Indian and Pacific Oceans where the temperature of the water surface has increased (east of about 150 E in the Pacific Ocean). Horizontal advection is minimal along the ITCZ, allowing relatively small heating perturbations to propagate vertically into the atmosphere. As a more intense ITCZ develops over the ocean, extra moisture is not only generated by local increase in evaporation, but predominantly by advection from neighbouring areas, such as south-eastern Africa and the Indonesian region. Steeper meridional SST gradients in the Indian Ocean, as a result of negative SST anomalies to the south, further enhance these changes. It is proposed therefore that the Indian Ocean SSTs influence south-eastern African summer rainfall, mainly through a redistribution of moisture transport in the region. The SSTs in the western Pacific Ocean (west of 180 E) contribute to an eastward shift of the Walker Circulation ascending branch over the southern African region. Anomalously warm surface waters in the central and eastern Pacific generate strong upper-level westerly anomalies which propagate to the east across South America into the Atlantic and over southern Africa. It is not the objective of this study to investigate the synoptic-scale phenomena whereby these upper-level westerlies modulate south-eastern Africa rainfall, but Harrison (1986) has identified such physical mechanisms. It is believed that although ENSO dominates summer rainfall over south-eastern Africa, independent Indian Ocean SST variability may explain the relatively weak SOI-rainfall relationship identified in RS. Our study has implications for the predictability of south-eastern African summer rainfall using SSTs. It has been shown in RS that coherent SST anomaly patterns develop well before, and are associated with, anomalies in the rainy season and that the strongest relationships exist 1 to 3 months before. Assuming that SST anomalies larger than those used here develop in some years, one would hope, despite the absence of some interactive aspects of air-sea processes in the MUGCM, that the model would be able to successfully simulate large scale rainfall anomaly patterns over the subcontinent. However, the dependence of model solutions on initial conditions is a consideration. Predictability is hampered, or even lost, if integrations with the same SST but with different initial conditions give rather unrelated solutions. Predictability may be affected by how the initial soil moisture is prescribed. This may be an important factor during individual years and needs to be studied further. There is some degree of consistency in all solutions (not displayed)in the sense that wet and dry conditions are simulated over the central Indian Ocean, and southern Africa (particularly in its eastern parts), and the Indonesian region, respectively. However, positive anomalies are modelled in some integrations over the subcontinent. The results suggest that SST anomalies, particularly those over the Indian Ocean, can be used to forecast largescale south-eastern African summer rainfall anomalies. However, it may not be possible to provide a definite quantitative forecast, merely a qualitative forecast.
22 288 A. ROCHA AND I. SIMMONDS 6. CONCLUDING REMARKS A major aim of this study has been to investigate whether SST anomalies could be used to understand the variability of summer rainfall over south-eastern Africa. This has been tested in a series of GCM experiments in which the model has been forced with spatial- and time-evolving SST anomalies characteristic of dry summers over south-eastern Africa. Three experiments have been performed to assess the relative importance of certain spatial features of the global SST anomaly pattern. In the first of these, where the global SST anomaly pattern has been used, positive anomalies are present over the tropical Indian and over the central and eastern Pacific Ocean, whereas anomalously cool waters cover much of the western Pacific and south Indian Oceans. The spatial configuration and the magnitude of the SST anomaly field is reminiscent of that occurring in a moderate ENSO. The second experiment considers SST anomalies only in the Indian and western Pacific Oceans. In the third simulation, only the positive SST anomalies in the Indian Ocean are used. The conceptual model shown in Figure 17 summarizes the effects of SST anomalies in particular Figure 17. Conceptual model of SST influence on southern African summer rainfall