Climate Dynamics of Easterly Waves in the North Atlantic and East Pacific

The frequency and intensity of easterly waves is of particular interest in context of their role in the formation of tropical cyclones. This analysis examines the relationship between easterly wave frequency estimated using a new and improved tracking scheme (Belanger et al. 2014) and various climate modes including the Atlantic Multidecadal Oscillation (AMO), Atlantic Meridional Mode (AMM), North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), and El Nino Southern Oscillation (ENSO). Analysis of this new climatology of easterly waves shows that both the AMO and AMM exhibit the strongest positive covariability with Atlantic easterly wave frequency. This is associated with the north–south dipole structure of mid-tropospheric relative vorticity in western Africa in positive versus negative years that favors African easterly wave amplification just prior to entry into the tropical Atlantic basin. The easterly wave–tropical cyclone pathway is likely a more important mechanism governing how the AMO and AMM modulate North Atlantic TC frequency than previously thought. In contrast, for the East Pacific, it is shown that ENSO plays an important role in the median location of easterly wave genesis.