Does the South American Monsoon Influence African Rainfall?

Transcription

1 1226 J O U R N A L O F C L I M A T E VOLUME 24 Does the South American Monsoon Influence African Rainfall? ALICE M. GRIMM Department of Physics, Federal University of Parana, Curitiba, Brazil CHRIS J. C. REASON Department of Oceanography, University of Cape Town, Cape Town, South Africa (Manuscript received 10 March 2010, in final form 3 August 2010) ABSTRACT Teleconnections between the South American monsoon and southern African rainfall are investigated for years with Benguela Niño or Niña events in the South Atlantic. During these events, it is found that substantial rainfall anomalies also occur over South America in addition to those previously known for southern Africa. The appearance of large rainfall anomalies in the South American monsoon region prior to the onset of the Benguela Niño proper suggests that anomalous convection over South America may influence the evolution of both the SST anomalies and the African rainfall anomalies associated with Benguela Niño events. This teleconnection between South America and southern African rainfall may occur directly, via atmospheric circulation anomalies induced by convection over South America, or indirectly, via the effect of induced circulation anomalies on regional SST. To investigate these teleconnections, a vorticity equation model, which is linearized about a realistic basic state and which includes the divergence in this state and the advection of vorticity by the divergent wind, is applied to the events. The model is forced with anomalous divergence patterns observed during the events, and the steady-state solutions show that anomalies of convection during the South American monsoon produce the main circulation anomalies observed during the Benguela Niño events and hence influence rainfall and circulation patterns over Angola and other southern African countries. An influence function analysis confirms this result, indicating that South America is the most efficient source region to produce the observed anomalies, and also shows that there is no influence of convection over Africa on the South American monsoon. Based on these linear model and observational results, it is concluded that the South American monsoon can influence the evolution of Benguela Niños and associated rainfall anomalies in southern Africa. 1. Introduction Significant relationships exist between the Atlantic Ocean and climate variability in South America and Africa (Nobre et al. 2006; Reason et al. 2006; Grimm and Zilli 2009). However, teleconnections between South American and African climate are poorly understood. Through AGCM simulations, K. Cook et al. (2004) concluded that South America and Africa influence each other s climate and suggested a stronger influence of Africa on South America. The influence of South American variability on African climate has not been documented. Corresponding author address: Alice M. Grimm, Department of Physics, Federal University of Paraná, Caixa Postal 19044, Curitiba, PR, Brazil. grimm@fisica.ufpr.br The South American monsoon could influence southern Africa during episodes of Benguela Niño, a climate mode that causes anomalous sea surface temperatures (SSTs) in the tropical southeastern Atlantic (Shannon et al. 1986). For example, the second mode of precipitation variability in South America shows high correlation with Benguela Niño like SST anomalies (Fig. 1). Benguela Niños involve a weakening in the trade winds in the equatorial western Atlantic during the early monsoon, generation of downwelling equatorial Kelvin waves that transport a warming signal toward equatorial Africa, and then a coastally trapped signal propagating toward Angola (Florenchie et al. 2003, 2004). The strongest SST anomaly tends to occur off Angola where the thermocline shoals toward the surface. Associations between tropical southeastern Atlantic SST and southern African rainfall have been documented DOI: /2010JCLI Ó 2011 American Meteorological Society

2 15 FEBRUARY 2011 G R I M M A N D R E A S O N 1227 FIG. 1. (left) Second variability mode of annual precipitation over South America (14% of the variance, ) and (right) its correlation with SST. Light (dark) shades indicate positive (negative) correlation coefficients with levels of confidence higher than 90%. (e.g., Hirst and Hastenrath 1983; Nicholson and Entekhabi 1987; Rouault et al. 2003; Hansingo and Reason 2009). The strongest relationships tend to occur in late summer [February April (FMA)], the Angolan wet season, when regional SST reaches its maximum (about 308C). However, here we show that there are also strong South American precipitation anomalies associated with Benguela Niño events as well as prior to their onset, and that these anomalies can influence Benguela Niño evolution as well as the associated African precipitation. This anomalous South American rainfall during these events has not been previously documented, nor has the temporal evolution of the African rainfall anomalies from the spring (prior to the Benguela Niño) to autumn. data was also done with Climatic Research Unit (CRU) gridded precipitation data (Hulme et al. 1998) and the University of Delaware gridded precipitation data ( ) (Legates and Willmott 1990) and the results are qualitatively the same, leading to the same conclusions. Benguela Niño (Niña) events are defined as those showing FMA SST standardized anomalies about or greater (less) than one standard deviation (minus one standard deviation) in a box off Angola ( S, E) (Rouault et al. 2009). Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST; Rayner et al. 2003) and National Centers for Environmental Prediction (NCEP) National Center for Atmospheric 2. Methodology Although data from around rain gauges over South America were used to construct Fig. 1, the joint analysis of precipitation in South America and Africa is carried out with Global Precipitation Climatology Centre (GPCC) gridded data for (Schneider et al. 2008), since station data for Angola (the main region of rainfall impact in southern Africa) were not available to us. Figure 2 shows the boxes that contain data in the GPCC set for five or more of the Benguela Niña events included in this study. The corresponding map for Benguela Niños is similar. White areas have less than five events included in the average series (or no data at all) and are therefore not included in the plot. All of the analysis carried out with GPCC FIG. 2. Number of Benguela Niña events during in each GPCC grid box average series used for the computation of the differences between average precipitation during Benguela Niños and Niñas. White areas have less than five events included in the average series (or no data at all) and are therefore not included in the plot.

3 1228 J O U R N A L O F C L I M A T E VOLUME 24 Research (NCAR) reanalysis data (Kalnay et al. 1996) are used for SST and atmospheric fields, respectively. The warm events (Benguela Niños) are 1959, 1963, 1965, 1984, 1986, 1995, 1998, 1999, 2001, and 2006; the cold events (Benguela Niñas) are 1956, 1958, 1972, 1978, 1980, 1981, 1982, 1992, 1997, and SST and circulation anomalies are composited for these events from November through March. For brevity, only their differences and corresponding statistical significance are shown for November, January and March, which are representative of the evolution of Benguela Niños. The possible origin of atmospheric circulation anomalies preceding the peak phase of Benguela Niño that can influence its evolution and the southern African rainfall anomalies is analyzed with influence functions (IFs) of a vorticity equation model with a divergence source (Grimm and Silva Dias 1995a,b). The model is linearized about a realistic basic state and includes the divergence of the basic state and the advection of vorticity by divergent wind. It can be written as z9 t 1 V c $z91v9 c $z 1 V x $z91z9d A95F9, (1a) with F95 zd9 V9 x $z, (1b) where F9 depends only on the anomalous divergence. In these equations, z is absolute vorticity, D is divergence, V x and V c are the divergent and rotational components of the wind, and A9 is the damping term, including linear damping and biharmonic diffusion. The model is applied at 200 hpa, near the level of maximum divergence associated with convective outflow in the tropics and an equivalent barotropic level in the extratropics. The stationary version of the model may be written as Mc95D9, (2) with M being a linear operator and c9 the anomalous streamfunction. Then the IF based on divergence forcing is defined by G D (l, f, l9, f9) 5 M 1 [d(l, f, l9, f9)], (3) where d(l, f, l9, f9) is the delta function. Thus, the IF G D (l, f, l9, f9) for the target point with longitude and latitude (l, u) is, at each point (l9, u9), equal to the model response at (l, u) to an upper-level divergence located at (l9, u9). Maps with contours of influence function for a given target point indicate the tropical/ subtropical regions in which the anomalous upper-level divergence is most efficient in producing streamfunction anomalies at the target point. In other words, the IF for a given target point summarizes the efficiency of divergence anomalies with different locations in producing streamfunction anomalies at the target point. Large values at midlatitudes do not imply that these regions are important for generating rotational flow by anomalous upper-level divergence, but simply mean that anomalous divergence in these regions is consistent with high values of the streamfunction at the target. In these latitudes, divergence and vorticity are coupled, and the upperlevel anomalous divergence is not directly related to anomalous heat sources, unlike in the tropics, where upper-level divergence is nearly independent of the rotational flow. Thus, more attention is given to the tropics/ subtropics, where the anomalous upper-level divergence/ convergence is directly related to anomalous tropical heat sources and nearly independent of the rotational flow. Besides the IF analysis, the influence of anomalous convection during the South American summer monsoon on circulation anomalies associated with Africa rainfall during Benguela Niño events and the possible influence of convection anomalies in Africa over the South American monsoon are tested by simulations with the linearized vorticity equation model forced by different anomalous divergence patterns observed during these events. What is shown are the steady-state solutions after integration for 30 days, although those patterns are already well defined after approximately one week. The simulations and the computation of the IFs are made with T42 resolution (approximately 2.88). The vorticity equation model described above is essentially an equation that links the rotational and divergent components of the flow at one pressure level. It is useful as a prognostic model if the divergence can be specified without knowing the vorticity at this level. This is possible in the tropics, since the upper-level divergence is directly related to the tropical heating, but in midlatitudes the divergence and vorticity are coupled, as mentioned before, and therefore it should be applied in an equivalent barotropic level. The 200-hPa level is both near the level of maximum divergence associated with convective outflow in the tropics and near an equivalent barotropic level in the extratropics. More information about the model and the IFs, including their usefulness and drawbacks, can be found in Grimm and Silva Dias (1995a,b). 3. SST and rainfall anomalies Weak warm anomalies are already present off the Angolan coast in November and there is a band of warming

4 15 FEBRUARY 2011 G R I M M A N D R E A S O N 1229 FIG. 3. Difference in (left) SST (K) and (right) rainfall (mm) between Benguela Niños and Niñas for (a),(d) November; (b),(e) January; and (c),(f) March. Contour interval for rainfall is 25 mm. Dark and light shades indicate levels of confidence (higher than 90%) for positive and negative differences, respectively. stretching from the South Atlantic convergence zone off Brazil toward the Benguela Current (Fig. 3a). By January, the warming off Angola strengthens considerably and the warming stretching from the coast of Brazil is more zonal (Fig. 3b). In March, the SST anomaly off the Angolan coast is near maximum and most of the South Atlantic displays positive SST anomalies (Fig. 3c). Although the rainfall anomalies over southwestern Africa are weak and interspersed with opposite anomalies, it is apparent that the increased (decreased) rainfall during Benguela Niños (Niñas) begins in the northwestern Angola/western Congo basin and western Cameroon in November (Fig. 3d), strengthens and spreads southward by January preceding an event (Fig. 3e), and by March covers most of western and northern Angola, as well as the southwestern Congo (Fig. 3f). The main rainy season in Angola is FMA, when the strongest SST anomalies occur off the coast and the rainfall anomalies are greatest (Rouault et al. 2003). Rainfall anomalies are also seen in the central and eastern parts of southern Africa during all those months, but especially in January, the height of the main rainy season there. In South America, relatively weak positive rainfall anomalies occur in equatorial Amazonia and southeastern South America in November with negative anomalies in the central regions (Fig. 3d). By January, this tripolar pattern is clear, with a large area of strong negative anomalies in east-central regions, surrounded by positive anomalies to the south and to the north in equatorial Amazonia and the Brazilian Nordeste (Fig. 3e). The dry anomalies in the east-central regions contract and weaken in March, while the wet anomalies in Nordeste strengthen and extend southward (Fig. 3f). March is close to the demise of the South American monsoon in the east-central regions and to the peak rainy season in Nordeste.

5 1230 J O U R N A L O F C L I M A T E VOLUME 24 Figure 3 shows clearly that the South American rainfall anomalies strengthen before those in the Angolan region. The rainfall anomalies in Angola/Congo (our focus) are much weaker in November than those over South America. They are also weaker in January than in March, while the anomalies in South America are stronger in January than in March, especially in the eastcentral areas. Also the SST anomalies off Angola are stronger in March than in January. These relationships (and signs of this South American pattern already in September; not shown) suggest that the rainfall patterns associated with Benguela Niños appear in South America before they occur in Angola/Congo. 4. Circulation anomalies In November, a low-level high pressure anomaly extending from the western equatorial Atlantic/northeastern South America to central South America (not shown), with concurrent lower pressure over southern Africa, weakens the Atlantic trade winds. Consistent with this pattern, low-level cyclonic streamfunction anomalies straddle the equatorial Atlantic, extending eastward into Africa (Fig. 4a), producing some warming off Angola (Fig. 3a) and enhanced rainfall in the northwest of that country (Fig. 3d). The warming in the South Atlantic stretching from Namibia to Brazil (Fig. 3a) may produce favorable conditions for rainfall in the equatorial Amazon, since the easterlies become warmer and moister, but in central South America low-level high pressure anomalies (not shown) causes subsidence and dry conditions. At higher levels, anticyclonic streamfunction differences straddle the equator over the Atlantic and Africa (Fig. 4d). Plots (not shown) of reanalysis differences at the 500-hPa level in omega (pressure tendency) between Benguela Niños and Niñas show positive values (relative subsidence) over east-central Brazil with weaker positive values over central southern Angola, consistent with the rainfall differences in Fig. 3d. There are also strong negative omega differences (relative ascent) over Cameroon that are consistent with the increased rainfall in the western part of that country. By January, the strongest low-level high pressure anomaly has moved south into central South America, strengthening the anticyclonic anomalies over this region and the South Atlantic convergence zone (Fig. 4b), consistent with most of east-central Brazil being dry (Fig. 3e). There is a large area of positive omega anomaly (not shown) centered over the east-central areas, reflecting strong relative subsidence there. The cyclonic streamfunction anomalies over southern Africa have moved north over Angola (Fig. 4b), and the negative omega anomalies now cover most of Angola, being strongest in the west, which is consistent with the strengthening of the positive rainfall anomalies there (Fig. 3e). At 200 hpa, cyclonic (anticyclonic) streamfunction anomalies are centered over central Brazil (equatorial Atlantic and Angola).Thestrongcyclonic barotropic anomaly over the South Atlantic in November has shifted toward Africa and weakened near South America. In March, the streamfunction anomalies over central Brazil are much reduced at both levels, as is the lowlevel cyclonic anomaly over Angola (Figs. 4c,f). The strongest anomalies are now over the western tropical Atlantic, consistent with the strong precipitation anomalies over Nordeste, but they still persist over Africa and are reflected in the strong precipitation anomalies there (Fig. 3f). These patterns are also consistent with the omega anomalies (not shown), which are still positive over east-central Brazil (but weaker than in January) but strongly negative (relative ascent) over western Angola/the western Congo basin, consistent with the positive rainfall differences. Negative omega anomalies also exist over Nordeste where there are positive rainfall differences. The dipolar character of the rainfall anomalies in southern Africa (wetter Angola region, drier central southern Africa) is reflected in the upperlevel streamfunction dipole. Besides examining the rotational component of the wind, represented by the streamfunction fields, it is also convenient to inspect the divergent component, more specifically the perturbation of the Walker circulation in the S band during January of Benguela Niño events. Climatologically, there is ascending motion over South America and eastern Africa in this latitudinal belt (Fig. 5a). It is important to note that the climatological moisture flux (integrated from the surface to 300 hpa) over southern Africa, including Angola, during FMA is almost entirely easterly from the Indian Ocean (see Fig. 1 in Rouault et al. 2003). During intraseasonal wet spells during the summer half of the year in southern Africa, a secondary source of moisture is a low-level flow from the tropical southeastern Atlantic into western Angola and northwestern Namibia (C. Cook et al. 2004; Reason et al. 2006). However, as discussed by Rouault et al. (2003) and further shown in Hansingo and Reason (2009). It is the orientation in latitude and zonal extent of the low-level circulation anomaly relative to the mean flux, and hence the contribution of low-level moisture from the tropical southeastern Atlantic, that helps to determine the size of the rainfall impact over the Angolan region during Benguela Niño events. Rouault et al. (2003) did not show how or where these circulation anomalies arose during the four Benguela Niño events they analyzed. However, as argued here, we believe that it is the

6 15 FEBRUARY 2011 G R I M M A N D R E A S O N 1231 FIG. 4. Difference in zonally asymmetric streamfunction (10 6 m 2 s 21 ) at (left) 850 and (right) 200 hpa between Benguela Niños and Niñas for (a),(d) November; (b),(e) January; and (c),(f) March. Dark and light shades indicate levels of confidence (higher than 90%) for positive and negative differences, respectively. influence of the South American monsoon that helps generate the observed circulation and hence rainfall anomalies. During Benguela Niños, it is apparent (Fig. 5b) that there is strong ascent relative to the Benguela Niña composite over most of southern Africa, whereas anomalous subsidence prevails at various levels over central South America. The upper-level easterly divergent wind is visible in Fig. 5c, while the low-level westerly divergent wind is evident in Fig. 5d. These figures also show the upper-level anomalous convergence (divergence) over east-central Brazil (Africa, particularly Angola and the Congo basin) and the low-level anomalous divergence (convergence) over east-central Brazil (Africa, particularly Angola and the Congo basin) during January. Consistent with these divergent wind and velocity

7 1232 J O U R N A L O F C L I M A T E VOLUME 24 FIG. 5. (a) Longitude height picture of the climatological zonal and vertical winds in January averaged over the S latitudinal belt. The vertical winds are multiplied by 300. (b) Longitude height picture of the difference between Benguela Niño and Niña zonal and vertical winds averaged over the S latitudinal belt. The vertical winds are multiplied by 300. (c),(d) Differences in divergent wind and velocity potential at 200 and 850 hpa, respectively, between Benguela Niños and Niñas. Shaded areas indicate wind differences with levels of confidence higher than 90%.

8 15 FEBRUARY 2011 G R I M M A N D R E A S O N 1233 potential anomalies, Fig. 3b shows that east-central Brazil (western Angola/the western Congo basin) has negative (positive) rainfall anomalies in January. 5. Potential mechanisms a. Circulation and SST anomalies There are several possible connections between the observed circulation anomalies and SST anomalies. First, the South Atlantic SST anomalies associated with Benguela Niños (Fig. 3) are consistent with the weakening of the trade winds in the equatorial Atlantic, which then generates downwelling equatorial Kelvin waves that transport a warming signal toward equatorial Africa and then a coastally trapped signal propagating toward Angola (Florenchie et al. 2003, 2004). The strongest SST anomaly tends to occur off Angola where the thermocline shoals toward the surface. This trade wind weakening and low-level increasing pressure over east-central South America is already visible in September (not shown), a season before the strong SST anomalies appear off Angola, but is much stronger in January because of the strong convective anomalies over central South America. The SST warming off the Angolan coast may be enhanced by the relative wind convergence off the coast of Angola in January and the slightly more onshore direction of the low-level wind than average (Fig. 5d). This anomaly leads to less Ekman transport away from the coast and less divergence in the upper ocean than average, thereby reducing the mean upwelling and hence producing local SST warming. There mayalsobeaneffectonthemarineboundarylayer since the relative convergence at low levels and cyclonic streamfunction anomalies (Fig. 4b) imply a weaker marine subsidence inversion over the coastal ocean and a weakening in the marine stratocumulus deck that is climatologically present. As a result, more insolation may reach the upper ocean, thereby further warming it. In addition, the downwelling wave in the coastal ocean during Benguela Niños further enhances the SST warming (Florenchie et al. 2003, 2004). Besides the intensification of the SST anomalies off the Angolan coast from January to March, there is also an extension of the positive anomalies southward into the midlatitudes of southeastern South Atlantic southwest of South Africa (Fig. 3c), which is probably due to the strong cyclonic anomalies in the southeastern Atlantic (Figs. 4b,e, around point 3), via advection and less latent heat loss, since those anomalies weaken the climatological low-level southwesterly winds in that region. On the other hand, cold anomalies develop east of Argentina (Fig. 3c) in association with the cyclonic anomalies in the midlatitudes of the southwestern South Atlantic (Fig. 4c), which enhance the climatological lowlevel westerlies there. b. Influence of South American monsoon on African rainfall As shown above, the South American rainfall anomalies precede those in Angola. This timing might occur if the large-scale forcing that causes the high pressure anomaly extending from the western equatorial Atlantic/ northeastern South America to central South America and consequent western Atlantic wind anomalies in spring, which start the Benguela Niño, also produces anomalous convection over South America in spring/ summer. From the observed lag among the strongest anomalies in South America and Africa, influence function analysis, and model experiments, we argue that these anomalies do not simply have a common origin; rather, the South American anomalies influence those in Africa. The former generate circulation anomalies that can affect southern African rainfall directly and indirectly (via their influence on SST). To investigate this hypothesis, we determine the IFs (Fig. 6) for the action centers of a wave train emanating from tropical South America in January (Fig. 4e) and for another target point in southwestern Africa. The vertical structure of that wave train is baroclinic in the tropics (center 1) and equivalent barotropic at higher latitudes (centers 2 4), as expected for a tropical extratropical Rossby wave. In the regions with higher positive (negative) values of the IF in the tropics/subtropics (Fig. 6), the anomalous upper-level divergence is most efficient in producing positive (negative) streamfunction anomalies at the target point. The upper-level anomalous convergence produces opposite streamfunction anomalies. Large IF values at midlatitudes do not imply that anomalous divergence at upper levels there are important for generating rotational flow (cf section 2). Therefore our analysis focuses on the tropical and subtropical upper-level divergence. IFs for centers 1 3 all have high absolute values in the tropics/subtropics over central South America, suggesting that anomalous convection (and associated upper-level divergence) there is a preferred candidate to produce the Rossby wave train. Over this region, there is suppressed (enhanced) convection during Benguela Niños (Niñas) (Fig. 3e), consistent with the significant upper-level convergence in Fig. 7 (dashed ellipse). Since the IFs in central South America show negative values for target points 1 and 3 and positive values for points 2 and 4, this convergence should produce anomalous positive streamfunction around points 1 and 3 and

9 1234 J O U R N A L O F C L I M A T E VOLUME 24 FIG. 6. Influence functions for January atmospheric basic state of target points 1 5 (shown in Fig. 4e). negative streamfunction anomalies around points 2 and 4, as observed (Fig. 3e). Since the IFs change sign over southeastern South America, the anomalous upper-level divergence here (solid ellipse in Fig. 7) contributes to the same streamfunction anomalies. There are also extensive divergence/convergence anomalies over the equatorial Pacific Ocean, but the IFs of the wave train action centers are not strong there. Although there are high values of some IFs over the South Atlantic, the divergence anomalies are not strong there, and IF values of same sign cover divergence anomalies of opposite signs, causing little streamfunction response at the target points. Thus, the analysis indicates the anomalous convection over central South America as the main source of the wave train. The circulation anomalies around action center 3 (Figs. 4b,e) are important for weakening the climatological southerly winds near the southwestern African coastal zone, contributing to large SST increases in January March (cf. Figs. 3b,c). Although the effect of South American convection is largest in January (note the strong IF and divergence anomalies then), a similar influence exists from spring to early autumn in southeastern South Atlantic (IFs not shown). The IF at center 4 also shows high values in subtropical Africa, indicating that the upper-level divergence/ convergence here (Fig. 7) may also influence the streamfunction anomalies around this center. While circulation anomalies around point 3 affect SST anomalies southwest of Africa, the anomalies around point 5 (just off the coast of Angola in Fig. 6e) have a more direct effect. The IF at point 5 indicates that these anticyclonic anomalies can be caused both by anomalous local convection (with upper-level divergence) and by remote anomalous convergence over central South America. Thus, perturbations in the Walker circulation (Fig. 5) resulting from the anomalous convection in South America can influence southern African rainfall. The greater influence of anomalous convection over South America on southern Africa than the other way round is clearly illustrated by the IFs for points 1 and 5 (Fig. 6). They indicate that, at least during Benguela Niños, South America has a stronger impact on Africa than Africa on South America, since there are strong IF values over South America for point 5, while the IF is zero over Africa for point 1. It is worth mentioning that the conditions evident in January over South America during Benguela Niños (relative subsidence with dry conditions in the eastcentral area and relative ascent with wet conditions in the subtropics; Fig. 3e) are already visible in November (Fig. 3d). Also visible are some upper-level streamfunction anomalies that were analyzed for January as possibly connected with the evolution of the Benguela Niño SST anomalies and the precipitation anomalies in the Angolan region: the cyclonic barotropic anomaly over central Atlantic midlatitudes (around point 3 in Fig. 4e) and the upper-level anticyclonic anomalies (and lower-level cyclonic anomalies) over the Angolan region (around point 5 in Fig. 4e). All these circulation anomalies can be related to the convection anomalies over South America also in November, as indicated by the November IFs for points 3 and 5 (not shown, but rather similar to those for January), but the upper-level cyclonic (anticyclonic) anomalies over central (subtropical) South America can hardly be ascribed to convection over Africa or over the Benguela Niño region, as indicated from the November IFs for points 1 and 2. Further indication of the possible influence of the South American monsoon anomalous convection on the

10 15 FEBRUARY 2011 G R I M M A N D R E A S O N 1235 FIG. 7. The difference in divergence at 200 hpa between Benguela Niños and Niñas for January contoured at intervals of s 21. Dark and light shades indicate levels of confidence (higher than 90%) for positive and negative differences, respectively. The solid (dashed) ellipses indicate regions of significant positive (negative) divergence difference in South America, discussed in the text. Benguela Niño related circulation (and consequently precipitation) anomalies is given by model simulation of the response to idealized upper-level divergence/ convergence anomalies representing some of the observed features in Fig. 7. Figure 8a shows the response to anomalous convergence over central South America. The resulting wave train over South Atlantic represents all the centers (1 4) in Fig. 4e, with small shifts. Even anticyclonic anomalies near point 5 are reproduced. The inclusion of the anomalous divergence over subtropical South America (Fig. 8b) enhances the wave train and improves some features, such as the anticyclonic anomalies around point 5 and over Africa. The inclusion of the anomalous divergence over Africa (Fig. 8c) improves these features further and reproduces well the anomalies around point 5 and extending over Africa, confirming the indication of IF for point 5 that this circulation anomaly is due to remote influence from South America and local influence from Africa. Figure 8d shows the response to upper-level divergence over southern Africa, including the divergence associated with anomalous convection in the east-central portion of southern Africa (Mozambique and Tanzania in Fig. 3e). It is clear that these divergence anomalies produce weak streamfunction anomalies, but their influence on the circulation around point 5 is visible, although not strong. This figure shows clearly that the southern African rainfall and associated upper-level divergence has very little influence and does not determine the South American circulation anomalies, since the response to this divergence shows weak anticyclonic anomalies over South America, while the observed response is cyclonic. Figure 9a shows the response to the same anomalies as before plus a set of divergence/convergence anomalies that are present in Fig. 7 over regions with weaker influence functions, such as the Pacific Ocean, North Atlantic, and northern Africa. The response over South America, the South Atlantic, and southern Africa is almost unchanged with respect to the previous response (Fig. 8c). Some patterns in the eastern Pacific and Northern Hemisphere are improved. It is worth noting how this simple model is able, from some idealized patterns of observed upper-level divergence, to reproduce the main features of the observed anomalous global streamfunction anomalies (Fig. 9b) rather well, even in the Northern Hemisphere. It is also worth noting the strong influence of the South American monsoon convection anomalies over the Northern Hemisphere circulation anomalies. The circulation anomalies around point 5 are related both to the extension of near-equatorial anticyclonic anomalies to the east of the upper-level convergence anomaly (associated with convection anomaly) over central South America [seen in the observations (Fig. 4e) and in the model experiments (Fig. 8)] and to the local effect of the anomalous convection over southern Africa, as shown by the IF of point 4 and the model experiments (see the differences between the experiments in Figs. 8b and 8c). An eastward equatorial propagating atmospheric Kelvin wave from the South American convection anomaly produces a perturbation in the Walker circulation, since there are opposite senses of the circulation in the upper and lower troposphere in the equatorial region, with associated divergence and convergence anomalies (e.g., Gill 1980; Jin and Hoskins 1995). In our case, there are anomalous low-level westerlies and upper-level easterlies between South America and Africa (Figs. 4b,e). However, our simple model only shows the corresponding streamfunction anomalies at upper levels (Fig. 8). This perturbation of the Walker circulation can be seen

11 1236 JOURNAL OF CLIMATE VOLUME 24 FIG. 8. Anomalous upper-level divergence/convergence and corresponding steady anomalous streamfunction, with zonal components removed for several experiments. in Fig. 5, where anomalous subsidence prevails over central South America (where there is climatological ascending motion; Fig. 5a), and anomalous ascent is clear over southern Africa (Fig. 5b). The upper-level easterly divergent wind is visible in Fig. 5c, while the low-level westerly divergent wind is evident in Fig. 5d. These figures also show the upper-level convergence (divergence) over South America (southern Africa) and the low-level divergence (convergence) over South America (southern Africa). Although the Benguela Nin o related SST anomalies are able to produce rainfall anomalies over the Angola

12 15 FEBRUARY 2011 G R I M M A N D R E A S O N 1237 anomalies over South America during the summer monsoon, which can then produce circulation anomalies that modulate Benguela Niños. The timing of the strongest rainfall and circulation anomalies in South America and Africa suggests that these rainfall anomalies do not simply have a common forcing, but that the anomalies over South America can influence the evolution of the Benguela Niño and the rainfall anomalies over Africa through the circulation anomalies that they produce. The association between these circulation anomalies and the anomalous South American convection was confirmed through influence function analysis and simple model experiments. The effect of these circulation anomalies on southern African rainfall during Benguela Niños can be direct, by modulating uplift over the affected area (around point 5 and inland southern Africa), or indirect, through their influence on South Atlantic SST. Acknowledgments. Funding came from the Brazilian CNPq (Brazilian National Council for Scientific and Technological Development) and the European Community s Seventh Framework Programme (FP7/ ) under Grant Agreement (CLARIS LPB: A Europe South America Network for Climate Change Assessment and Impact Studies in La Plata Basin). REFERENCES FIG. 9. (a) Anomalous upper-level divergence/convergence and corresponding steady anomalous streamfunction, with zonal components removed for the experiment with additional divergence features. (b) Observed Benguela Niño Niña difference in streamfunction. region, they are not enough to explain the strong rainfall anomalies observed over southern Africa (Hansingo and Reason 2009). Our results show that these rainfall anomalies are most probably enhanced by the perturbations in the Walker circulation produced by the anomalous convection over South America. Furthermore, also the SST anomalies are enhanced (via the mechanisms described in section 5a) by the circulation anomalies produced by this anomalous convection. 6. Conclusions The same forcing of the equatorial Atlantic anomalies that initiate the Benguela Niño also yields rainfall Cook, C., C. J. C. Reason, and B. C. Hewitson, 2004: Wet and dry spells within particular wet and dry summers in the South African summer rainfall region. Climate Res., 26, Cook, K. H., J.-S. Hsieh, and S. M. Hagos, 2004: The Africa South America intercontinental teleconnection. J. Climate, 17, Florenchie, P., J. R. E. Lutjeharms, C. J. C. Reason, S. Masson, and M. Rouault, 2003: The source of Benguela Ninos in the South Atlantic Ocean. Geophys. Res. Lett., 30, 1505, doi: / 2003GL , C. J. C. Reason, J. R. E. Lutjeharms, M. Rouault, C. Roy, and S. Masson, 2004: Evolution of interannual warm and cold events in the southeast Atlantic Ocean. J. Climate, 17, Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulations. Quart. J. Roy. Meteor. Soc., 106, Grimm, A. M., and P. L. Silva Dias, 1995a: Use of barotropic models in the study of the extratropical response to tropical heat sources. J. Meteor. Soc. Japan, 73, , and, 1995b: Analysis of tropical extratropical interactions with influence functions of a barotropic model. J. Atmos. Sci., 52, , and M. T. Zilli, 2009: Interannual variability and seasonal evolution of summer monsoon rainfall in South America. J. Climate, 22, Hansingo, K., and C. J. C. Reason, 2009: Modelling the atmospheric response over southern Africa to SST forcing in the southeast tropical Atlantic and southwest subtropical Indian Oceans. Int. J. Climatol., 29,

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