PHYSICAL-MECHANISMS FOR THE ASSOCIATION OF EL-NINO AND WEST-AFRICAN RAINFALL WITH ATLANTIC MAJOR HURRICANE ACTIVITY

Physical mechanisms responsible for the contemporaneous association, s hown in earlier studies, of North Atlantic basin major hurricane (MH) activity with western Sahelian monsoon rainfall and an equatorial east ern Pacific sea surface temperature index of El Nino are examined, usi ng correlations with 200- and 700-mb level wind data for the period 19 68-92. The use of partial correlations isolates some of the relationsh ips associated with the various parameters. The results support previo us suggestions that the upper- and lower-level winds over the region i n the basin between similar to 10 degrees and 20 degrees N where most MHs begin developing are critical determinants of the MH activity in e ach hurricane season. In particular, interannual fluctuations in the w inds that produce changes in the magnitude of vertical shear are one o f the most important factors, with reduced shear being associated with increased activity and stronger shear with decreased activity. The re sults show that most of these critical wind fluctuations are explained by their relationship to the SST and rainfall fluctuations. Results c onfirm previous findings that positive (warm) eastern Pacific SST and negative (drought) Sahelian rainfall anomalies are associated with sup pressed Atlantic basin tropical cyclone activity through an equatorial ly confined near-zonal circulation with upper-level westerlies and low er-level easterlies that act to increase the climatological westerly v ertical shear in the main development region. SST and rainfall anomali es of the opposite sense are related to MH activity through a zonal ci rculation with upper-level easterly and lower-level westerly wind anom alies that act to cancel out some of the climatological westerly verti cal shear. The results also show that changes in vertical shear to the north of the main development region are unrelated to, or possibly ev en out of phase with, changes in the development region, providing a p ossible physical explanation for the observations from recent studies of the out-of-phase relationship of interannual fluctuations in MH act ivity in the region poleward of similar to 25 degrees N with fluctuati ons in activity to the south. The interannual variability of MH activi ty 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 equat orially confined, so that the changes in shear in the main development region have a stronger association with the rainfall than with the SS Ts.