Sensitivity of the southern African circulation to dipole sea‐surface temperature patterns in the south Indian Ocean

Abstract

Previous observational work suggests that when sea-surface temperature (SST) is warm (cool) in the southwest Indian Ocean and cool (warm) in the southeast Indian Ocean, increased (decreased) summer rains may occur over large areas of southeastern Africa. In this study, an atmospheric general circulation model is used to investigate the sensitivity of the regional circulation and rainfall over southern Africa to these dipole SST anomalies in the subtropical south Indian Ocean. When the model is forced with positive SST anomalies in the west and negative SST anomalies in the east, increased rainfall occurs over southeastern Africa as a result of the enhanced convergence of moister than average air over the region. Enhanced evaporation occurs over the warm pole in the southwest Indian Ocean and this moist air is advected towards southeastern Africa as a result of the low-pressure anomaly generated over this pole, which strengthens the onshore flow. Increased and more intense extra-tropical cyclones occur to the southeast of South Africa, favourable for tropical-temperate trough formation. When the SST poles are reversed in sign, decreased precipitation occurs over southeastern Africa as a result of increased low-level divergence of low-level flow and this flow being drier than average. Weaker and fewer extra-tropical cyclones occur southeast of South Africa in this case.The model results are sensitive to the proximity of the southwest Indian Ocean pole to southeastern Africa. There is also sensitivity in the model low-level wind changes and precipitation anomaly over low-latitude southern Africa (but not over South Africa to any significant extent) to the presence or absence of the SST pole over the southeast Indian Ocean. Although the model resolution does not capture the details of the local SST and topographic gradients as well as one would like, the changes in model circulation and precipitation in the experiments with different SST anomalies are consistent with previous observational and theoretical work, hence increasing confidence in the robustness of the results.

Interannual variability in rainy season characteristics over the Limpopo region of southern Africa

“…In addition, a strong positive South Indian Ocean dipole-like sea surface temperature (SST) event occurred during 1980/81. These dipole events have been shown both observationally and in atmospheric model experiments to lead to above average summer rainfall over the region (Behera and Yamagata, 2001;Reason, 2001Reason, , 2002. The relationship between dry spell frequency and seasonal rainfall anomaly during DJF for Limpopo is not linear.…”

Water Balance Modeling in a Semi-Arid Environment with Limited in situ Data Using Remote Sensing in Lake Manyara, East African Rift, Tanzania

“…During positive phase there is flooding in East Africa. The Indian summer monsoon is more intense than normal and there is drought in Indonesia and in several regions of Australia [23,24].…”

Hydrological changes in southern Africa over the last 200 Ka as recorded in lake sediments from the Tswaing impact crater

“…The fact that intensities of detrital elements decrease during the Holocene, when the SST gradient is still low, might be explained by low summer insolation during this period. Modelling studies indicate that present day rainfall over south-eastern Africa is sensitive to changing SSTs in the south-western Indian Ocean (Reason, 2002a;Reason, 2002b;Washington and Preston, 2006), but it is not possible to test whether this connection exists on glacial/interglacial time scales, due to a lack of records from the Indian Ocean.…”