An index for tropical temperate troughs over southern Africa
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Strong cases of the tropical temperate troughs (TTT) that are responsible for the most of the summer rainfall over subtropical southern Africa are analyzed. An index for identifying the TTT is introduced for the first time using anomalies of outgoing longwave radiation (OLR) and the wind. The TTT is associated with a ridge-trough-ridge wave-like structure in the lower troposphere over southern Africa and the adjoining Indian Ocean. Therefore, the index considers physical processes that occur over southern Africa, adjoining the Atlantic and Indian Oceans to depict the variability of the TTT events. Unusually strong TTT events are identified when the standard deviations of the TTT indices defined by the OLR and wind anomalies in the selected regions are above 1.5 and 0.5 respectively. After applying this criterion and filtering out consecutive events, 55 TTT events are identified during the study period of December–January–February seasons from 1980–1981 to 2009–2010. From the composite analyses of those 55 events, it is found that the TTTs evolve with suppressed (enhanced) convection over the southwest Indian Ocean adjacent to Madagascar (southern Africa). The suppressed convection is, in turn, found to be associated with the enhanced convection around Sumatra in the southeast tropical Indian Ocean. This may explain why more TTT events occur in La Niña years as compared to El Niño years. Time evolution of the canonical TTT event shows that it starts 3 days prior to the mature phase of the event, suggesting possible predictability. After reaching a matured state, the system moves east toward the Indian Ocean and decays within the subsequent couple of days. In addition, the intertropical convergence zone (ITCZ) structure changes over Southern Africa/Madagascar during the TTT event and remains similar to climatology over other regions. The results indicate that the continental part of the ITCZ intensifies prior to the TTT event and then spreads southward following the mid-latitude influence during and after the event.
KeywordsTropical temperate trough TTT Index Southern Africa Southwest Indian Ocean El Niño/La Niña Floods
This research is supported by The Japan Science and Technology Agency (JST)/Japan International Cooperation Agency (JICA) through Science and Technology Research Partnership for Sustainable Development (SATREPS). The NCEP reanalysis, interpolated OLR and the high resolution SST data were provided by NOAA/OAR/ESRL/PSD, Boulder, Colorado, USA through their web site at http://www.esrl.noaa.gov/psd. The authors thank Dr. Willem Landman and Prof. Hannes Rautenbach for their valuable suggestions. Thanks to Dr. Desmond Manatsa and Dr. Wing-Le Chan for their help in improving the manuscript. We also thank three anonymous reviewers for their constructive comments.
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