Southeast Pacific Subtropical Cyclones

The Southeast Pacific Ocean (SEPO) is the region of the South Pacific located between the west coast of South America and 120°W. In recent years, several subtropical cyclones (SCs) have occurred in this region of the world, which is unprecedented. The scientific literature has not identified the SEPO as an ocean basin capable of producing SCs and laments that the region is devoid of any tropical cyclone (TC) activity. The SEPO is not monitored by any World Meteorological Organization (WMO) Regional Specialized Meteorological Centre (RSMC) or TC Warning Centre (TCWC) for subtropical or tropical development.

This work examines and discusses SC occurrence in the SEPO. First, the several SCs known to have occurred in recent years are analyzed and discussed. These cyclones have been dubbed Katie, Lexi, and Humberto by independent researcher Steven H. Young and occurred in April/May 2015, May 2018, and January 2022 respectively. ERA5 reanalysis data was utilized to study the environment which allowed for the development of Katie, Lexi, and Humberto. Each cyclone was found to be a product of a favorable upper-level blocking pattern, which allowed for the transition of its precursor disturbance into a SC over marginally favorable sea surface temperatures (SSTs). The story (i.e., life cycle) of each cyclone was revealed with utilization of cyclone phase space diagrams.

The question as to the prevalence of SCs in the SEPO was also addressed. MODIS imagery from the Terra and Aqua satellites was used to hunt for other SCs which may have occurred since March 2000. At least a dozen other cyclone candidates were identified as potential SCs and were analyzed, along with their environments, using ERA5. Notably, cyclone phase space diagrams suggested many of these cyclones were indeed SCs.

The time of year most common for development of a SC in the SEPO appears to be during later austral autumn to early austral winter (i.e., April to June). Specifically, the month of May appears to be the most favorable time of year for the development of a SC in the SEPO. A favorable meteorological/climatological situation occurs particularly in the month of May which allows for extratropical disturbances to protrude more readily into the subtropics of the basin and undergo short-lived SC transitions over marginally favorable SSTs, if a favorable upper-level blocking pattern is present.

This work suggests that while SC occurrence in the SEPO is certainly rare, it may not be as exceptionally rare as once thought. The main goal of this work is to raise awareness of SC formation in the SEPO within the scientific community. The SEPO is a cyclogenetic region of the world which can on rare occasions support the development of a SC. Life and property may be better protected if the basin is monitored for SC development under the jurisdiction of a WMO RSMC or TCWC.