Examination of the influence of the Saharan Air Layer on hurricanes using data from TRMM, MODIS, and AIRS

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. However, the impact of the SAL on tropical cyclones has only recently become an area of active research. The nature of this impact remains unclear, with some researchers arguing that the SAL amplifies cyclone development while others arguing that it inhibits it, with the determining factor possibly being the amount of dust present. In this study, we combine data for 2003-2006 from the TRMM multi-satellite precipitation analysis product, used to characterize the rainfall associated with storms that may evolve into tropical cyclones, with aerosol information from MODIS and thermodynamic data from AIRS on Aqua (temperature and humidity profiles) in order to characterize the dustiness and thermodynamic properties of the SAL. Data from the NASA GEOS-5 analysis system is used to characterize the wind fields.

We use these data sets to generate daily-to-monthly climatologies of the SAL to characterize its properties and evolution. With daily analyses, we track specific rainfall and dust events from the coast of Africa to possible development of individual tropical cyclones. For hurricanes developing from African systems, many are seen to occur in association with dust events. In some cases, the developing storms form and move westward at the leading edge of the SAL, a scenario that has been identified previously as favorable for development. In other cases, the dust and SAL air move ahead of the developing storm, with the storm moving more northward in the wake of the dust outbreak, often not strengthening until moving out of the dust layer belt. Several examples are found of hurricanes with substantial amounts of dust in their immediate environment having no trouble developing into intense hurricanes. Monthly climatologies are used to quantify whether mean dust amount or relative humidity is indicative of an enhanced/reduced probability of cyclone development. Also, it is found that the dustiest air is not necessarily the driest air; instead, the driest air appears to originate from higher latitudes just north of the dust layer.