Recurrent daily OLR patterns in the Southern Africa/Southwest Indian Ocean region, implications for South African rainfall and teleconnections Nicolas Fauchereau Æ B. Pohl Æ C. J. C. Reason Æ M. Rouault Æ Y. Richard Received: 14 October 2007 / Accepted: 19 May 2008 / Published online: 6 June 2008 Ó Springer-Verlag 2008 Abstract A cluster analysis of daily outgoing longwave radiation (OLR) anomalies from 1979 to 2002 over the Southern Africa/Southwest Indian Ocean (SWIO) region for the November to February season reveals seven robust and statistically well separated recurrent patterns of large- scale organized convection. Among them are three regimes indicative of well defined tropical–temperate interactions linking the hinterland parts of Southern Africa to the mid- latitudes of the SWIO. Preferred transitions show a ten- dency for an eastward propagation of these systems. Analysis of daily rainfall records for South Africa shows that six of the OLR regimes are associated with spatially coherent and significant patterns of enhanced or reduced daily rainfall over the country. Atmospheric anomalies from the NCEP/DOE II reanalysis dataset show that the OLR regimes are associated with either regional or near- global adjustments of the atmospheric circulation, the three regimes representative of tropical–temperate interactions being in particular related to a well-defined wave structure encompassing the subtropical and temperate latitudes, featuring strong vertical anomalies and strong poleward export of momentum in the lee of the location of the cloud- band. The time-series of OLR regimes seasonal frequency are correlated to distinctive anomaly patterns in the global sea-surface-temperature field, among which are shown to be those corresponding to El Nino and La Nina conditions. The spatial signature of El Nino Southern Oscillation’s (ENSO) influence is related to the combination of an increased/decreased frequency of these regimes. It is shown in particular that the well-known ‘‘dipole’’ in con- vection anomalies contrasting Southern Africa and the SWIO during ENSO events arises as an effect of seasonal averaging and is therefore not valid at the synoptic scale. This study also provides a framework to better understand the observed non-linearities between ENSO and the sea- sonal convection and rainfall anomalies over the region. Keywords Southern Africa and Southwest Indian Ocean Á Atmospheric convection Á Cluster Analysis Á Tropical-temperate-troughs Á Rainfall variability Á Scale interactions 1 Introduction Southern Africa (‘‘SA’’, south of 10S) experiences its main rainfall season during the austral summer half-year, except for the Western Cape region where winter rainfall prevails. Because of the predominance of rain-fed agriculture (Mason and jury 1997; Jury 2002; Reason and Jagadheesha 2005), large departures in the seasonal rainfall amount (either drought or floods) are liable to have particularly detrimental effects on the economies and societies of the region. According to Jury (2002), an analysis of food and water supplies and economic growth in South Africa emphasizes the major role played by climate variability. Summer rainfall in the period of 1980–1999 is closely associated with year-to-year changes in the gross domestic product. It is estimated that over U.S.$1 billion could be saved annually with reliable long range seasonal forecasts. Such predictions are however not easy to produce, as heavy N. Fauchereau (&) Á C. J. C. Reason Á M. Rouault Department of Oceanography, University of Cape Town, Rondebosch, Cape Town 7701, South Africa e-mail: Nicolas.Fauchereau@uct.ac.za B. Pohl Á Y. Richard Centre de Recherches de Climatologie, CNRS/Universite ´ de Bourgogne, Dijon, France 123 Clim Dyn (2009) 32:575–591 DOI 10.1007/s00382-008-0426-2