The collapse of transitioning mesoscale convective systems off the coast of West Africa and links to downstream tropical cyclogenesis

Abstract

While most MCSs dissipate upon approaching the coast of West Africa and transitioning to the East Atlantic, a few cases have been shown to strengthen, such as Hurricane Cindy (1999). A better understanding of how some MCSs decay is essential for identifying other MCSs that go on to play a role in tropical cyclogenesis. A six- year (2000-2006) climatology of TRMM PR data were used to construct CFADs of three regimes (continental, transition, and oceanic) in the region of West Africa and the Eastern Atlantic. After establishing the differences between MCSs in these three regimes, datasets from the NASA African Monsoon Multidisciplinary Analyses (2006) field campaign were used to investigate the environments of transitioning MCSs as well as small-scale factors contributing to localized convection. The results show MCSs that initiate over continental West Africa fail to transition successfully into the Eastern Atlantic Ocean, which previous studies using satellite data have not been able to resolve. The inability to support additional convection offshore is primarily due to decreased environmental instability between continental West Africa and the Eastern Atlantic and cold pool expansion ahead of the MCS, which results in a tilt in updraft away from the ideal vertical orientation. Although the MCS storm structure does not survive the transition from land to ocean, dynamic mid-level rotation associated with African Easterly Waves that initiated these MCSs remains intact and is responsible for tropical cyclogenesis downstream from the African continent. These results should prove useful for incorporation into tropical cyclone models with domains including regions east of the Cape Verde Islands.