A Southern Hemisphere Wave Response to ENSO with Implications for Southern Africa Precipitation

Abstract

Ensemble GCM simulations with an imposed, idealized warming of the eastern Pacific Ocean reveal two wave anomalies in the Southern Hemisphere, one in the eastern and one in the western hemisphere. Both are statistically significant at the 99% confidence level. Application of a steady-state linear model and a Rossby wave source analysis is used to diagnose the causes of the waves. The western hemisphere wave is forced by the advection and stretching of planetary vorticity by the divergent flow from the Southern Hemisphere component of the central Pacific ?twin anticyclones? that straddle the equator during warm events. The eastern hemisphere wave is a result of the northeastward shift of the South Indian convergence zone (SICZ) that, in turn, is forced from the upper troposphere by convergence to the north. An upper-level convergence maximum over the equatorial Indian Ocean induces divergence to the south, encouraging vertical motion and precipitation to the northeast of the SICZ's normal position. The resulting anomalous upper-level convergence in the climatological position of the SICZ, as well as the anomalous vorticity flux convergence by the transients associated with an equatorward shift of the storm track behind the SICZ, force the eastern hemisphere Rossby wave. Since a shift of the SICZ is a fairly robust observed consequence of ENSO events, these results suggest the mechanism by which drought conditions develop over southern Africa at the height of many warm events. Seasonal prediction capabilities in this region can be improved by monitoring and understanding the details and consequences of the adjustment of the Walker circulation near the equator outside of the Pacific Ocean basin.