The seasonal and interannual variability of Atlantic Basin hurricane activity

The investigation of El Niño's effect on hurricane activity has long been a popular topic of study. It is well established that there are fewer Atlantic Basin hurricanes during an El Nino year than during a La Nina year. Various atmospheric and oceanic parameters that influence hurricane development become significantly altered during an El Niño event, leading to the suppression easterly wave development and growth, thereby reducing the yearly total of named storms. However, the effect of the El Niño/La Niña cycle on hurricane intensity is not as straightforward. Studies addressing this aspect of hurricane intensity have captured far less attention than the more generalized subject of the connection between hurricane occurrence and El Niño. This study examines the intensity (measured as wind speed and interpreted through the Saffir-Simpson Scale) of every hurricane over a 61 year (1938-98) time frame. These data are then compared with the occurrence of El Niño/La Niña events as defined using the Japan Meteorological Association (JMA) index. In addition, we examined El Nino/La Nina variability superimposed on variability associated with the North Pacific Oscillation (NPO). The North Pacific Oscillation has recently been shown to reinforce or weaken El Nino/LaNina-induced teleconnection patterns. Not surprisingly, during an El Niño year the intensity of Atlantic hurricanes were found to be weaker than during a neutral year or a La Niña year. However, when examining this variability superimposed on NPO variability, it was found that there were significant differences in hurricane intensity between El Nino and La Nina years when the NPO was in phase 1, rather than when the NPO was in phase 2. This study also examines the interannual variation in hurricane intensity by season, and by genesis region (i.e. the eastern and western Atlantic Ocean Basins, the Caribbean, and the Gulf of Mexico).