Patterns of Atlantic Multidecadal Variability and Hurricane Intensity Change Along the U.S. Coast

Basin-wide North Atlantic hurricane activity is suppressed and enhanced, respectively, during periods of cooler and warmer tropical North Atlantic sea surface temperature (SST), and mean SST in these periods is well-separated on multidecadal timescales. For example, a period of comparatively cool SST and suppressed hurricane activity was observed in the period from the late 1960s to the mid-1990s while warmer SSTs and greater hurricane activity have been observed, on average, from the mid-1940s to late 1960s, and again from the mid-1990s to present. The Atlantic is somewhat unique from other tropical cyclone-prone ocean basins in that SST variability is also strongly correlated (inversely) with vertical wind shear (VWS), which also modulates hurricane development and intensification. This correlation ostensibly manifests through a wind-evaporation-SST (WES) feedback mechanism, which drives tropical Atlantic variability via the Atlantic Meridional Mode (AMM). The simple upshot is that when SST is favorable for hurricane development and intensification (i.e., warm SST), so is the VWS (i.e., low VWS), and vice versa.

The decadal co-variability of basin-wide SST, VWS, and hurricane activity described above is well documented, but the regional patterns of co-variability has received less attention. In this talk, I’ll show that while decadal periods of warm SST and low VWS in the tropical Atlantic correlate with increased basin-wide activity, the dipole pattern of the AMM creates conditions along the U.S. coast that are unfavorable for maintaining hurricanes. Conversely, when tropical Atlantic conditions are unfavorable and basin-wide hurricane activity is low, the region near and along the U.S. coast becomes substantially more favorable for maintaining and intensifying hurricanes. During these periods of reduced basin-wide activity, hurricanes along the U.S. coast are 2–3 times more likely to rapidly intensify and major hurricanes are 3–6 times more likely. Thus although there are fewer hurricanes that move near or along the U.S. coast during periods of basin-wide quiescence, those that do approach the coast are much more likely to be intensifying, sometimes rapidly, at that time. This poses a significant challenge for forecasters and increases coastal risk during periods when risk is generally considered to be comparatively low.