Variability in global-scale circulations and their impacts on Atlantic tropical cyclone activity

Abstract

evel relative vorticity, a decrease in westerly vertical wind shear, and increased convection in the West African monsoon (WAM). The second and third modes of an empirical orthogonal function (EOF) analysis of the 700-hPa height anomalies identify a distinct Rossby-wave pattern. Significant variability in the Southern Hemisphere mid-latitude circulations is related to the two EOF modes and to equatorward Rossby-wave dispersion. Formation of a large cyclonic anomaly over the southeast Pacific, west of Chile, is related to equatorward propagation of a Rossby-like wave across South America, toward the equatorial Atlantic. The cyclonic anomaly precedes an increase in WAM convection by an average of two days, which then precedes westerly wind anomalies over the equatorial North Atlantic by several days. Tropical cyclone formation is found to be enhanced when the increased equatorial westerly anomalies coincide with reduced vertical wind shear, which is related to Northern Hemisphere midlatitude circulations