A Case Study of Heavy Rainfall Associated with Weak Subtropical Disturbances in the Gulf of Mexico

Abstract:

Mid-leveldisturbances in the Gulf of Mexico (GoM) led to excessive rainfall across the western Caribbean from 10–16 May 2020. These disturbances interacted with weak eastward-propagating upper-level potential vorticity (PV) anomalies of eastern Pacific (EPAC) origin that crossed Mexico and reached the GoM and western Caribbean. These upper-level PV anomalies were associated with cutoff cyclones that fractured from weak zonally elongated PV streamers in the subtropical central Pacific due to anticyclonic Rossby wave breaking. Over the GoM, these PV anomalies induced ascent in a moister and more unstable atmosphere over warmer SSTs in an environment that was increasingly conducive to the formation of subtropical disturbances and regional orographically enhanced heavy rainfall. The purpose of this study is to investigate the origin of the EPAC upper-level PV anomalies, determine how a pre-existing PV waveguide enabled these PV anomalies to reach the GoM, and establish how these PV anomalies established a more favorable environment conducive to subtropical cyclogenesis and excessive rainfall.

Weak upper-level PV maxima that initially moved eastward in weak upper-level westerly flow over the EPAC did not induce low-level cyclonic disturbances due to unfavorable atmospheric conditions. However, it is hypothesized that the PV features were able to induce surface cyclogenesis in a moister environment characterized by reduced static stability, pre-existing mid-level disturbances (vorticity) and higher SSTs over the GoM. Mid-level disturbancesfostered convective organization and heavy rainfall in areas with sustained upslope flow near elevated terrain. Gridded ERA5 datasets will be used to quantify and characterize how the GoM environment became conducive for organized convection and mid-level vorticity growth. Our analysis will focus on quantifying dynamical processes that govern mid- and upper-level PV interactions, lower troposphere moisture convergence, thermal advection, and frontogenesis, and atmospheric destabilization and low-level vorticity growth. Our ultimate goal is to better understand how interactions between upper-tropospheric PV maxima and mid-level disturbances fostered excessive rainfall during 10–16 May 2020.