Elevated storm surge differs in origin along the US East and Gulf Coasts

June 1, 2020
Time of emergence of the anthropogenic signal in storm-related extreme sea level at New York, Miami, and New Orleans in the GFDL CM4 simulations.
Figure 1. Time of emergence of the anthropogenic signal in storm-related extreme sea level at New York, Miami, and New Orleans in the GFDL CM4 simulations. In response to a 1% per year atmospheric CO2 concentration increase (1pctCO2), anthropogenic signals (relative to piControl) in daily surge height and frequency can emerge from the background variability in as little as 20 years along the US Northeast and Southeast Coasts. It is delayed along the Gulf Coast because of the large natural variability. Blue and red colors denote the daily sea level anomaly in the unperturbed (piControl) and perturbed (1pctCO2) simulations, respectively. Horizontal dashed lines denote the return levels of 1-, 10-, and 100-year events in the 150-year unperturbed simulation. Triangles and diamonds indicate time of emergence in extreme sea level height and frequency, respectively. Rectangles denote permanent exceedance by the rising mean sea level (click image to enlarge).
 
The US East and Gulf Coasts are vulnerable regions to storm surge and coastal flooding induced by tropical and extratropical cyclones. These storms and related extreme sea level (ESL) events are critically influenced by various factors including chaotic/stochastic weather processes, internal climate variability, and external climate forcing. Thus, it would be desirable to study characteristics of these ESLs and their future evolution in a fully coupled weather, climate, sea level, and storm surge modeling system, in which important interaction and feedback processes among the ESL events and impact factors could be better represented.
 
Recently, the study of ESL in such a modeling system became possible thanks to the latest development of the high-resolution global climate model. Yin and coauthors use the new GFDL CM4 and CM4HR models with a 1° or 0.5° atmospheric model resolution and a 0.25° oceanic model resolution, and consider a series of climate change experiments under the CMIP6 protocol. Due to the refined model resolution and improved model physics such as for atmospheric convection and oceanic mesoscale eddies, tropical and extratropical cyclones and related storm surge are better simulated in CM4 and CM4HR compared with previous model generations.
 
By analyzing daily sea level in the CM4 and CM4HR model simulations, they find that even in the absence of global warming, the Gulf Coast is most vulnerable to hurricane-induced storm surge. New Orleans is a striking hotspot with the highest surge efficiency in response to storm winds (Figure 1). Under a 1% per year atmospheric CO2 concentration increase on centennial time scales, the elevated storm surge along the East Coast is mainly caused by the background sea level rise, while that along the Gulf Coast is sensitive to the modification of hurricane characteristics by the external forcing. Along the East Coast, the anthropogenic signal in ESL can emerge from the background variability as soon as in 20 years, or even before global sea level rise is taken into account (Figure 1). The regional dynamic sea level rise induced by the weakening of the Atlantic meridional overturning circulation facilitates this early emergence, especially during wintertime coastal flooding associated with nor’easters
 
As caveats, CM4 and CM4HR do not simulate tides and ice sheet melt. Including these important processes would further increase future sea level rise and heighten ESLs associated with severe storms along the US East and Gulf Coasts.
Written by 
Jianjun Yin, University of Arizona

Jianjun Yin1, Stephen M. Griffies2,3, Michael Winton2, Ming Zhao2, Laure Zanna4,5

1Department of Geosciences, University of Arizona
2NOAA/GFDL
3Atmospheric and Oceanic Sciences Program, Princeton University
4Courant Institute, New York University
5Department of Physics, University of Oxford