Interannual to Interdecadal variability of winter and summer Southern African rainfall, and their teleconnections

Abstract:

This study examines for the first time the changing characteristics of summer and winter Southern African rainfall, and their teleconnections with large-scale climate through the dominant timescales of variability since 1901. As determined by wavelet analysis, the summer and winter rainfall indices exhibit three significant timescales of variability over the century: interdecadal (15–28 year), quasi-decadal (8–13 year) and interannual (2–8 year). Teleconnections with global sea-surface temperature and atmospheric circulation anomalies are established here, but are different for each timescale. El Niño Southern Oscillation (ENSO) is the main driver of summer rainfall variability, even at the decadal timescale through its forcing on the Pacific Decadal Oscillation and its collinearity with the Interdecadal Pacific Oscillation. This ENSO influence leads to shifts in the Walker circulation, which, at the regional scale, impacts the position of the Southern Indian Convergence Zone modulating the development of deep-convection and the synoptic-scale rain bearing systems over the northeastern regions of Southern Africa. Summer rainfall variability is also related to latitudinal shifts in the subtropical atmospheric circulation, which are, for instance, generating an anomalous low-level easterly moisture flux. Winter rainfall variability, however, is more influenced by the mid-latitude atmospheric variability, in particular the Southern Annular Mode, but interactions with ENSO remain, especially in the subtropics. Asymmetrical changes in the mid-latitude westerlies between the Atlantic and Indian Oceans are thus impacting preferentially the southwestern regions of Southern Africa.