A Case Study of Mesoscale Cyclonic Vortices Associated with the South Atlantic Convergence Zone

Abstract:

The objective of this work is to study the behavior of mesoscale cyclonic vortices (MCVs) over South America, principally those that form in association with the South Atlantic Convergence Zone (SACZ), with a view toward identifying the basic characteristics of the formation of these MCVs. Two case studies were conducted over the Continental Amazonia Zone, simulated using the BRAMS model, showing the relationship between the mesovortices formation and the convective activity near its formation region. A thermodynamic analysis of two selected MCVs, embedded in the SACZ, highlights some salient features of these intense MCVs. Both systems are associated with strong upward vertical motion throughout practically the whole troposphere, before and during formation. This motion creates a transport of moisture into the upper troposphere and the surface fluxes of sensible and latent heat are reduced when the MCVs are operating. These systems that form in more than one level in the troposphere are more intense and are associated with greater precipitation rates (over 150 mm). Another striking feature is that these systems dissipate quickly. With respect to the horizontal wind, there is no similarity in pattern between the two cases. The first case, where the base of the vortex formed at 925 hPa and extended to 875 hPa, was characterized by convergence of winds from the south - west of the vortex - with winds from the north - east of the vortex. In the second case, which extended from 800 hPa to 775 hPa, we can clearly see the motion, originating in central Brazil, transporting moisture toward the vortex and certainly serving as a local factor contributing significantly to the moisture balance in the region.