Previous studies have indicated substantial interannual and interdecadal variability of Atlantic tropical storm and hurricane activity. This variability is heavily influenced by the vertical wind shear over the central subtropical North Atlantic, a feature which is well-known to be strongly influenced the ENSO phenomenon. In addition to interannual variations in the vertical wind shear, we identify strong interannual variations in the location and structure of the African easterly jet during August and September (~600 hPa level), and show that these variations are also strongly linked to ENSO. It is suggested that these variations in the African easterly jet significantly impact the potential for development of easterly waves as the waves propagate westward over the subtropical North Atlantic. Thus, they represent a second extremely important mechanism influencing the interannual variability of North Atlantic tropical storm and hurricane activity.
We present case studies indicating that active hurricane seasons feature a well-defined easterly jet with large values of cyclonic relative vorticity extended westward over the central subtropical North Atlantic between 10- 15N. Also, this jet structure exhibits large regions which satisfy the necessary condition for linear barotropic instability, suggesting that the jet is quite efficient at suppling energy to developing African easterly waves. Moreover, these favorable jet conditions often exhibit a strong overlap with the region of low vertical wind shear across the central and eastern North Atlantic, thus allowing for extended periods in which the developing easterly waves experience conditions favoring intensification leading to organized convective development and eventual tropical cyclone formation.
In contrast, inactive hurricane years feature a poorly-defined African easterly jet shifted south of normal with little attendant cyclonic relative vorticity. In fact, this region of cyclonic vorticity is often located equatorward of 10 latitude, which is generally considered too far south to favor efficient tropical cyclogenesis. Also, the jet in these seasons often exhibits no region which satisfies the necessary condition for linear barotropic instability, suggesting that it cannot efficiently supply energy to developing African easterly waves. Finally, the inactive seasons feature little to no overlap between the regions of cyclonic relative vorticity and low vertical wind shear, indicating an almost complete lack of conditions favorable for easterly wave intensification and tropical cyclone development across most of the subtropical North Atlantic.