Physical Mechanisms for the Association of El Niño and West African Rainfall with Atlantic Major Hurricane Activity

Abstract

Physical mechanisms responsible for the contemporaneous association, shown in earlier studies, of North Atlantic basin major hurricane (MH) activity with western Sahelian monsoon rainfall and an equatorial eastern Pacific sea surface temperature index of El Niño are examined, using correlations with 200- and 700-mb level wind data for the period 1968–92. The use of partial correlations isolates some of the relationships associated with the various parameters. The results support previous suggestions that the upper- and lower-level winds over the region in the basin between ∼10° and 20°N where most MHs begin developing are critical determinants of the MH activity in each hurricane season. In particular, interannual fluctuations in the winds that produce changes in the magnitude of vertical shear are one of the most important factors, with reduced shear being associated with increased activity and stronger shear with decreased activity. The results show that most of these critical wind fluctuations are explained by their relationship to the SST and rainfall fluctuations. Results confirm previous findings that positive (warm) eastern Pacific SST and negative (drought) Sahelian rainfall anomalies are associated with suppressed Atlantic basin tropical cyclone activity through an equatorially confined near-zonal circulation with upper-level westerlies and lower-level easterlies that act to increase the climatological westerly vertical shear in the main development region. SST and rainfall anomalies of the opposite sense are related to MH activity through a zonal circulation with upper-level easterly and lower-level westerly wind anomalies that act to cancel out some of the climatological westerly vertical shear. The results also show that changes in vertical shear to the north of the main development region are unrelated to, or possibly even out of phase with, changes in the development region, providing a possible physical explanation for the observations from recent studies of the out-of-phase relationship of interannual fluctuations in MH activity in the region poleward of ∼25°N with fluctuations in activity to the south. The interannual variability of MH activity explained by Sahel rainfall is almost three times that explained by the eastern Pacific SSTs. It is demonstrated that a likely reason for this result is that the SST-associated vertical shears are more equatorially confined, so that the changes in shear in the main development region have a stronger association with the rainfall than with the SSTs.