Month-to-Month Variability of the Atlantic Tropical Circulation and Its Relationship to Tropical Storm Formation

Abstract

Monthly mean winds have been derived from 200 mb and Analysis of the Tropical Oceanic Lower Layer (ATOLL) winds over the southern North Atlantic, Caribbean, Gulf of Mexico and eastern Pacific during the hurricane seasons (June-November) of 1975 through 1985. After removal of the seasonal cycle, the winds are expressed in terms of empirical orthogonal functions. The dominant mode of variability for the combined 200 mb/ATOLL circulation strongly resembles part of a Walker cell confined near the equator. This mode is strongly correlated with El Niñ index (Weare, 1986), and is associated with the.El Niñ/Southern Oscillation. A positive (El Niñ-like) index tends to be associated with more anticyclonic vorticity at the ATOLL level in the tropics and increases in the vertical shear between about 10° and 30°N. This circulation is unfavorable for tropical storm formation. Correlations are derived between the monthly mean winds and monthly tropical storm frequency in the Atlantic basin. Contemporaneous correlations in August, September and October, the three most active mouths, as well as correlations between winds and tropical storm formation 1 and 2 months later, are computed. Predictability of monthly tropical storm frequency at the 2-month lead is statistically significant, with true skill approximately 45% of the variance. Only one-sixth of this skill is associated with the El Niñ/Southern Oscillation A favorable environment for storm formation is apparently established atleast 2 months before the given month of formation. The results extend and complement predictions of seasonal tropical storm activity and previous hypotheses concerning the influence of El Niñ on stern formation.