The Relationship between Autumn Cold Surge Activity and Tropical Cyclones in the Eastern North Pacific

Eastern North Pacific (EPAC) tropical cyclones (TCs) have a significant impact on both Mexico and Central America’s weather and climate. TC Patricia (2015) and TC Sandra (2015) were examples of major TCs that inflicted over $450 million in damage and killed 15 people. Storms like Patricia and Sandra that developed in complex environments towards the end of the Pacific TC season during October-November remind us of the TC threat in the EPAC. The intensity and track of TCs like Patricia and Sandra were influenced by the combined effects of midlatitude cold surges and the variability of MJO and ENSO background states throughout their lifespans. The purpose of the study is to illustrate a spectrum of EPAC TCs during October-November that develop within spatially and temporally varying environments of the climatological, large-scale, and synoptic-scale conditions favorable for the development of EPAC tropical cyclogenesis.

The study is divided in two parts. The first part presentsareanalysis-based TC climatology during October-November 1980-2017 to capture both synoptic and planetary scale environmental features that lead to TC development over the Gulf of Papagayo (PP) and the Gulf of Tehuantepec. Because cold surges can have significant impacts on the predictability of TC genesis during the transition season, we will focus on analyzing TC genesis events that occur in tandem with cold surge events. Cold surge cases will be defined based on sustained surface wind speeds and daily maximum lower-tropospheric temperature change. The relative importance of different background states and dynamic characteristics of cold surges on subseasonal time scales will also be explored to account for different physical and dynamical processes and weather phenomena that significantly contribute to the genesis of TCs within the region.

A case study of Tropical Storm (TS) Selma (27-28 October 2017) will also be conducted. TS Selma made an historical landfall near Playa El Pimental, El Salvador as the only TS on record to make landfall in El Salvador. In combination with TS Philippe and a cold surge that moved through Honduras, Selma produced rainfall amounts exceeding 15 cm, which resulted in life-threatening landslides and river flooding throughout the region. TS Selma was a short-lived storm that was very challenging to forecast. A cold surge prior to Selma’s genesis produced a PP gap wind event that generated anomalous cyclonic relative vorticity near Selma’s formation region.Because TC Selma’s generation process was environmentally complex on different time and space scales, further evaluation of this storm is needed to better understand the various linkages between EPAC cold surges and TC formation in this particular case.

For perspective purposes, a general synopsis of the synoptic-scale and local mesoscale interactions and origins that contribute to EPAC tropical cyclogenesis and fully developed TCs are presented. Rainfall distribution associated with the combination of TCs and cold surges throughout Central America will be quantified from satellite-based estimations using the PERSIANN-CDR 0.25 gridded dataset. The ECMWF fifth-generation reanalysis (ERA5) 0.25 gridded dataset will be used to objectively examine the evolution of TCs that are influenced by cold surges and composite cold surge events to illustrate the different categories of Central American cold surges in association with tropical cyclogenesis in the EPAC.