The impact of the positive Indian Ocean dipole on Zimbabwe droughts

Abstract

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.

ENSO and IOD teleconnections for African ecosystems: evidence of destructive interference between climate oscillations

“…During this period, the southern Africa regions (Namibian, SE African) show unusual positive association with ENSO, likely owing to the typically positive response to IOD characteristic of the region and indicating destructive interference with a dominant response to IOD forcing. This is consistent with a recent analysis of rainfall in Zimbabwe by Manatsa et al (2008) that similarly suggests IOD dominance over ENSO when the two are in competition.…”

Spatial and temporal variability of Standardized Precipitation Index over Indochina Peninsula

“…The correlation coefficient ranges from -0.4 to 0.4. This range is considered to be reasonable for correlation between SST and local precipitation (Evan et al, 1998;Manatsa et al, 2008;Zhang and Zhou, 2015). As seen in annual and most of the seasonal plots (Fig.…”

On the decoupling of the IODZM from southern Africa Summer rainfall variability

“…As a result, one may assume that the decoupling process between the IODZM and rainfall of southern Africa took place around this time. In the operational context, this observation suggests severe limitations to the usefulness of the IODZM index as a regional rainfall predictor after 1997, contrary to the recommendations of Manatsa et al (2007Manatsa et al ( , 2008.…”

“…Monthly mean NCEP/NCAR reanalysis data for the period 1948-2008(Kalnay et al, 1996 are used to assess changes in the atmospheric circulation over the Indian Ocean region. Only the sea surface temperature (SST), wind stress and mean sea level pressure fields (MSLP) have been used herein because there is a general agreement that these variables are relatively reliable (Poccard et al, 2000;Nicholson and Webster, 2007).…”

“…Black et al, 2003;Saji and Yamagata et al, 2003). Manatsa et al, (2007Manatsa et al, ( , 2008 showed that the IODZM could have provided a potentially more robust rainfall predictor for Zimbabwe summer rainfall than ENSO for the last half of the 20th century. In addition to the IODZM, a dipole SST pattern in the south Indian Ocean may also be important to the late summer rainfall of the eastern areas of southern Africa (Behera and Yamagata, 2001;Reason, 2001).…”