The Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution Using MERRA2 Reanalyses with Aerosols

Abstract:

This work follows up on a study by Braun (2010, Mon. Wea. Rev.) on the interaction of tropical cyclones (TCs) with the Saharan Air Layer (SAL). Braun used NCEP reanalyses for 41 TCs to form composites for weakening and intensifying storms, while the current study updates results with MERRA2 reanalyses with fully-coupled aerosols for 57 Atlantic TCs during 2001-2011. Storm centers are defined as the location of the maximum of the 950-600-hPa layer-mean Okubo-Weiss parameter so that storms can be tracked in a consistent manner both before and after storm genesis. For each storm, the center is identified for days spanning two days prior to two days after genesis, with the date and time of genesis determined from National Hurricane Center best-track information. For each storm and date/time, data is extracted for a 30°x30° box centered on the storms. Composites fields are formed by averaging over all storms in this storm-centered reference frame. Designating genesis time as t0, fields from all storms are correlated with the intensity change occurring between days t0+2 and t0+4. Key findings include: 1) There is no significant correlation prior to or at genesis between dust loading and storm intensification, 2) negative correlations for zonal winds on the north side of the storms suggest that either a broader vortex or weaker vertical wind shear favors stronger intensification, and 3) correlations with temperature and relative humidity on the western sides of storms suggest that entrainment of warmer and dryer air into TCs is associated with weakening storms, although the region of correlations also extends into non-SAL air to the south of the SAL air mass.