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A comparative study of the impact of the anomalous positive Indian Ocean SST gradient, referred to as the Indian Ocean Dipole/Zonal Mode (IODZM), and El Niño-Southern Oscillation (ENSO) on Zimbabwe seasonal rainfall variability for the period 1940-1999, is documented. Composite techniques together with simple and partial correlation analyses are employed to segregate the unique association related to IODZM/ENSO with respect to the Zimbabwe seasonal rainfall.The analysis reveals that the IODZM impact on the country's summer rainfall is overwhelming as compared to that of ENSO when the two are in competition. The IODZM influence remains high (significant above 99% confidence level), even after the influence of ENSO has been removed, while that of ENSO collapses to insignificance (even at 90% confidence level) when the IODZM contribution is eliminated. The relationship between ENSO and Zimbabwe seasonal rainfall seems to be sustained through El Niño occurring in the presence of positive IODZM events. However, when the co-occurring positive IODZM and El Niño events are removed from the analysis, it is apparently clear that ENSO has little to do with the country's rainfall variability. On the other hand, positive IODZM is mostly associated with the rainfall deficits, whether or not it co-occurs with El Niño. However, the co-occurrence of the two events does not necessarily suggest that El Niño influences droughts through the positive IODZM events. The El Niño event components during co-occurrence seem to be unrelated (at least linearly) to the droughts, while the positive IODZM events display a relatively strong relationship that is significant above the 95% confidence level. It thus becomes important to extend the study of this nature to cover the whole of southern Africa, so that the extent of the impact of the phenomena can be realized over the whole region.

This study attempts to identify meteorological conditions that lead to dry and wet episodes over Zimbabwe. Correlation analyses are carried out between the Southern Oscillation Index and rainfall over south-eastern central Africa. The results show significant correlation between the Southern Oscillation (SO) and seasonal rainfall over the region. The results also indicate that the SO signal is stronger over the south-east of Zimbabwe and along the coastal areas of Mozambique. El Nifio events seem to be associated with below normal rainfall over Zimbabwe, while cold events, which are the opposite extreme of the SO, are accompanied by above average rainfall.Three-month seasonal (December-February) deviations of 1000-mbar temperature, surface pressure, and the 500-mbar geopotential height from the long-term mean are analysed during wet and drought years. The anomaly pressure gradient force is directed towards the subcontinent during wet periods and towards the ocean area during dry episodes. A similar pattern is observed at the 500-mbar level.A comparison of wet and dry seasons in relation to the temperature anomaly field points to an association between warmer 1000-mbar surface temperatures around the African subcontinent and above normal seasonal rainfall. Correlation analysis shows that a high incidence of tropical cyclones in the Mozambique Channel is associated with below normal rainfall over Zimbabwe.

ABSTRACT:The current paper is an observational study that investigates the October to December (OND) rainfall variability over the east Southern African Development Community (SADC) mainland region in relation to El Niño-Southern Oscillation (ENSO) and the Indian Ocean dipole zonal mode (IODZM) for the period 1950-1999. An empirical orthogonal function (EOF) analysis of OND rainfall field revealed that the north-south aligned areas of the eastern SADC are located in different covariability regions. This meridionally aligned dipole rainfall anomaly configuration is captured only in the dominant principal component (PC1), making it possible for the opposing rainfall anomalies of the two regions to have a common trigger. However, ENSO which is the standard attribute for regional rainfall variability failed dismally to adequately explain this dipole rainfall anomaly pattern. Instead, there appears to be consistent evidence through statistical techniques which strongly indicates the likelihood of the participation of only the positive IODZM phase events in the creation of the positive dipole rainfall phase (i.e. simultaneous floods over the northeast and droughts over the southeast of the SADC region). Since the negative IODZM phase events can hardly be linked to the reverse rainfall pattern, it implies that the positive and negative rainfall dipoles have fundamentally different causes. Thus, contrary to convectional knowledge, the ENSO association to this dipole rainfall anomaly pattern is by no means robust and could be symptomatic of the well-known ENSO-IODZM connection. Interestingly, however, when analysed over the 31-year overlapping segments, IODZM's once significant independent influence on this dipole rainfall seems to be diminishing gradually as from the early 1990s, whereas that of ENSO is correspondingly being reinforced.

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The meteorological structure of contrasting wet and dry spells in Zimbabwe, south-east Africa is studied using European Centre for Medium Range Weather Forecasts composites. Diagnostic examination of composite synoptic-scale features indicates that lower northerly and upper easterly flow is enhanced during wet spells, corresponding with a tropical low over Zambia and an anticyclone off the south-east coast of Africa, respectively. Dry spells exhibit an increase in midlatitude cyclones off the south-west coast of Africa and tropical cyclones near Madagascar. Mid-tropospheric troughs are located over the east and west coasts of Africa near 25"s and serve to increase anticyclonic vorticity over Zimbabwe in dry spells. Specific humidity fields reveal that the maximum shifts southward 5" and eastward 10" in the wet spells. Cyclonic vorticity and low geopotential heights occur on two or three days in the wet spell, coincident with intense cumulus convection and high rain rates.The eddy covariances of zonal wind and specific humidity are dominated by westward (eastward) fluxes in wet (dry) spells; while poleward moisture fluxes contributed by transient eddies are largest over the south Mozambique Channel (south-east Atlantic) in wet (dry) spells. The eddy fluxes are shifted 15-20" longitude eastward in the wet spells. The relative contribution of thermodynamic and kinematic inputs are evaluated in the mean flows and transient eddies for contrasting wet and dry spells. Much of the difference between wet and dry spells is explained by the mean flow, while the transient flux in the mid-latitudes assumes a subordinate role. KEY WORDS Fluctuations Wet and dry spells Intra-seasonal events Rainfall Zimbabwe

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