Relating Mid-Level Meridional Water Vapor Transport by Stationary Eddies to Seasonal Atlantic Tropical Cyclone Activity

Previous studies highlight mid-level moisture as an important environmental characteristic in seasonal tropical cyclone (TC) development. The distribution of mid-level moisture and subsequent TC development, for a given season, in the Atlantic basin is the result of many different factors, one of which is the transport of moisture by large-scale seasonal circulations. For example, the transport of dry, midlatitude air into the tropics, characterized the 2013 Atlantic hurricane season with an anomalously dry mid-troposphere and fewer TCs, despite otherwise favorable conditions.

The role of moisture transport by large-scale seasonal features during the peak of the Atlantic hurricane season (August-September-October) is examined through analysis of the stationary eddy component of the meridional water vapor flux (i.e., “moisture flux”) averaged over various parts of the Atlantic Basin using ERA-Interim analysis data from 1979-2014. The Atlantic subtropical high appears as the primary eddy responsible for mean seasonal moisture transport within the basin, with additional contributions from local seasonal circulations in the Caribbean Sea and off the West African coast.

Furthermore, the role of interannual variability in the moisture flux on TC activity, defined by the accumulated cyclone energy (ACE), is assessed. The least active (bottom 20%) TC seasons are characterized by large moisture fluxes away from the main development region, relative to the most active (top 20%) of seasons. These differences are mainly concentrated on the eastern side of the subtropical ridge and the local Caribbean circulation.