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relatively symmetric according to Ropelewski and Hal-pert (1987, 1989) and Richard (1996) who investigatedseparately the impact of the low and the high SO phaseson rainfall.

3. Mason (1995) argued that the lack of South Afri-can rainfall response to some El Nin o Southern Oscil-lation (ENSO) events could be explained with an Indian/Pacic disconnection. The disconnection could be atleast partly associated with the east phase of the quasi-biennial oscillation (QBO). The correlation between SOIand summer rainfall is at its maximum when the QBO isin its west phase (Mason and Lindesay 1993), followingthe hypothesis suggested by Gray et al. (1991).

4. An alternative hypothesis concerns the associa-tion between the SST of the subtropical/mid-latitudeIndian Ocean region and southern African (Walker1990). This association is conrmed by numerical ex-periments (Reason 1998; Reason and Mulenga 1999).Interannual warm and cool events not associated withextreme phases of the SO (Reason 1999) and leading to

occasional failing of the association between SO andrainfall over South Africa (Mason 1995) have beenidentied.

5. The fth hypothesis involves the time-evolution ofthe relationship between SO and the rainfall since WorldWar II. The recent (19611990) 30-year southern Afri-can rainfall mean is not signicantly dierent from theprevious one (19311960) (Hulme 1992). However,Flohn and Kapala (1989) noted that warming of thetropical troposphere leads to an acceleration of the de-layed ocean warming in low latitudes and to a greaterfrequency and/or intensity of warm ENSO events. Thistropospheric warming is correlated with an enhanced

latent heat release into the atmosphere when the seasurface temperature (SST) rises above 27 C. Wang andRopelewski (1995) also found signicant changes inENSO amplitude and frequency associated with themean global climate base-state. According to some SSTanalyses, the year 1970 roughly corresponds to a modi-cation of the global ocean interhemispheric SST gra-dient (Trzaska et al. 1996). It is also concomitant withdecadal scale rainfall vagaries in West Africa in theSahelian and Soudanian regions of West Africa (Janicotand Fontaine 1993).

This study investigates the fth hypothesis using twocomplementary approaches. The stability of the tele-

connections between southern African rainfall and SO isinvestigated relative to 1970 and links between SO andSST are detailed. Numerical experiments are also per-formed in order to evaluate the impact of the mainglobal SST anomalies on the relationships betweensouthern African rainfall and ENSO. The study focuseson the late summer season. The spatial coherence of theinterannual rainfall variability associated with SO iscarried over the subcontinental scale which is supposedto be the most appropriate.

Data, analyses and numerical experiments are de-tailed in Sect. 2. Section 3 shows the main results ofdiagnostic and numerical studies.

2 Data and methods

2.1 Diagnostic analyses

Diagnostic analyses are mainly based on principal component an-alyses (PCA) for rainfall and SST and correlation between rainfall,SOI and SST anomalies. The dierent datasets used are described.

2.1.1 Rainfall

Most of the southern Africa's rainfall occurs during the SouthernHemisphere summer, from October to March. Two seasons areusually distinguished according to the main features of the atmo-spheric circulation: early summer season (October to December),with more inuence from the extratropical atmospheric circulationand late summer season (January to March), when the circulationis more tropical in nature (D'abreton and Lindesay 1993). The SOis a typical tropical phenomenon and its impact concerns mainlylate summer rainfall (Lindesay 1988). Hence in this study focus isplaced on the late summer season, the wettest and the most directlyrelated to tropical circulation. This season contributes more than40% to the annual totals, except in southwestern Africa (Fig. 1).

An index, the Southern African Rainfall Index (SARI), hasbeen computed from dierent data for the period 194694 to reectthe main interannual late summer rainfall variability on a sub-continental scale. Previous investigations of southern Africa rain-fall variability used various rainfall indices. Some of them werebased on annual totals or seasonal regimes (Lindesay 1988;Lindesay and Vogel 1990; Shinoda and Kawamura 1996), otherswere calculated for boxes or relatively small geographical regions(Jury et al. 1994, 1996; Nicholson and Kim 1997). Other rainfallindices are based on SO-related rainfall sensitivity (Ropelewski andHalpert 1987, 1989, 1996; Kiladis and Diaz 1989; Mason andLindesay 1993; Mason 1995). In contrast, coherent areas based onthe interannual rainfall variability over the whole continent wereidentied by Janowiak (1988) and over the southern subcontinentby Rocha and Simmonds (1997a). In the same way we propose asynthetic Southern African Rainfall index based on the interannual

rainfall variability which is achieved through PCA of the monthlyrainfall data.The data were compiled from ONRL/CDIAC 1946 to 1988

monthly raingauge data, completed with original data for SouthAfrica, Namibia and Zimbabwe. A total of 149 stations with lessthan 5% of missing values were rst selected and the missing valueslled in from a stepwise multiple linear regression. Therefore, the

Fig. 1 Raingauge network and late summer (JanuaryMarch) con-tribution (in %) to the annual rainfall in southern Africa

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statistical distribution of the series was not modied. A PCA per-formed on these data for 1946 to 1988 for all JanuaryMarchseasons denes the rst mode of interannual late-summer rainfallvariability. The rst eigenvector (PC1) extracts 31.9% of the totalvariance (Fig. 2); the others describe less than 10% each. Thespatial coherence of the rst PC is high on the plateau (Fig. 2). Thishas already been noticed in Bigot et al. (1995) who performedanalyses on the whole of tropical Africa and found that it is clearly

the third largest mode of variability over the period 19511988.These authors also showed that unlike the Sahelian mode and de-spite small modications of its spatial extent, this mode is stablewhen the period is split into two sub-periods (19511967 and 19681988). Thus, it can be considered as the main mode of rainfallvariability over southern Africa.

In order to extend this initial period through to the 1990s,complementary rainfall data have been included. They were ex-tracted from the Climate Research Unit (CRU) dataset with a 3.75 longitude by 2.5 latitude resolution (Hulme 1992). A total of 28gridpoints were selected covering the area of the PC1 of interest(thick contour on Fig. 2). The timeseries of each of these 28 grid-points was standardized using the 19461994 period. The spatialaverage computed over this area is strongly correlated with PC1(r = 0.99). Hence it may be considered as representative of themain variability over the subcontinent between 1946 and 1994 and

will be used hereafter as the Synthetic Southern African RainfallIndex (SARI).

2.1.2 Southern Oscillation Index

The usual Tahiti minus Darwin sea-level pressure (SLP) dierenceis used for SOI. ENSO and La Nin a Southern Oscillation (LNSO)events are selected following Ropelewski and Halpert (1996)(Table 1). Years of Table 1 correspond to years 0 in Rasmussonand Carpenter (1982). For the post World War II ENSO events,

more common elements are found in recent studies attesting to arobust composite (Harrison and Larkin 1996, 1998). According tothe latter classication, the late summer seasons are characterizedby noticeable rainfall anomalies and correspond to the ``decayingENSO'' phase when SST anomalies in Pacic are well established.This corresponds to the maximum of Indian Ocean warming oc-curring in JanuaryMarch of year +1 (Nicholson 1997; Reasonet al. 2000).

2.1.3 Sea surface temperatures

In this work, the MOHSST4 SST gridded dataset (5 5) for the194594 period provided by the United Kingdom MeteorologicalOce are used. The leading interannual variability modes of theglobal SST determined from a Varimax PCA on monthly nor-malized data are used (Trzaska et al. 1996). The rst mode depictsa Global Tropical-wide mode (GTm) clearly dominated by anENSO-like pattern (Fig. 3a). It shows quasi-regular oscillations,with a main periodicity of 4 to 5 years and accounts for 10.2% ofthe total variance (Fig. 3b). The second (5.24%) and the fourth(5.03%) modes reect variability in the Atlantic (not shown). Thethird mode (5.21%) describes the variability of the extratropics(Fig. 3c) and is hereafter referred to as the GEm (Global Extra-tropical mode). This mode shows an out of phase evolution of theSST between the northern high latitudes (negative loadings), andthe South Atlantic and Indian oceans (positive loadings), con-rming that pronounced multidecadal SST variations appear to bein phase at midlatitudes of the South Indian, South Atlantic, NorthPacic, and North Atlantic oceans (Allan et al. 1995; Reason2000). This mode is similar to the mode dened by Folland et al.(1986). It is marked by a slow warming of southern oceans from1965 to the 1980s and mainly reects the evolution of the climaticSST background (Fig. 3d).

2.2 Numerical experiments

2.2.1 Denition of experiments

The main goal of the numerical experiments is to study the impact

of objectively dened ENSO-like SST anomaly patterns onsouthern African rainfall through short range, equilibrium statesimulations in March. As in Trzaska et al. (1996), the SST forcingwas objectively dened using two of the aforementioned loadingpatterns, namely, GTm and GEm. Two experiments were per-formed:

1. ``Early ENSO'' experiment using (2*GTm ) GEm) * c SSTanomalies. It corresponds to ENSO events superimposed oncolder SST conditions in southern latitudes that occurred dur-ing the 1950s and the 1960s (Fig. 4a);

2. ``Recent ENSO'' experiment i.e. (2*GTm + GEm) * c. It cor-responds to ENSO events superimposed on warmer SST con-ditions in southern latitudes that occurred during the 1970s andthe 1980s (Fig. 4b).

The numerical constants c allows one to account for the greater

part of variance described by GTm and to convert the standarddeviation elds into realistic anomaly elds (i.e. maximum of about3 K in the Pacic and 1 K in the Indian Ocean). In this way thehigher-frequency ENSO-like mode is embedded in two dierentglobal SST contexts.

2.2.2 Model and simulations

The model used in the experiments is the ARPEGE-ClimatAtmospheric General Circulation Model (AGCM) developedby CNRM (Centre National de Recherches Me te orologiques ofMe te o-France). It is a spectral AGCM with a 30-layer verticalresolution (De que et al. 1994). The T42 truncation correspondingto a regular grid (64 latitudes and 128 longitudes) was used. Thevegetation scheme ISBA was also included.

Fig. 2 Loading pattern of the rst principal component on the 149southern African JanuaryMarch rainfall amounts (19461988).Absolute values greater than 0.4 (0.3) are signicant at 99% (95%)level. Thick contour is the area selected for SARI

Table 1 Ten ENSO and 9 LNSO during 195094 period (afterRopelewski and Halpert 1996)

SO episode year ``0''

10 ENSO 51, 53, 57, 65, 69, 72, 76, 82, 86, 919 LNSO 50, 55, 56, 64, 70, 71, 73, 75, 88

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Each experiment consisted of 50-day runs in an evolutionarymode, i.e. the seasonal cycle (radiation, SST) is included but theanomalies are kept constant. The rst 20 days (approximately) wereused to spin up the model and only the results of the last 31 days(March) are considered meaningful. As in Mason et al. (1994) andJury et al. (1996), the model was initialized using the SST anomalyelds of February, month of highest SST in the Southern Hemi-sphere. Thus, the rst experiment is similar to those of DecemberFebruary in Rocha and Simmonds (1997b), with positive SSTanomalies in the global tropics and negative anomalies south of30, but SST anomalies are global and not restricted to a box. Theexperiments are designed over FebruaryMarch and are represen-tative of the late summer season as a whole.

Because AGCMs are very sensitive to initial conditions(Palmer and Anderson 1994), three integrations were performedfor each SST eld, starting with dierent initial atmosphericconditions i.e. the middle of February 1979, 1980 and 1981. Short-term and simultaneous responses to SST are analyzed. Monthlymeans and variances for selected climatological elds for each SSTpattern are compared to the March climatology of ARPEGE-Climat, which is the Atmospheric Model Intercomparison Project(AMIP) 19791988 mean (Gates 1992). Student's t-tests orHotelling T2 tests enable us to assess whether the prescribedanomalies do have any signicant impact on the simulated resultsor not.

3 Results

3.1 Diagnostic studies

3.1.1 Stability of SARISOI relationships

During 19461994, the correlation between January

March SOI and SARI is signicant at the 95% level(r = 0.3). This conrms the statistical links between SOIand southern African rainfall already noticed in severalprevious studies. However, the relationship is weak andthe SOI explains less than 10% (r2 = 0.09) of the in-terannual rainfall variability. Furthermore, Fig. 5 showsthat the relationship is not robust. Five ``decayingENSO'' events coincide with drier than normal condi-tions and ve with wetter. Four LNSO lead to negativerainfall anomalies and ve to positive anomalies. Notethat 1970 is the rst JanuaryMarch season recording astrong drought synchronous with a ``decaying ENSO''.Hence two periods can be distinguished: 19461969 and

Fig. 4a, b Sea surface temperature anomaly elds (in K) used in thetwo numerical experiments. Left 2GTm GEm: ``Early ENSO'';2GTm + GEm: ``Recent ENSO''. Right Light grey and dashed

contours for positive anomalies warmer than 0.5 K. Dark grey andsolid contours for negative anomalies colder than )0.5 K. Isolinesevery 0.5 K

Fig. 3ad Loading patterns and time-series of the rst and third sea

surface temperature anomaly (SSTA) modes obtained from varimaxrotated principal component analysis. a Correlation with the rstmode ``GTm'' (10.2% of explained variance); b scores of the rst

mode ``GTm''; c correlation with the third mode ``GEm'' (5.21% of

explained variance); d scores of the third mode ``GEm''. a and cshaded areas and solid (dashed) lines show positive (negative) linearcorrelation coecients jRj > 0.2


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19701994. During the rst period (19461969), all``decaying ENSO'' seasons are relatively wet (1952,1954, 1958 and 1966). By contrast, between 1970 and1994, most ``decaying ENSO'' seasons are abnormallydry (1970, 1973, 1983, 1987 and 1992). Only the 19761977 event, which is the least characteristic of the ten

post World War II ENSO (Harrison and Larkin 1998),is followed by a wet season over southern Africa (Jan-uaryMarch 1977). Dierences between the two sub-periods are also observed for the cold Pacic events(LNSO). During the 1950s and 1960s, the end of LNSOevents is associated with three abnormally dry JanuaryMarch (1951, 1957 and 1965) and one abnormally wet(1956). During the 1970s and 1980s the end of the coldevents is frequently wet (1972, 1974, 1976 and 1989).Furthermore, after 1970 the four driest JanuaryMarchseasons (1970, 1983, 1987, 1992) correspond to ``decay-ing ENSO'' and the two wettest (1974, 1976) to the endof LNSO events (Fig. 5 and Table 1). Therefore, the

impact of the Southern Oscillation on southern Africanrainfall seems to have been modied since the end of the1960s.

The stability of the SOSARI relationship is assessedthrough correlation coecients on 20-year runningwindows (Fig. 6). The rst value, calculated for the19371956 window is indexed as 1946, the last, calcu-lated for the 19751994 period, as 1984. Local statisticalsignicance at 5% level is obtained with Monte Carlosimulations computed independently on each 20-yearrunning window. Correlation coecients are generallypositive, except during the 1950s. Since the late 1960scoecients have become signicant at the 5% level. A

similar recent reinforcement of the correlation with theSOI was observed for the Sahelian rainfall index (Janicotet al. 1996).

Further investigation of the evolution of the spatialpattern of the relationship between SOI and gaugerainfall was conducted by computing correlation coe-cients on the 149 rain-gauge series for both sub-periods(Fig. 7). Before 1970, coecients are positive in thesouthwestern part of the subcontinent, and negative insouthwestern Zambia and in Zimbabwe (Fig. 7a). Theybecome positive further north. During the recent period,positive correlation coecients are widespread (Fig. 7b).The southern African rainfall response to SOI becomes

more spatially coherent after 1970. This evolution of therelationship through time explains the weakness of thecorrelation coecients calculated for the entire period.Particularly strong over southwestern Zambia andZimbabwe, this evolution facilitates understanding ofthe lack of relationship between the ENSOSST modeand rainfall over the semi-arid northeastern part of

southern Africa depicted by Shinoda and Kawamura(1996).

The high SO phase is associated with widespreadregional-scale rainfall anomalies after 1970 but thespatial coherence of rainfall variability was already ef-fective during the 19461969 period. PC1 for 19461969(not shown) is spatially similar to the one computed onthe entire period, but explains only 25.7% of varianceagainst 31.9% for the PC1 computed over the wholeperiod and 41.5% for 19701988. When ENSO/LNSOyears are considered separately from non-ENSO/LNSOyears, the PC1 explains 40% of the variance (27.4% fornon-ENSO/LNSO years).

Fig. 5 Southern African Rain-fall Index (SARI) and SouthernOscillation (SO) events (19461994). ENSO (LNSO) eventsin black (grey)

Fig. 6 Time evolution of correlation between Southern African

Rainfall Index (SARI) and Southern Oscillation Index (SOI) inJanuaryMarch along the 19461994 period with a 20-year runningwindow. 1970 means period 19611980. The dotted line represents the5% local statistical signicance level according to Monte Carlosimulations calculated for each 20-year running window


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3.1.2 SST-rainfall relationships

Southern African rainfall is known to be very sensitiveto SST anomalies in central and southwestern IndianOcean and, to a less extend, the southern mid-latitudesand southern Atlantic (Walker 1989; Jury 1995; Mason1995; Reason and Mulenga 1999).

In order to evaluate the evolution of the teleconnec-tions between SARI and global SST, linear correlationwith gridded SST is computed on both sub-periodsseparately. The 19461969 period is characterized bypositive correlation between SST and rainfall (Fig. 8a).Correlations are signicant in the north equatorialAtlantic and the southwestern Indian Ocean. Therelationship between this part of the Indian Ocean, andrainfall over eastern South Africa is well known: wetconditions are associated with positive SST anomalies

(Walker 1989). The correlation pattern for the 19701994 period is dierent (Fig. 8b). The positive coe-cients disappear and tropical SST is now negativelycorrelated with late summer rainfall in southern Africa.The most striking feature is the emergence of correlationwith eastern Pacic during this period. This pattern isusually associated with ENSO/LNSO events and high-lights the enhanced impact of the SO on southernAfrican late summer rainfall.

3.1.3 SOISST relationships

Previous composite analyses (Trzaska et al. 1996)showed noticeable changes in the worldwide SSTanomalies related to ENSO; mainly a warmer IndianOcean after 1970. The strengthening of the relationship

Fig. 7a, b Correlation coecients between JanuaryMarch gauge rainfalls and Southern Oscillation Index. a 19461969; b 19701988. Absolutevalues greater than 0.55 (0.4) are signicant at 99% (95%) level

Fig. 8a, b Correlation coe-cients between JanuaryMarchSouthern African Rainfall In-dex (SARI) and sea surfacetemperature. a 19461969;b 19701992. 95% signicancelevel is shaded: dark (light) for

positive (negative) values


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between the central Indian Ocean and the central PacicSST anomalies during the more recent period was alsoobserved by Lanzante (1996). However, in an ENSOcomposite sample, Harrison and Larkin (1996) showedthe weak robustness of this teleconnection. In order todetect possible changes in SO related SST anomalypatterns, the latter are correlated with SOI over eachsub-period. The results (Fig. 9) show stronger and moreextended negative correlation coecients in the Pacicand Indian Oceans after 1970. These results are consis-tent with previous ndings. They can also be related tothe work of Wang and Ropelewski (1995) who foundthat ENSO-scale variability is higher (both in frequencyand amplitude) when the climate low-frequency ``basestate'' is relatively warm. Thus higher SOSARI tele-connections could be related to changes in the SO-re-lated SST anomaly patterns. This question is assessedthrough numerical experiments which take into accountthe changing SST background depicted by the GlobalExtratropical mode.

3.2 Numerical experiments

3.2.1 Rainfall and outgoing longwave radiation

The mean March rainfall computed from AMIP 19791988 simulations correctly reproduces the spatial dis-tribution of the observed precipitation (Fig. 10a, inmm/day). Rainfall maxima are clustered around 10S.They correspond to the mean location of the Inter-tropical Convergence Zone (ITCZ). Other rainy areasare located on the eastern slopes of the major moun-

tain ranges: Madagascar, Manicaland and Drakens-berg; leesides, like Western Madagascar, are drier. Thedriest areas, such as the Kalahari and Namib deserts,are correctly featured, with a minimum of simulatedprecipitation. This is consistent with the results ofJoubert (1997) who found that the AMIP generation

of models provide an improved simulation of meancirculation around southern Africa when comparedwith earlier AGCM, mainly due to increased resolu-tion allowing better representation of southern Africaorography.

In the ``early ENSO'' experiment, where ENSO SSTanomalies are associated with relatively cool tempera-tures in the southern Indian and extratropical oceans,rainfall is close to the AMIP mean (Fig. 10b). Non-signicant decits appear in the southwestern part of thesubcontinent, and excedents are observed in the centralpart. These features are consistent with observations(Fig. 7a). Over the Indian Ocean, the positive anomaliesdo not spread beyond 20S and are not signicant. Inthe ``recent ENSO'' experiment, in which ENSO wassuperimposed on a warmer Southern Hemisphere SST,rainfall is below the AMIP mean over the whole sub-continent although signicant anomalies do not coverlarge areas (Fig. 10c). These results are again consistentwith observation (Fig. 7b). Rainfall is above normal

over the entire Indian Ocean and a bipolar feature issimulated with decits (excedents) west (east) of 35 E.

The AMIP mean for outgoing longwave radiationux (Fig. 11a) exhibits its smallest values (deep con-vection maxima) over land along a northwest to south-east orientated ridge from the Congo Basin toDrakensberg. Mean deep convection has a spatial con-guration similar to that of rainfall (Fig. 10a), which isconsistent with the mainly convective origin of the pre-cipitation. Bias found in observational data when tryingto estimate precipitation from the OLR signal is alsopresent. Rainfall is overestimated over the rainforest andunderestimated over mountainous and coastal areas

(Richard et al. 1995). The ENSO conditions from therst experiment (Fig. 11b) have little impact on deepconvection. In contrast, signicant anomalies develop inthe second experiment (Fig. 11c) and show a clear op-position between land (decrease of deep convection) andocean (increase). These results are consistent with rain-

Fig. 9a, b Correlation coe-cients between JanuaryMarchSouthern Oscillation Index(SOI) and sea surface tempera-ture. a 19461969; b 19701992.95% signicance level is shaded:

dark (light) for positive(negative) values


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Fig. 10ac Simulated rainfall. a Monthly average (AMIP 19791988);b ``Early ENSO'' experiment: % of the 19791988 AMIP average;c ``Recent ENSO'' experiment: % of the 19791988 AMIP averagein mm/day. bc Shaded: signicant value at the 95% level (t-test);Light shading for below normal, dark shading for above normal

Fig. 11ac Outgoing longwave radiation at the top-of-the-atmo-sphere. a March monthly mean values (AMIP 19791988); b ``EarlyENSO'' experiments minus AMIP average; c ``Recent ENSO''experiment minus AMIP average; in W/m2 s)1. bc Shaded: signi-cant value at the 95% level (t-test); Light shading for below normal,dark shading for above normal


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fall anomalies (Fig. 10c) and with the results of Rochaand Simmonds (1997b). The simulated eastward shift ofthe deep convection area is also consistent with previousdiagnostic studies (Harrison 1986; Tyson 1986; Juryet al. 1994) which identied this feature as a major fac-tor of southern African droughts. Adjustments in thelocation of convection and the alignment of cloud bandshave been linked to changes in Hadley Cell mass over-turning over the east coast of southern Africa and to areversal of the Walker Cell over the western tropicalIndian Ocean between wet and dry years (Lindesay1988). This eastward shift is strongly coupled withwarmer temperatures in the Indian Ocean. This mayexplain the stronger link between ENSO and southernAfrican droughts during the recent period.

3.2.2 Atmospheric circulation

The eastward shift of the main deep convective region is

directly related to anomalies in both low-level and high-level dynamics. At the 850 hPa level, AMIP reproducesthe mean meridional geopotential height gradient. Asubtropical high-pressure belt is located between 30 and35S, although a little less dened over the western partof the subcontinent between 15 and 25E (Fig. 12a). Inthe `èarly ENSO'' experiment, the meridional gradientis slightly reduced because of abnormally high equato-rial and abnormally low mid-latitude pressures(Fig. 12b). This result is consistent with the above nor-mal global 700-hPa geopotential height pattern observedduring the warm phase of ENSO by Shinoda and Ka-wamura (1996). In the ``recent ENSO'' experiment, a

zonal gradient of signicant geopotential height anom-alies is simulated. The pressure is enhanced over south-eastern Africa and reduced in the Madagascar region(Fig. 12c). The experiment leads to the same geopoten-tial anomalies as those simulated by Jury et al. (1996)and corresponds to a negative pressure gradient betweenBrandon and Marion Islands, which is positively cor-related with rainfall in the Zambezi basin as shown inRocha and Simmonds (1997a). Furthermore, anoma-lously high rainfall January 1997 reects a reversalpattern (Reason and Lutjeharms 1998). This scenario isconsistent with an eastward shift of the preferred cloudband mean position towards the Indian Ocean during

dry summers, as reported by Harrison (1986). Only the``recent ENSO'' experiment reproduces well-identiedcirculation anomalies that are known to be responsiblefor dry conditions over the subcontinent. The high-pressure belt is reinforced over the western part ofsouthern Africa, between 15 and 25E and the relativeweakness in the subtropical high-pressure belt is shiftedeastward towards the south of Madagascar.

The mean wind ow analysis shows that the moisttrade winds penetrate deeply into the continent (notshown). For the `èarly ENSO'' experiment this situationis roughly maintained. On the contrary, for the ``recentENSO'' experiment, wind anomalies become stronger

over the southwestern Indian Ocean (not shown). Ex-ceptionally strong southerly winds are simulated overthe eastern part of southern Africa and the MozambiqueChannel. These winds prevent the Indian Ocean trade

Fig. 12ac 850 hPa geopotential height; a March monthly meanvalues (AMIP 19791988); b `Èarly ENSO'' experiment minus AMIPaverage; c ``Recent ENSO'' experiment minus AMIP average, in m.bc Shaded: signicant value at the 95% level (t-test); Light shadingfor below normal, dark shading for above normal


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winds that bring moist air to the continent from reach-ing the core of southern Africa.

In the middle and high latitudes of the SouthernHemisphere, a semi-annual wave dominates. Van Loonet al. (1993) have shown a weakening of this featureduring the 1970s and 1980s. Dry southern African yearsare associated with decreased ridging in the vicinity ofGough Island in the mid-latitudes, indicating the pres-ence of a smaller amplitude westerly wave, with a lack oftrough over the west coast (Taljaard 1986). Therefore,the observed semi-annual wave weakening could aectthe ENSO/rainfall relationship. Decrease in pressurearound Gough Island (not shown) and increases alongthe west coast (Fig. 12) are simulated for both experi-ments. Nevertheless, similar results at 500 hPa (notshown) conrm that the simulated relationship betweenENSO and mid-latitude circulation was not signicantlymodied after 1970.

A meridional cross section of the zonal wind isconstructed along 30E. Figure 13a shows the `èarlyENSO'' experiment, Fig. 13b the ``recent ENSO'' ex-periment and Fig. 13c displays the dierence. In Jan-uaryMarch the two subtropical high-level westerlyjets and the middle latitude low-level westerliessurround easterly winds which reach their maximumvelocity at 900 hPa by 20 of latitude in both hemi-spheres, with a weak high level (200 hPa) easterly jetover southern Africa (not shown). For the `èarlyENSO'' experiment, this conguration is maintained(Fig. 13a) but it is modied for the ``recent ENSO''experiment in the ITCZ region i.e. near 10S at thislongitude (Fig. 13b). At 200 hPa the tropical easterlyjet is enhanced, while westerly winds develop in thelow levels. Enhanced upper-level easterly winds havebeen observed in association with dry spells inZimbabwe (Matarira and Jury 1992). The dierence

Fig. 13ac Meridional crosssection of the zonal wind at 30East between 950 and 50 hPa.a `Èarly ENSO'' experiment;b ``Recent ENSO'' experiment;c (`Èarly ENSO'' experiment) (``Recent ENSO'' experiment)values, in m/s. c Light shading:negative dierences, dark shad-ing: positive dierences


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between the two experiments highlights the enhancedwesterly winds between 15S and 5N (ITCZ) and thetropical easterly jet of the ``recent ENSO'' experiment(Fig. 13c).

These modications of the atmospheric circulationover southern Africa and western Indian Ocean are partof global scale circulation changes. The velocity poten-tial reproduces most of the global scale ux conver-gence/divergence. At the 850-hPa level, the meanconvergence maximum (positive values) is located eastof Indonesia and two secondary maxima appear overAfrica and Amazonia (Fig. 14a). Divergence maxima(negative values) are located close to the main subtrop-ical highs of the two hemispheres. For the ``earlyENSO'' experiment, the spatial distribution of thesecentres is modied. Negative anomalies show than theWest Pacic convergence maximum weakens (Fig. 14b)

whereas positive anomalies signal that some conver-gence anomalies develop in east Pacic. The African andIndian Ocean area is unaected. For the ``recent ENSO''experiment, these changes are more complex and involvethe peri-African region (Fig. 14c). Positive (convergent)anomalies of the same amplitude as those of the eastPacic develop over the Indian Ocean. Simultaneouslydivergent anomalies can be observed over the AtlanticOcean. A pattern exactly opposite to that of the low-level anomalies is found at the 200 hPa level (notshown). The dipolar structure existing between Africaand the Indian Ocean already observed on rainfall(Fig. 10), OLR (Fig. 11) and 850 hPa geopotentialheight (Fig. 12) patterns, is reproduced. Four anomalycentres are present for the ``recent ENSO'' experiment.Warmer Indian Ocean SSTs modify the Indian eastwestcirculation, in such a way that during recent ENSO

Fig. 14ac Velocity potential at850 hPa. a March monthlymeans values (AMIP 19791988); b ``Early ENSO'' exper-iment minus AMIP average;c ``Recent ENSO'' experimentsminus AMIP average, in m2/s.b and c Shaded: signicantvalue at the 95% level (t-test);light shading for below normal,dark shading for above normal


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events, not only the Pacic, but also the Indian Oceansector, experience strong velocity potential anomalies.

4 Conclusion

The principal component analysis performed on latesummer season (JanuaryMarch) gures indicates highspatial coherency of interannual rainfall variability oversouthern Africa. The relationship between the associatedindex (SARI) and the SO is signicant but moderate. Nosimultaneous occurrence of dry (wet) late summer andENSO (LNSO) events is recorded before 1970. This isconsistent with ndings by Kruger (1999) that identiednear-decadal epochs of below and above-normal rainfallamounts over South Africa. Thus the severity of theimpact of El Nin o during wet periods and La Nin a duringdry period are moderated. However, after 1970, duringabove normal 1970s as well as below normal 1980s ENSO(LNSO) are clearly associated with abnormally dry (wet)

conditions. Increased spatial coherency of the rainfallvariability seems also to be linked to ENSO events.

The study of global SST anomalies shows that thespatial pattern of anomalies associated with SO since1970 has been more extensive, and that recent ENSOevents are embedded in dierent global scale climateconditions that the previous ones. The impact of thesenew conditions is studied through numerical experi-ments using SST anomalies dened from rst eigen-modes of SST variability, reproducing ENSO variabilityand a slow evolution of the tropical south Indian Oceanand southern oceans. Rainfall anomalies are well re-produced with weak anomalies for the simulated ``early

ENSO'' conditions, and a clear opposition betweena dry continent and wet Indian Ocean in the ``recentENSO'' experiment. The response is enhanced both inmagnitude and in spatial extension. This eastwest op-position is characteristic of dry conditions over southernAfrica, as reported by many authors. It is associatedwith an eastward shift of the simulated main deep con-vection area over the Indian Ocean. This shift seemsforced by warm anomalies in the tropical south IndianOcean leading to a weakened subtropical high belt at thelongitude of Madagascar. Moisture-bearing easterliescannot penetrate over the continent but fuel the deepconvection in the ITCZ over the Indian Ocean. Because

of a warmer Indian Ocean, eastwest circulation ismodied, not only in the Pacic as for pre 1970 ENSOevents but also, with approximately the same magnitude,in the Indian Ocean sector. As shown in Rocha andSimmonds (1997b) and in Reason and Mulenga (1999),the Indian Ocean plays a key role in the rainfall vari-ability in southern Africa. Its long-term warming coin-cides with signicant ENSO-related rainfall decits. Thiscan be part of the Indian Ocean modulation of ENSOimpact pointed out by these authors.

Further diagnostic and numerical investigations ofrainfall decits related to Indian Ocean warming havealready been undertaken. The impact of the cold LNSO

phase on southern Africa late summer rainfall, associ-ated with post 1970 warmer southern oceans has also tobe assessed.

Acknowledgements C.R.C. Reason, School of Earth Sciences,University of Melbourne, Australia, and Department of Ocean-ography, University of Cape Town, South Africa. J.A. Lindesay,

Australian National University, Canberra, Australia and the Cli-mate Research Group of the University of Witwatersrand, Jo-hannesburg, South Africa, are thanked for supplying data whichhelped to complete the raingauge dataset compiled by the ORNL/CDIAC, Tennessee, United States of America. M. Hulme, Uni-versity of East Anglia, United Kingdom, for the rainfall grid-pointdataset. The Water Research Commission is acknowledged. Thisstudy is part of the FRANCE/SOUTH AFRICA co-operativeproject. We are grateful to M. De que and the CNRM sta forproviding ARPEGE-Climat AGCM. P. Camberlin and B. Fon-taine, Centre de Recherches de Climatologie, CNRS Universitede Bourgogne, Dijon, France, and the anonymous reviewers fortheir useful comments on the manuscript.

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