Bimodal Distribution of Tropical Cyclogenesis in the Caribbean: Characteristics and Environmental Factors

Abstract

Tropical cyclogenesis critically depends on the presence of warm water at the sea surface. For the North Atlantic basin as a whole, the tropical storm season starts in May, peaks in September, and then declines, generally following the seasonal warming and cooling of sea surface temperature. In the Caribbean, in contrast, there is a distinct bimodal distribution in the number of tropical storms formed, with peaks in June and October separated by a significant minimum in July. The timing of the observed minimum in tropical cyclogenesis appears to be related to the strengthening of the easterly trade winds over the Caribbean associated with the onset of the so-called veranillo, or midsummer drought (MSD), previously recognized over south-central Mexico, Central America, and parts of the Caribbean. It appears that the observed minimum in cyclogenesis is caused by a combination of environmental factors related to the strengthening of the easterly trade winds across the Caribbean Basin. The strengthening easterly trade winds and their associated changes in wind stress curl give rise to enhanced upwelling in the southwestern Caribbean. This appears to trigger an enhanced local atmosphere?ocean coupling, giving rise to very unfavorable conditions in several environmental variables including cooler sea surface temperature (SST), higher sea level pressure (SLP), increase in outgoing longwave radiation (OLR), and decrease in precipitable water content (PRW). Moreover, strengthening trade winds result in increases in tropospheric vertical wind shear (VSH). Except for OLR, these environmental variables become least favorable for southwestern Caribbean cyclogenesis in July. In contrast, the transition from weak to intense convective activity in the eastern Pacific results in weaker trade winds in the Caribbean in October. The resulting westerly wind anomalies lead to weakening upwelling, warmer SST, enhanced convection, and moist air coupled with weaker VSH in the southwestern Caribbean. All variables, except OLR, then become most favorable for cyclogenesis. In the rest of the Caribbean, some of the conditions, primarily SST related, are not fully met. Nevertheless, the southwestern Caribbean appears to dominate the rest of the Caribbean in terms of setting the stage for either favorable or unfavorable conditions for cyclogenesis in the whole Caribbean Basin. Therefore, ocean?atmosphere interaction over the southwestern Caribbean appears to play an integral role in both suppressing and enhancing tropical cyclogenesis in the Caribbean on an annual basis.