The Influence of a Precursor Central American Gyre and a Northerly Surge into the Gulf of Tehuantepec on the Formation of Hurricane Patricia in October 2015

The National Hurricane Center (NHC) initiated advisories on Tropical Depression (TD) Patricia at 1500 UTC 20 October 2015. TD Patricia originated from a pre-existing area of disturbed weather over the eastern Gulf of Tehuantepec (GoT). Over the next 72 h, Patricia rapidly intensified from a TD (30 kt) to a Category 5 tropical cyclone (TC; 175 kt). TC Patricia rapidly intensified while moving northwestward parallel to the Mexican coast over anomalously warm sea surface temperatures (SSTs) in a region of anomalously high oceanic heat content (OHC). A relatively novel aspect of TC Patricia's development was that the storm appeared to form in conjunction with the breakdown of a Central American gyre (CAG), and a surge of northerly flow through a mountain gap (Chivela Pass) across the Isthmus of Tehuantepec and into the GoT. Pre-existing cyclonic vorticity associated with the CAG and the gap flow through the Chivela Pass likely contributed to the initial classification of the storm as a TD. The purpose of this presentation is to provide an overview of the physical processes that helped to establish a favorable pre-genesis and genesis environment for TC Patricia.

A CAG was first apparent over Nicaragua by 0600 UTC 15 October. The CAG was defined by weak southwesterly flow east of the GoT in the far eastern Pacific, easterly flow across the southeastern Gulf of Mexico and western Caribbean Sea, and a surge of northerly flow through the Chivela Pass into the GoT. The aforementioned surge of northerly flow into the GoT was driven by strong low-level height rises over the northern Gulf of Mexico behind a southeastward-moving cold front. Strong low-level anticyclogenesis over the Gulf of Mexico and the southeastern United States behind the cold front and CAG-related surface pressure falls over Central America contributed to the development of an anomalously strong meridional surface pressure gradient that drove the aforementioned surge of northerly flow into the GoT. This wind surge resulted in the formation of an elongated vorticity strip in the GoT by 0000 UTC 18 October. Subsequently, this vorticity strip broke down into a cyclonic vortex by 0600 UTC 20 October and transitioned into TD Patricia by 1500 UTC 20 October as it moved over a region of anomalously warm SSTs and high OHC in the presence of large oceanic heat and moisture fluxes.