The Impacts of Column Water Vapor Variability on Atlantic Basin Tropical Cyclone Activity

The presence of dry air at the middle and upper troposphere in the tropics is well known to affect tropical cyclone formation in all ocean basins. In the Atlantic basin, dry air may be associated with the Saharan Air Layer (SAL) or large-scale subsidence. The seasonal variability of the occurrence of middle and upper tropospheric dry air and impacts on tropical cyclone activity over the Atlantic are examined in this study. Empirical Orthogonal Function (EOF) analysis reveals dominant modes of seasonal variability of the dry air occurrence over the tropical north Atlantic, with strong variations over the central and eastern Atlantic between 15-30°N. In the positive (negative) phase of the EOF modes, tropical cyclone (TC) activity is reduced (increased) over the Atlantic basin, particularly over the Main Development Region (MDR). On the origin of dry air, the SAL does not explain the middle or upper tropospheric EOF patterns, but the patterns are shown to be related to large-scale subsidence in association with anticyclones. The leading modes show significant correlations to several large-scale climate oscillations, including the Atlantic Multi-decadal Oscillation (AMO), Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), Pacific North American Pattern (PNA) and others. The EOF modes also indicate a relationship to the large scale circulation via the Hadley cell. An evaluation of the steering flow for tropical cyclones from the modulation of the subtropical high and the values of deep layer vertical wind shear (VWS) are consistent with the distribution of TC activity only in limited areas, suggesting that the impacts of the dry air patterns are required to provide a more complete explanation of the TC activity.