P1.8
Convectively coupled Kelvin waves over tropical Africa during the boreal summer: Structure and variability
Ademe Mekonnen, SUNY, Albany, NY; and C. D. Thorncroft and G. N. Kiladis
The structure and variability of convectively coupled Kelvin waves during the boreal summer is explored based on satellite brightness temperature data and the ECMWF reanalyses system. Composite analysis using a regression technique reveals Kelvin wave propagation from central and eastern Pacific through Africa towards the Indian Ocean. The convectively coupled Kelvin wave propagates faster (~24ms-1) at the primary source regions over the Pacific and western Atlantic, and slows down over tropical Africa (~14ms-1). Low-level convergence leads the enhanced Kelvin convection, an important feature of the eastward moving wave. Over Africa, while convection is observed north of the equator, dynamic measures show near symmetry about the equator, consistent with theory and observations.
Analysis of weather events during July-September 1987 has shown that convective development and daily rainfall increase when enhanced Kelvin wave approaches regions in tropical Africa. Rainfall increased from about 5mm/day to 12-20mm/day when the Kelvin wave approaches regions, while it sharply fell when the Kelvin wave passed by. This weather event also shows a series of African easterly waves (AEWs) being initiated or enhanced in association with the enhanced Kelvin wave over Africa. After a period of relatively weak AEW activity during the first week of August 1987, about 5 AEWs were triggered in association with enhanced Kelvin wave during the second week. One of these became Tropical Storm Bret, the first tropical storm of the season.
Results show marked interannual variability of Kelvin wave activity over Africa. Anomalously high Kelvin wave activity is associated with dry years over Africa, while anomalously low Kelvin years tend to occur during wet years. On the other hand, anomalously high Kelvin wave variability is related to anomalously high sea surface temperature (SST) anomalies over equatorial eastern Pacific, indicating the importance of warm episodic years on Kelvin wave development there. This suggests that warming of SST leads to increased local convection, and convection in turn is favorable for an increase in Kelvin wave activity over equatorial east Pacific. The same SSTs which favor increased Kelvin wave activity are negatively related to the rainfall over Africa, explaining the fact that active Kelvin wave years are also dry years over Africa.
Poster Session 1, African Climate Poster Session
Monday, 21 January 2008, 2:30 PM-4:00 PM, Exhibit Hall B
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