Summary of 1993 Atlantic Tropical Cyclone Activity and Verification of Author'sForecast. A Year of Unanticipated Light Hurricane Activity and a Poor Forecast: The Expected Dissipation of the El Nino Did Not Take Place and Caribbean Basin Surface Pressure

摘要

This paper summarized the tropical cyclone (TC) activity which occurred in theAtlantic Basin during 1993, and verifies the author's seasonal forecast of this activity which were issued on 24 November of last year and updated on 5 June and 4 August of this year. The 1993 hurricane season was relatively inactive with eight named storms (average 9.3) and four hurricanes (average 5.7). There was only one major (or intense) Saffir/Simpson category 3-4-5 hurricane (average is 2.2) and this storm (Emily) was only a marginal category 3 intensity for slightly less than one day. The seasonal total of hurricane days was only 10 (average is 23) and the seasonal total of named storm days was just 30 (average is 46). Only two systems (tropical cyclone Arlene in south Texas and Hurricane Emily near Cape Hatteras) affected the U.S. mainland. The author had forecast a substantially more active hurricane season. This forecast failure was primarily the result of the misjudgment of the remarkably persistent El Nino conditions which occurred this year and of the high Caribbean basin surface pressure which became established in August. The Caribbean pressure changes are believed to be unrelated to the El Nino and are a result of the intertropical covergence line being shifted southward over the South American continent. Additional unfavorable hurricane conditions included a continuing West Sahel drought. It is quite unusual for an El Nino to last through three consecutive seasons as the current El Nino event has. Actually, warm water has persisted in the equatorial Pacific through the last four hurricane seasons. The El Nino is the most dominant of the climate influences affecting Atlantic hurricane activity, tending to strongly suppress the activity. The long lasting (1991-1993) El Nino conditions had finally started weakening in June and July and this weakening was expected to continue. But, beginning in late August, a surprising resurgence of warming SST conditions occurred in the eastern Pacific. This warming led to a stronger upper tropospheric trough over the western Atlantic and Caribbean sea, and with it, enhanced vertical wind shear. All tropical cyclone activity ended after the 21st of September, a date which is normally only 60% of the way through the Atlantic cyclone season.