On Southeastern Pacific Subtropical Cyclones

The Southeastern Pacific Ocean (SEPO) is the region of the South Pacific 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 the occurrence of SCs 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 were utilized to study the environments that allowed for the development of Katie, Lexi, and Humberto. Each cyclone was found to be a product of a favorable upper-level pattern, facilitating the transition of its precursor disturbance into a SC over marginally favorable sea surface temperatures (SSTs). The life cycle of each cyclone was then reconstructed through use of cyclone phase space diagrams.

This work then addressed the question of the prevalence of SCs in the SEPO through use of observational data. MODIS imagery from the Terra and Aqua satellites was used to hunt for other SCs that may have occurred since 24 February 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 that 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 seems 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 May, allowing precursor disturbances to protrude more readily into the subtropics of the basin and undergo short-lived SC transitions over marginally favorable SSTs, given a favorable upper-level environment.

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