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Intraseasonal Teleconnections between and

ALICE M. GRIMM Department of Physics, Federal University of Paraná, ,

C. J. C. REASON Department of Oceanography, University of , Rondebosch, South Africa

(Manuscript received 31 January 2015, in final form 25 September 2015)

ABSTRACT

Teleconnection of anomalies between various parts of the and extratropics is a well- established feature of the climate system. Building on previous work showing that a teleconnection exists between the South American monsoon system and interannual summer rainfall variability over , this study considers intraseasonal variability over these landmasses. It is shown that strong tele- connections exist between South African daily rainfall and that over various areas of South America, with the latter leading by 4–5 days, for both winter and summer, involving regions with strong rainfall in these . During the summer, the mechanisms involve both a modulation of the local Walker cell as well as extra- tropical Rossby wave trains. For winter, the latter mechanism is more important. While in summer tropical convective anomalies over South America play an important role, in winter the subtropics become more important. In both cases, these modulations lead to regional changes in circulation over southern Africa that are favorable for the dominant synoptic rainfall-producing weather systems such as cutoff lows and tropical extratropical cloud bands.

1. Introduction seasons, which generate Rossby wave trains across the Atlantic and, hence, affect southern African circulation Situated in the subtropics, South Africa mainly re- and rainfall. ceives rainfall during austral summer (Fig. 1) except in Here, we focus on intraseasonal time scales, less well the southwest (winter rainfall) and the south coast (all- studied over South Africa than interannual variability, rainfall). Characteristic of southern African cli- and on teleconnections between South American - mate is marked variability on intraseasonal (e.g., Pohl fall and that in South Africa, although it is recognized et al. 2007; Mapande and Reason 2005), interannual that there are other important influences such as ENSO. (e.g., Lindesay 1988; Mason and Jury 1997; Reason et al. There is significant intraseasonal variability over South 2006), interdecadal (e.g., Tyson et al. 1975; Reason and America and the Pacific (e.g., Paegle et al. 2000) that can Rouault 2002; Allan et al. 2003), and longer time scales. modulate synoptic-scale variability. The enhanced syn- Grimm and Reason (2011) suggested an interannual optic anomalies can then produce teleconnections that teleconnection between Brazilian and southern African perturb weather and circulation over Africa. Evidence is rainfall during summers when there are Benguela Niño given of such teleconnections during summer (December– (Niña) events, anomalous warm (cool) sea surface February) and winter (June–August) for those parts temperature (SST) in the South Atlantic near Angola of South Africa that receive significant rainfall during and Namibia. This teleconnection involves anomalous these seasons. rainfall during particular South American monsoon

2. Data and methods Corresponding author address: Alice M. Grimm, Department of Physics, Federal University of Paraná, Caixa Postal 19044, CEP Observed daily gauge data for 1970–99 81531-980, Curitiba, Brazil. are gridded to 18 resolution for South America and 2.58 E-mail: grimm@fisica.ufpr.br for South Africa. At each grid point, anomalies of daily

DOI: 10.1175/JCLI-D-15-0116.1

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FIG. 1. (top) Selected regions and annual cycles of precipitation in South Africa. (middle)– (bottom) The 18 boxes in South America with precipitation significantly correlated to the lagged precipitation (5 days) in each selected region in South Africa. Dark squares (triangles) indicate confidence levels higher than 90% for positive (negative) correlation; open squares (triangles) are for confidence levels between 85% and 90%. Ellipses indicate the regions with maximum correlation. White areas are void of data. The highest correlation coefficient for box 1 is 0.48, and for boxes 2 and 3, it is 0.42.

Unauthenticated | Downloaded 09/29/21 10:58 AM UTC 1DECEMBER 2015 G R I M M A N D R E A S O N 9491 precipitation are calculated and submitted to a bandpass The possible origin of the atmospheric circulation Lanczos filter to isolate intraseasonal oscillations in the anomalies associated with those positive phases is de- 20–90-day band. For each season, the filtered pre- termined using influence functions (IFs) of a vorticity cipitation anomalies for the South African grid boxes equation model with a divergence source (Grimm and are correlated with filtered precipitation anomalies in Silva Dias 1995a, b). The model is linearized about a the grid boxes over South America. Lags from 0 up to realistic basic state and includes the divergence of this 12 days are applied to the South African data, in order to state and vorticity advection by divergent wind: investigate convection anomalies over South America ›z0 that could produce atmospheric perturbations associ- 1 V =z0 1 V0 =z 1 V =z0 1 z 0 2 0 5 0 c c x D A F , ated with South African precipitation anomalies. The ›t significance of correlation between the filtered data (1a) takes autocorrelation into account and uses an effec- tive sample size calculated according to Dawdy and where Matalas (1964). 0 0 0 F 52zD 2 V =z. (1b) Although significant correlations are revealed for x several regions, the results shown represent the best Here F 0 depends only on the anomalous divergent flow. correlations for different precipitation regimes: the In these equations, z is absolute vorticity; D is divergence; Limpopo (summer), southwestern Cape (winter), and Vx and Vc are the divergent and rotational components South Coast (all-season) (Fig. 1). The correlation of the wind, respectively; and A0 is the damping term, maps show the South American grid boxes with sig- including linear damping and biharmonic diffusion. The nificant correlation, to confidence levels of 85% and model is applied at 200 hPa, near the level of maximum 90%. The isolines of correlation coefficients are not divergence associated with convective outflow in the shown, to keep the figures cleaner. The correlation tropics and an equivalent barotropic level in the extra- coefficients may be slightly affected by different tropics. Its stationary version may be written as quantities of data at each grid point, since there may be different missing daily data at each grid point and Mc0 5 D0 , (2) each season. On the other hand, the significance levels take into account the actual number of data in each where M is a linear operator and c0 is the anomalous grid box and therefore are comparable. Nevertheless, streamfunction. Then the IF based on divergence forc- it is convenient to give an idea of the magnitude of the ing is defined by correlation coefficients corresponding to given confi- l u l0 u0 5 21 d l u l0 u0 dence levels, and their maximum value for each case. GD( , , , ) M [ ( , , , )], (3) It may vary a little for each grid box (because of dif- ferent autocorrelations). When using around 2500 where d(l, u, l0, u0) is the delta function. Thus, the IF 0 0 data points, the correlation coefficients corresponding GD(l, u, l , u ) for a target point with longitude and to confidence levels of 85% and 90% are 0.19 and 0.23, (l, u) is, at each point (l0, u0), equal to the respectively. The maximum correlation coefficient model response at (l, u) to an upper-level divergence forbox1(summer)is0.48andforboxes2and3 located at (l0, u0). Maps with contours of IF for a given (winter) it is 0.42. target point indicate the regions in which the anomalous NCEP–NCAR reanalyses (Kalnay et al. 1996) are used upper-level divergence is most efficient in producing to composite intraseasonal anomalies in OLR, 200-hPa streamfunction anomalies around that target point. streamfunction, and vertically integrated moisture flux Upper-level anomalous divergence (convergence) in associated with South African positive filtered pre- regions with positive values of the IF produces positive cipitation anomalies above one standard deviation (pos- (negative) streamfunction anomalies around the target itive phases). The OLR composite anomalies are also point, and the opposite is true for negative values of the calculated for 5 days before the days of the positive phase, IF. More information about the model and the IFs, their in order to capture the previous OLR anomalies over and usefulness and drawbacks can be found in Grimm and near South America. The statistical significance of the Silva Dias (1995a, b). This model is also used for simple composite analysis is calculated with a t test. Since the simulations of the observed streamfunction anomalies at daily anomalies are filtered, they have some serial de- 200 hPa in response to anomalous convection over pendence, and therefore, the effective sample size must South America. take into consideration the autocorrelation of the series Here, the IFs are shown only for the action cen- (Wilks 1995). ters of circulation anomalies directly associated with

Unauthenticated | Downloaded 09/29/21 10:58 AM UTC 9492 JOURNAL OF CLIMATE VOLUME 28 anomalous convection in southern Africa, usually cyclonic Indian Ocean over Mozambique toward South Africa. anomalies southwest of the analyzed regions. Before the Anomalous moisture flux also occurs from the Atlantic, IF analysis, selection of regions whose upper-level anom- through Angola/Namibia, consistent with Cook et al. alous divergence might contribute to generate stream- (2004). Relative convergence occurs over South Africa, function anomalies leading to anomalous South African Zimbabwe, and Botswana, including the TTT region. precipitation is primarily based on the correlation patterns The cyclonic moisture flux anomaly over western in Fig. 1 (regions within the ellipses). The OLR composite southern Africa and the southeastern Atlantic favors anomalies calculated for 5 days before the days of positive both increased uplift over subtropical southern Africa phase in South Africa (Figs. 2b, 3b,and4b) are used only and inflow of warm tropical air from the tropical South to confirm the results of the correlations, and to add in- Atlantic toward the TTT source region, the Angola low formation about regions within or nearby South America (Reason et al. 2006). An enhanced cloud band is evi- (e.g., the Atlantic) where precipitation data are not avail- dent in the negative OLR anomaly band (Fig. 2a)that able and anomalous convection might be important for the stretches southeastward from southeastern Angola, teleconnections. It is convenient to remark that these OLR across the Limpopo box to the Agulhas Current region anomaly composites for 5 days before also show enhanced where it joins with another band of relative uplift. TTT convection in South Africa because the anomalies are fil- formation is triggered by the arrival of an upper-level tered (smoothed) by a 20–90-day bandpass filter. trough over southern Africa and associated planetary waves (Hart et al. 2010), which may originate from 3. Summer teleconnections South America. A first indication of South American influence is the There are significant correlations for a lag of few days wave train visible in Fig. 2c, originating south of the between filtered daily precipitation anomalies in several Brazilian precipitation anomalies (Fig. 1). As a second summer rainfall regions of South Africa and South indication, the IF for the action center 4 of this wave America [for precipitation regimes in South America, train (Fig. 2e) shows clearly that the anomalous upper- see Grimm (2011)]. As an example, we use box 1, in the level divergence and convergence in the ellipses in Fig. 1 north of the country (Fig. 1). are in the right position to produce cyclonic anomalies For box 1, the strongest correlation is at a 4–5-day lag around point 4. The IF is positive (negative) in the re- with rainfall over parts of northeastern and central gion with anomalous upper-level divergence (conver- Brazil. The OLR anomalies for positive phases in box 1 gence) over central-eastern (southeastern) South show a northwest–southeast (NW–SE)-oriented con- America, indicating that both positive and negative di- nection of negative values between tropical southern vergence anomalies are able to produce cyclonic Africa and the southwestern Indian Ocean (Fig. 2a), anomalies (positive streamfunction anomalies) around consistent with the formation of tropical-temperate point 4. Although there are also OLR anomalies in the troughs (TTT), the main summer synoptic rainfall sys- Pacific and the Atlantic, either they are in regions with tem (Harrison 1984; Hart et al. 2010, 2013). Over South weaker IF or the associated upper-level divergence does America 5 days before (Fig. 2b), they are predominantly not have the correct sign, according to the IF, to negative (positive) within the ellipse with positive produce a cyclonic anomaly around point 4. No other (negative) correlations in Fig. 1, coherent with enhanced regions with strong OLR anomalies have such strong IF (suppressed) precipitation associated with positive values with such coherent signs. A third indication of the phases in box 1. influence of South America anomalous convection is the The 200-hPa streamfunction cyclonic (anticyclonic) upper-level streamfunction response (Fig. 2g) to upper- anomalies behind (ahead) box 1 are coherent with a level divergence anomalies in the marked ellipses TTT (Fig. 2c), being part of a wave train that originated (Figs. 1 and 2f). The wave trains in Figs. 2c and 2g are from the strongest South American convective anoma- similar, although complete agreement is not possible lies. In the tropics, there is also a Walker cell–like pat- with such an idealized experiment. tern across the South American–African sector (pairs of Our results are also consistent with Macron et al. anticyclones/cyclones either side of the equator). Al- (2014), who showed that not all extratropical waves though the filtered data do not contain isolated synoptic produce a TTT; Rossby waves leading to TTT systems rainfall systems, their positive phases occur when these are already stronger and show associated enhanced systems are more intense and frequent, and therefore, convection over the subtropical southern Atlantic and the anomalies during these phases are representative of eastern South America prior to the TTT development them. The moisture flux anomaly (Fig. 2d) shows in- over southern Africa, indicating the contribution of a creased inflow of moist marine air from the southwestern wave train that originated in the tropics.

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FIG. 2. Anomaly composites for the days of positive phases in box 1, in summer: (a) OLR and (b) OLR 5 days before (contour interval 2 2 1.5 W m 2), (c) 200-hPa streamfunction (contour interval 0.3 3 106 m2 s 1), and (d) vertically integrated moisture flux (arrows, 2 2 2 2 2 mgs 1 kg 1) and its divergence (contour interval is 0.75 3 10 2 gs 1 kg 1). Dark (light) gray shading indicates confidence levels higher than 90% for negative (positive) anomalies, and only significant moisture fluxes are shown. Zero isolines are not shown. (e) Influence function for summer basic state for the easternmost action center numbered in (c). The values shown in each location are proportional to the streamfunction response at the target point to a unitary upper-level divergence anomaly in this location. (f) The anomalous 200-hPa prescribed divergence and (g) corresponding steady anomalous streamfunction, with the zonal components removed, for the box 1 experiment.

4. Winter teleconnections regions have a significant relationship on intraseasonal scales with rainfall upstream over certain South American regions. Western South Africa receives mainly winter rainfall For the winter rainfall southwestern region (box 2), (Fig. 1). The only other region in southern Africa that has the strongest correlation with South American winter significant winter rainfall is the south coast. Both these rainfall regions is southern Brazil/Uruguay (positive),

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FIG.3.AsinFig. 2, but for box 2 in winter. at a 4–5-day lag (Fig. 1). The OLR anomaly composite features over western South Africa and southern Ar- confirms the enhanced convection in southwestern gentina and . The latter extends over the western South Africa (Fig. 3a) and 5 days before in a region off of South Atlantic important area for cyclogenesis, and thus southeastern South America, an important winter cy- favors more frontal systems approaching southwestern clogenesis region (Fig. 3b). Most of the rainfall in South Africa. On the other hand, an anticyclonic southwestern South Africa is brought by cold fronts, anomaly is present near southeastern South America. with cutoff lows that are sometimes important. Figure 3c These patterns are consistent with the positive correla- shows a wave pattern in the midlatitudes, similar to that tions between box 2 rainfall and southern Brazil/ identified by Weldon and Reason (2014) as important Uruguay, and negative OLR anomaly composites off of for southern South African rainfall, with large cyclonic southeastern South America (Fig. 3b). The cyclonic

Unauthenticated | Downloaded 09/29/21 10:58 AM UTC 1DECEMBER 2015 G R I M M A N D R E A S O N 9495 anomaly over and south of western South Africa is fa- The streamfunction anomalies at 200 hPa (Fig. 4c) vorable for strengthening the approaching westerly show a wave train between South America and southern systems. A similar pattern is evident in the moisture flux Africa. Over South Africa, a large area of negative OLR (Fig. 3d), indicating its barotropic characteristics, with anomalies (relative uplift) is apparent over the south enhanced westerly/northwesterly inflow from the South coast and inland, except in the far southwest (Fig. 4a). Atlantic toward western South Africa and relative Both the 850- (not shown) and 200-hPa streamfunction moisture convergence there, favorable for increased anomalies (Fig. 4c) are characteristic of cutoff low rainfall (Reason et al. 2002; Reason and Jagadheesha conditions since they have a large anticyclonic anomaly 2005). Negative OLR anomalies (uplift) predominate (center 2) to the south of South Africa and a cyclonic upstream over the west coast of South Africa/Namibia anomaly over the landmass (center 3) (Singleton and and the adjacent ocean area (Fig. 3a). Reason 2006, 2007a). The fact that these patterns are The streamfunction anomalies associated with the posi- similar at the lower and upper levels is consistent with tive phases of the winter precipitation in box 2 (Fig. 3c)could the development of deep convective storms such as be attributed to a wave train circling the globe and its mi- cutoff lows. Almost all of the flooding events in southern gratory extratropical cyclones. However, one can show that South Africa result from these weather systems the anomalous convection over southeastern South Amer- (Singleton and Reason 2007b). Relative moisture flux ica and in the cyclogenetic region off the southern Brazil/ convergence is evident over eastern South Africa, in- Uruguay coast is able to influence the circulation anomalies cluding the eastern part of the south coast where there that lead to stronger South African precipitation. There are are easterly onshore anomalies (Fig. 4d). three factors that suggest that the relationship between The influence of South American anomalous convec- South American and South African rainfall is not simply due tion in forcing the wave train between the two continents to an extratropical wave train across the Southern Hemi- is confirmed by the IF of center 3 (Fig. 4e), which displays sphere that equally influences South American and South strong positive values over southeastern and central Brazil, African rainfall, without influence of the South American around 208S, indicating that upper-level divergence in anomalous convection. First, the streamfunction anomalies this region is able to produce a positive streamfunction are much stronger in the Atlantic than in the Pacific and around center 3. The IFs of the other centers (not Indian Oceans. Second, the IF of the action center 2 of the shown) indicate that also the subtropical upper-level cyclonic anomaly near southern South Africa in Fig. 3c in- convergence is important in generating that wave train. dicates very clearly that anomalous convection (and upper- Furthermore, a model simulation of the streamfunction level divergence) over southeastern South America and the anomalies at 200 hPa forced by anomalous divergence cyclogenetic region off the coast is very efficient in pro- (convergence) within the continuous (dashed) ellipse ducing this cyclonic anomaly (Fig. 3e). Third, the simple on the correlation map for box 3 (Fig. 1), as represented model simulation with a forcing over the maximum corre- in Fig. 4f, shows a wave train over the southern Atlantic lation for box 2 (Fig. 1) and maximum OLR negative (Fig. 4g) similar to the observed one (Fig. 4c). anomalies (Fig. 3b), as indicated in Fig. 3f,reproducesthe main circulation features observed over the Atlantic 5. Summary and conclusions (around centers 1 and 2 in Fig. 3c), as shown in Fig. 3g.These features are important to produce rainfall over box 2. Even Grimm and Reason (2011) showed that a teleconnection the anticyclonic anomalies over tropical southern Africa exists between the South American monsoon and summer extending toward Madagascar and the cyclonic ones over rainfall over tropical southern Africa on interannual Indian Ocean are reproduced. These results indicate that if a scales. Here, evidence is shown that teleconnections exist wave train passes over southern South America without between South American rainfall variability and that of causing strong convective anomalies over the southeastern various South African regions for both summer and winter continent and off the coast, there is likely to be no strong on intraseasonal scales. Although not as strong as summer/ anomaly over southern South Africa. These connections winter, there is evidence of rainfall teleconnections for the may potentially be useful for prognostic purposes. transition seasons too. It, therefore, appears that the For the south coast (box 3), there is strong positive rainfall relationships between these two Southern Hemi- correlation at a 4–5-day lag with southeastern and cen- sphere landmasses can exist at any time of the . Since tral Brazil; a negative correlation appears over northern there are teleconnections at both intraseasonal and in- and southern Brazil, where winter rainfall is terannual scales, they might appear on interdecadal scales, strong (Fig. 1). Negative (positive) OLR anomalies also too, since there is strong interdecadal precipitation vari- appear 5 days before in these regions with positive ability over South America (Grimm and Saboia 2015)as (negative) correlations (Fig. 4b). well as southern Africa (Reason and Rouault 2002).

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FIG.4.AsinFig. 2, but for box 3 in winter.

The mechanisms by which these teleconnections occur South America and southern Africa some days after, it is involve the generation of wave trains across the South possible to show that the anomalous convection over Atlantic that then impact on regional circulation and South America and the neighboring Atlantic region is able moisture flux convergence over South Africa. In summer, to influence South African precipitation. there is also an anomalous Walker-type circulation in the Obviously there are other influences over southern tropical Atlantic region, associated with anomalous trop- Africa rainfall on interannual, interdecadal, and intra- ical convection. While in summer tropical convective seasonal time scales. The focus of this manuscript is on anomalies play an important role, in winter the subtropics the possible influence of the South American anomalous become more important. Even when there is a wave pat- convection on the precipitation variability in southern tern circling the globe in the subtropics and midlatitudes Africa on intraseasonal time scales. The strongest re- that could be associated with anomalous convection in lationships between the intraseasonal variability of

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