Kinematics, Dynamics, and Occasionally Moist Thermodynamics of Topographic Vorticity Streamers over the Caribbean Sea

The lower troposphere over the Caribbean Sea is home to the strongest persistent easterlies on Earth, and is bounded by Atlantic trades to the east, a Costa Rican mountain range to the west, the Island Chain to the north, and complex topography of Columbia and Venezuela to the south. The surrounding topography not only defines the regional environment of exceptionally strong Caribbean trades, but triggers vorticity streamers owing to concave gap flows and convex topographic obstacles. Furthermore, a diurnal land-sea breeze along the coast of South America excites a regular pulsation of the vorticity source and a “flickering flame” effect in the streamers. The meteorological role of these topographic streamers varies from virtually none, a mere curiosity, to a substantial source of vorticity in circularly symmetric disturbances, including tropical cyclones in boreal autumn. In the latter case, a moist thermodynamic signature is evident.

The underlying concept of dipolar vorticity source and attendant vorticity streamer, excited and modulated, in part, by the regional flow affords a nice model-theoretic picture of advective and non-advective fluxes of isobaric vorticity substance documented in the classic works of Haynes and McIntyre 1987 and Schär 1993. Also highlighted is the nonlinear downstream development of closed vortices, which requires an intermediate range of regional flow speed to develop within the target region (SW Caribbean). In this circumstance the phenomenon resembles a kind of one-sided von Karman vortex rollup, owing the spatial asymmetry and temporal pulsation of regional flow and source mechanisms. Weaker flows do not result in separation from the coast, while stronger flows do not allow sufficient room for downstream development. Most flows vary in strength and direction with height, affording additional degrees of freedom.

The dataset used in our study is a 0.25-degree version of the operational GFS model in 2015-2017, and the results are evaluated in the broader perspective of ERA-Interim re-analyses 1979-2017. Examples of validated development are illustrated with geostationary imagery.