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

During the Southern Hemisphere winter, the Antarctic Oscillation (AAO) represents a significant amount of the variability in the primary Southern Hemisphere midlatitude and subtropical jet streams. Typically, the AAO represents a meridional shift in the primary jet system. During periods of active meridional adjustments in the jet location, equatorward-propagating waves may occur over key longitudinal domains that are connected to regions of maximum jet strength. Preferred regions of wave activity near Australia propagate toward the equatorial western Pacific, and near South America propagate toward the tropical Atlantic.

This study examines Southern Hemisphere mid-latitude wave variations connected to the Antarctic Oscillation (AAO) to establish connections with variability in large-scale circulations that may be favorable or unfavorable for tropical cyclone formation over the tropical North Atlantic. The AAO index is defined from the leading empirical orthogonal functions of 700 hPa height anomalies. Favorable impacts on tropical Atlantic circulation characteristics are defined by an increase in low-level relative vorticity or a decrease in vertical wind shear. Periods of favorable and unfavorable conditions are related to positive, negative, and neutral phases of the AAO, which are defined by the AAO index derived from the EOF analysis. Significant connections between the Southern Hemisphere mid-latitude circulations related to the AAO and the large-scale circulations over the tropical Atlantic are examined via the mechanism of equatorward Rossby-wave dispersion. When wave energy flux in the Southern Hemisphere is directed zonally, no connection is established between the AAO and periods of favorable and unfavorable conditions for tropical cyclone formation over the tropical North Atlantic. The relative impact of the AAO on Atlantic tropical cyclone activity is assessed relative to other large-scale factors known to influence tropical cyclone activity.