The South Indian Convergence Zone and Interannual Rainfall Variability over Southern Africa

Abstract

The South Indian convergence zone (SICZ) is identified in this paper as a region of enhanced precipitation extending off the southeast coast of southern Africa during austral summer. Unlike the South Pacific convergence zone, the SICZ is a land-based convergence zone (LBCZ), with position and intensity at least partially determined by surface conditions over southern Africa. An idealized GCM simulation is used to explore the basic dynamics of LBCZs in the Tropics and subtropics. Output from a realistic GCM simulation and the National Centers for Environmental Prediction?National Center for Atmospheric Research 40-Year Reanalysis are analyzed to apply this basic dynamical framework to the case of the SICZ. In contrast to the intertropical convergence zone where column moisture convergence is primarily due to meridional wind convergence in the moist environment, precipitation within the SICZ and the LBCZs in general is also supported by zonal wind convergence, moisture convergence by transient eddy activity, and moisture convergence associated with moisture advection. This fact suggests that interactions between transient and stationary eddy features and between tropical and midlatitude disturbances are key to understanding variability of the SICZ. In a GCM ensemble simulation of the response to ENSO-like warming in the eastern Pacific, the SICZ shifts northeastward because of a weakening of the western portion of the South Indian high. This shift results in the dipole precipitation pattern, with higher precipitation to the northeast and lower precipitation to the southwest, that is observed in connection with drying over southern Africa during warm events.