When does the Saharan Air Layer impede the intensification of tropical cyclones?

Abstract

We investigate the circumstances under which the Saharan Air Layer (SAL) has a negative impact on the intensification of tropical cyclones (TCs) over the North Atlantic. Using hurricane tracking, aerosol optical depth (AOD) data, and meteorological analyses, we analyze the interaction of the SAL with 52 named TCs that formed over the east and central Atlantic south of the Cape Verde islands between 2004 and 2017. Following the categorization of negative SAL influences on TC intensification by Dunion and Velden (2004), only 21% of the investigated storms can be classified (28% of all storms that encountered the SAL), while 21% of the storms continue to intensify despite the presence of the SAL. We show that among TCs which encounter the SAL, there is evidence supporting a weak negative correlation between the magnitude of TC intensification and the ambient AOD. However, above-average Saharan dust abundance in the vicinity of TCs is not a good independent indicator for storm non-intensification. To better understand the specific processes involved, a composite study is carried out, contrasting storms which intensify in the presence of the SAL against those which do not. We find that sheared air masses on the north side and drier air from the northeast of the storm early on during its lifetime, in addition to higher AOD, are associated with TC non-intensification in proximity to the SAL.