Statistical Aspects of ENSO Events (1950-1997) and the El Niño-Atlantic Intense Hurricane Activity Relationship

On the basis of Kevin Trenberth's quantitative definition for marking the occurrence of an El Niño (or La Niña), one can precisely identify by month and year the starts and ends of some 15 El Niño and 10 La Niña events during the interval of 1950-1997, an interval corresponding to the most reliable for cataloguing intense hurricane activity in the Atlantic basin (i.e., those of category 3-5 on the Saffir-Simpson hurricane scale). The main purpose of this investigation is primarily two-fold: First, the statistical aspects of these identified extremes and the intervening periods between them (called "interludes") are examined and, second, the statistics of the seasonal frequency of intense hurricanes in comparison to the extremes and interludes are determined. This study clearly demonstrates that of the last 48 hurricane seasons, 20 (42 percent) can be described as being "El Niño-related" (i.e., an El Niño was in progress during all, or part, of the yearly hurricane season-June-November), 13 (27 percent) as "La Niña-related" (i.e., a La Niñ was in progress during all, or part, of the yearly hurricane season), and 15 (31 percent) as "interlude-related" (i.e., neither an El Niño nor a La Niña was in progress during any portion of the yearly hurricane season). Combining the latter two subgroups into a single grouping called "non-El Niño-related" seasons, one finds that they have had a mean frequency of intense hurricanes measuring 2.8 events per season, while the El Niño-related seasons have had a mean frequency of intense hurricanes measuring 1.3 events per season, where the observed difference in the means is inferred to be statistically important at the 99.8-percent level of confidence. Therefore, as previously shown by William Gray and colleagues more than a decade ago using a different data set, there undeniably exists an El Niño-Atlantic hurricane activity relationship, one which also extends to the class of intense hurricanes. During the interval of 1950-1997, fewer intense hurricanes occurred during El Niño-related seasons (always ?3 and usually ?2, this latter value having been true for 18 of the 20 El Niño-related seasons), while more usually occurred during non-El Niño-related seasons (typically greater than or equal to 2, having been true for 22 of the 28 non-El Niño-related seasons). Implications for the 1998 and 1999 hurricane seasons are discussed.