ENSO and Indian Ocean sea surface temperatures and their relationships with tropical temperate troughs over Mozambique and the Southwest Indian Ocean

An analysis of the inter-annual frequency of the main summer rainfall producing synoptic system [tropical temperate troughs (TTTs)] over Mozambique and the Southwest Indian Ocean and their relationships with regional sea surface temperatures and circulation patterns is presented for the January-March season. Correlations and composite anomalies based on annual frequency of occurrence of TTTs over Mozambique suggest that TTT occurrence is related to La Nina and Southern Hemisphere planetary waves (wavenumber-3 or -4) that modulate the subtropical high pressure cells in the three oceanic basins. These modulations lead to enhanced easterly winds over the South Indian Ocean and onshore moisture fluxes towards southeastern Africa. A stronger than average Angola low, particularly linked to La Nina, also plays an important role in the frequency and intensity of the TTT, by enhancing the wind and moisture convergence over southeastern Africa. Moisture fluxes over the warm Agulhas current and tropical southeast Atlantic also affect the rainfall variability over southern Africa to some extent during summers with a high frequency of TTT. Positive sea surface temperature (SST) anomalies over the tropical and subtropical South Indian Ocean that are linked to El Nino-Southern Oscillation appear to play a significant role in causing dry conditions over Mozambique. These positive SST anomalies help promote convective activity offshore, and also weaken the northern half of the South Indian Ocean high pressure cell. As a result, the easterly winds are weakened leading to less onshore moisture fluxes than in the wet seasons. Copyright (C) 2009 Royal Meteorological Society