A Mechanically Induced Center of Heavy Rainfall Off the East Coast of Central America

Some of the regions with heaviest rainfall regimes in the world are conspicuously located offshore concave and mountainous lands. These centers of convection are quasi static and can reach synoptic spatial length scales. Their causes are not well understood, but they are commonly speculated to be rooted in the geographically induced convergence of nighttime land sea breezes. Offshore the east coast of Central America is located such a center of convection. The accumulated rainfall in this region can reach 600 mm per month and its seasonal cycle is similar to that of the Caribbean Low Level Jet (CLLJ). This case however, could not to be explained by the convergence of land breezes for two reasons. First, a mountain barrier over land is absent, so the radiative cooling of land associated with the thermal breeze is weak. Second, the nighttime accumulated rainfall can only explain a fraction the total. Wind diagnostics from reanalysis show that the ageostrophyc circulation in this region resembles that of a tropical cyclone. This circulation appears to be mechanically induced by the northward deflection of the CLLJ. As suggested by composites of rain types observations from TRMM-PR, such cyclonic circulation seems to promote the organization of convection into powerful mesoescale convective systems, and it therefore, extends the life cycle of the convective activity there.