The Onset of Early Season Rainfall and its Mid-Summer Cessation in the Caribbean.

Friday, 19 December 2014
Theodore L Allen, University of Miami, Miami, FL, United States and Brian E Mapes, RSMAS, University of Miami, Atmospheric Sciences, Miami, FL, United States
The annual rainfall cycle for the Caribbean basin reveals a distinct bimodal pattern with peaks during the late spring and late summer months. A relative minimum during the mid-summer, known as the mid-summer drought (MSD) separates the early rainfall season (ERS) from the late rainfall season. Accumulated rainfall totals during the ERS appear as a quasi-stationary rain-belt stretching across the Caribbean from the southwest to the northeast. We place late spring rains in the Caribbean in context of other subtropical convergence zones in order to address the onset and cessation of the ERS while also offering an explanation of a Caribbean rain-belt pattern. Upper tropospheric westerlies, mid-tropospheric positive temperature advection, and moist low level poleward flow are the three primary ingredients that conspire to produce the first peak of the annual bimodal rain signal and the related Caribbean early season rain-belt. The MSD ensues as the primary ingredients weaken across the Caribbean and enhanced rainfall shifts north along the North Atlantic Convergence Zone (NACZ). Seasonal rainfall totals from the ERS through the MSD periods reveal a continuous rain-belt that extends from the Caribbean to the NACZ termed the Caribbean Atlantic Rain-belt (CAR-belt). The Car-belt is present in the long term mean, but has signs of interannual variability.