Impact of Northern Low-level Wave Mergers on the Pre-genesis Development of Atlantic Tropical Easterly Waves

Tropical cyclogenesis over the Eastern Atlantic often occurs on the south side of the African Easterly Jet (AEJ). These disturbances, referred to as Cape Verde storms, sometimes include a low-level (850 or 925 hPa) wave merger from the north side of the AEJ. This merger typically leads directly to cyclogenesis as a tropical depression by the time a disturbance reaches 40W (merging developers). However, there are many Cape Verde storms which develop without a wave merger (non-merging developers). Furthermore, there are a large number of disturbances which merge off the African coast but never develop (merging non-developers). We employ the marsupial paradigm of tropical cyclogenesis (Dunkerton et al., 2009) to evaluate the pre-genesis convective evolution and differential environmental conditions which separate the three categories of disturbances from one another. The Saharan heat low was found to be warmer and broader in areal extent for merging cases, leading to the formation of strong midlevel anticyclones which propagate over the Atlantic with the disturbances and provide steering flow for the northern wave merger. Non-mergers display a deep pouch for 2-3 days prior to genesis, while developing mergers only attain a deep pouch after the merger time, indicating that the wave merger is a necessary condition for the weaker merging TEWs to reach tropical cyclogenesis. Very dry air intrudes the pouch region from the west and north through much of the middle and upper troposphere for merging non-developers, inhibiting deep convection and leading to dissipation. This is the result of strong westerly shear in the 400-600hPa layer allowing dry air initially positioned west of the wave pouch to intrude, and the fact that the non-developer wave track is displaced northward about 3 degrees from the developing mergers.