Patterns of North Atlantic Decadal Variability and Hurricane Intensity Change

It is well documented that Atlantic hurricane activity is suppressed during periods of cooler tropical North Atlantic sea surface temperature (tNASST), and that mean tNASST in these periods is well-separated on decadal timescales. For example, a period of comparatively cool tNASST and suppressed hurricane activity was observed in the period from the late 1960s to the mid-1990s while warmer tNASST 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 tNASST 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). When tNASST is favorable for hurricane development and intensification (i.e., warm tNASST), so is VWS (i.e., low VWS), and vice versa.

Although it’s well documented that tNASST, VWS, and hurricane activity all co-vary decadally, the regional pattern of this co-variability has received less attention. In this talk, we will 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 about 2–3 times more likely to rapidly intensify and major hurricanes are about 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 operational forecasting challenge, which increases coastal risk during periods when risk is generally considered to be comparatively low.

The results described here raise important questions regarding what might be expected if environmental conditions were to shift back towards the pattern of the previous quiescent period as well as what might be expected as the tropics continue to warm. For example, can we plausibly expect internal variability to return us to cool tNASST conditions and if so, can we expect the last cool phase from the late 1960s to the mid-1990s to serve as an analogy for the next cool phase? If the last cool phase is a good analogy for a subsequent cool phase, which some have argued has already begun, then we would expect a reduction of overall risk, but significantly heightened risk on average during times when a hurricane is near the US coast. On the other hand, if decadal variability of the combined external forcing from greenhouse gases, sulfate aerosols, and African dust has dominated the signal identified here, then we may never again experience another quiescent period like the last one, at least within the next century or two. Still, even in such an extreme case, there is a general expectation that internal variability plays a substantial role in decadal variability, and any related future periods of cooler conditions in the tropical North Atlantic would increase the US coastal risk associated with individual hurricanes if the previous cool period is at least a partial analogy. To address this question, we will discuss new results that explore the internal and external forcing factors that contribute to the pattern of variability.

Bibliography:

Kossin, J. P., 2017: Hurricane intensification along United States coast suppressed during active hurricane periods. Nature, 541, 390–393.

Kossin, J. P., and D. J. Vimont, 2007: A more general framework for understanding Atlantic hurricane variability and trends. Bull. Amer. Meteor. Soc., 88, 1767-1781.

Ting, M., Y. Kushnir, R. Seager, and C. Li, 2009: Forced and natural 20th century SST trends in the North Atlantic. J. Climate, 22, 1469–1481.

Vimont, D. J., and J. P. Kossin, 2007: The Atlantic meridional mode and hurricane activity. Geophys. Res. Lett., 34, L07709, DOI:10.1029/2007GL029683.