Interannual Variability of the Atlantic Hadley Circulation and Its Impacts on Tropical Cyclone Activity

The interannual variability of the Hadley circulation and its impacts on tropical cyclone activities over the Atlantic basin are examined. A meridional mass streamfunction is derived by inverting the Laplacian of zonal vorticity in isobaric coordinates, which can well represent the vertical motion. The EOF analysis of the mass streamfunction shows that the interannual variability of the Hadley circulation is dominated by an equatorially asymmetric mode, which explains more than 40% of the total variance. In the positive (negative) phase of the mode, the ITCZ is stronger (weaker) and extends northward (retreats southward); the Atlantic Main Development Region is characterized by higher (lower) relative humidity throughout the troposphere, weaker (stronger) vertical wind shear, lower (higher) sea level pressure, and enhanced (reduced) tropical easterly wave activity. Tropical cyclones form more (less) frequently in the positive (negative) phase of the asymmetric mode, and they are more (less) likely to reach major hurricane strength.