The role of the Saharan Air Layer in the evolution of Hurricane Helene (2006)

The Saharan air layer (SAL) has received considerable attention in recent years as a potential negative influence on the formation and development of Atlantic tropical cyclones. The 2006 hurricane season saw several storms potentially influenced by the SAL, including Hurricane Helene, that has led to speculation about the suppressing influence of the SAL in this and other cases. Here, a suite of remote sensing data, global meteorological analyses, and high-resolution simulations are used to evaluate the environment of Helene and the possible role of the SAL in its evolution. In general, the data do not support the notion of significant suppression of storm development. The influence of the SAL appears to be limited to Helene's earliest stages of development. Saharan dust was observed on the periphery of the storm during the first two days of development after genesis when intensification was slow, but much of this dust moved well westward of the storm thereafter, with little indication of SAL air present during the remainder of the storm's lifetime and with the storm gradually becoming a category 3 strength storm four days later. Dry air observed to wrap around the periphery of Helene was diagnosed to be non-Saharan in origin (the result of subsidence) and likely have had little impact on storm intensity. The eventual weakening of the storm is suggested to result from increased vertical wind shear and substantial reduction of the sea-surface temperatures beneath the hurricane as its forward motion decreased.