Variations of the Mid-tropospheric Dry Air Layer and its Impacts on Tropical Cyclone Activity over the Atlantic Basin

The presence of dry air at the midtroposphere 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 mid-level dry air layer and its impacts on tropical cyclone activity over the Atlantic are examined in this study. EOF analysis revealed a dominant mode 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 mode, more tropical cyclones formed 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. Composite analysis shows that easterly vertical shear is enhanced (reduced) during the positive (negative) phase of the leading EOF mode, which also contributes to reduced (enhanced) tropical cyclone activity. Although the leading mode is strongly correlated with AMM on the decadal time scale, they are not correlated on the interannual time scales. The leading mode is not significantly correlated with NAO or ENSO either.