Interannual Variability of Middle and Upper Tropospheric Dry Air and Its Impacts on Tropical Cyclone Activity over the Atlantic Basin

Abstract:

The presence of dry air at the middle and upper troposphere in the tropics is well known to affect tropical cyclone formation in all the ocean basins. In the Atlantic basin, dry air may be associated with the Saharan Air Layer or mid-latitude frontal systems. The interannual variability of the frequency of the middle and upper level dry air layer and its impacts on tropical cyclone activity over the Atlantic are examined in this study. EOF analysis revealed dominant modes of interannual variability of dry air layer frequency over the tropical north Atlantic, with strong variations over the western and central Atlantic between 15-30N. In the negative phase of the modes, tropical cyclone activity is increased over the Atlantic basin, especially over the Main Development Region. In the positive phase, tropical cyclones form less frequently and the storm tracks shift westward. The leading modes show relatively weak correlation to the AMM and NAO, but the EOF modes do indicate a relationship to the large scale circulation through an analysis of deep layer shear shear and the subtropical high. The steering flow from the modulation of the subtropical high and the values of vertical wind shear are consistent with the distribution of TC activity in some areas, but the impacts of the dry air patterns are required to fully explain the TC activity.