Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

Abstract

The existence of the Saharan air layer (SAL), a layer of warm, dry, dusty air frequently present over the tropical Atlantic Ocean, has long been appreciated. The nature of its impacts on hurricanes remains unclear, with some researchers arguing that the SAL amplifies hurricane development and with others arguing that it inhibits it. The potential negative impacts of the SAL include 1) vertical wind shear associated with the African easterly jet; 2) warm air aloft, which increases thermodynamic stability at the base of the SAL; and 3) dry air, which produces cold downdrafts. Multiple NASA satellite datasets and NCEP global analyses are used to characterize the SAL’s properties and evolution in relation to tropical cyclones and to evaluate these potential negative influences. The SAL is shown to occur in a large-scale environment that is already characteristically dry as a result of large-scale subsidence. Strong surface heating and deep dry convective mixing enhance the dryness at low levels (primarily below ∼700 hPa), but moisten the air at midlevels. Therefore, mid- to-upper-level dryness is not generally a defining characteristic of the SAL, but is instead often a signature of subsidence. The results further show that storms generally form on the southern side of the jet, where the background cyclonic vorticity is high. Based upon its depiction in NCEP Global Forecast System meteorological analyses, the jet often helps to form the northern side of the storms and is present to equal extents for both strengthening and weakening storms, suggesting that jet-induced vertical wind shear may not be a frequent negative influence. Warm SAL air is confined to regions north of the jet and generally does not impact the tropical cyclone precipitation south of the jet. Composite analyses of the early stages of tropical cyclones occurring in association with the SAL support the inferences from the individual cases noted above. Furthermore, separate composites for strongly strengthening and for weakening storms show few substantial differences in the SAL characteristics between these two groups, suggesting that the SAL is not a determinant of whether a storm will intensify or weaken in the days after formation. Key differences between these cases are found mainly at upper levels where the flow over strengthening storms allows for an expansive outflow and produces little vertical shear, while for weakening storms, the shear is stronger and the outflow is significantly constrained.