Changing dependence of Zimbabwean rainfall variability on ENSO and the Indian Ocean dipole/zonal mode

Abstract

A novel approach of using variability as a major descriptive parameter for observational time series is adopted to investigate how southeastern African (Zimbabwe) summer rainfall may have been forced by the El Nino southern oscillation (ENSO) and/or the Indian Ocean dipole/zonal mode (IODZM), concurrently with the changing sea surface temperature (SST) background of the Indo-Pacific region. The period is from 1901 to 2007. Wavelet power spectrum and linear statistical analysis, including simple and partial correlation techniques, are utilized to achieve this end. The results reveal epochal changes in teleconnections and other statistical properties linking southeastern African rainfall variability to the Indo-Pacific SSTs. These epochs conspicuously exhibit the period around 1945/1946, 1960/1961, 1972/1973, and 1997/1998 as demarcating distinct systematic climate turning points during the century. The role of the 1976/1977 climate regime shift seems not to be that apparent but instead, 1960/1961 and 1997/1998 appear to be the major turning points, both in the teleconnections and Indo-Pacific SST temporal characteristics. From the early 1960s to 1997, as much as 28% of the rainfall variability was linearly related to the IODZM while the ENSO hardly explained a quarter of this value. During this period, droughts are especially strongly connected to the positive IODZM events but insignificantly linked to El Nino, thus contradicting the conventional knowledge attributing most of regional rainfall suppression to the warm ENSO phase. However, the post 1997 epoch saw the reversal of the two climate modes’ influences. ENSO influence evidently became activated, attaining the previously assumed dominant role which had proved elusive during the earlier epochs. But this increased ENSO control is attributed to only less than 12% of the rainfall variance while IODZM seem to be largely decoupled from the local rainfall influencing processes by hardly explaining a drastically reduced 1% of the rainfall variance. This transformation occurred despite of neither ENSO nor IODZM SST anomalies showing any statistically significant changes. As far as could be established, this epochal behavior may not be forced by the frequency and magnitude of ENSO and IODZM events but most probably by the slow processes inherent in the SST background, especially of the tropical Indian Ocean. Thus, the apparent simultaneous decoupling of the IODZM and coupling of the ENSO to the regional rainfall controlling mechanisms seems to be the result of the unprecedented warming of the intervening tropical western Indian Ocean SSTs in the last decade. Consequently, the knowledge of the states of epochal Indian Ocean SST background variability should be an important regional scientific issue, since these epochal variations may dictate the nature (and successful prediction) of interannual as well as decadal climate fluctuations over southeastern Africa.