Warmer and higher than normal sea surface contibutes to Mid-Atlantic flooding

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Piling waters Mid-Atlantic Coast caused by days of onshore flow, full moon but more importantly…persistent warm SSTA of the western North Atlantic.

Fig. 1: Current NOAA/NWS-issued coastal flood advisories and warnings.

Discussion: NOAA/NWS offices maintain coastal flood warnings along Chesapeake Bay and the Potomac River in Westmoreland, Lancaster, and Northumberland Counties in Virginia (Fig. 1). Up to 2 feet of water inundation above ground level continues in low-lying areas near the shoreline. The warning is currently forecast to expire at 6PM EDT today. Flood advisories extend from northeast North Carolina northward throughout the remainder of Chesapeake Bay and also to southeast coast of Maryland.

The cause of the coastal flooding is a persistent onshore flow caused by an offshore storm forming middle of last week south of Nantucket, becoming stationary and acquiring subtropical characteristics. NOAA/NHC named this storm Subtropical Storm Melissa late last week. Melissa is drifting east-northeastward near 14 mph now and is located about 370 miles south of Halifax, Nova Scotia. The strong onshore flow into the Mid-Atlantic region eases later today and tonight.

Also contributing to the high water rise is a full moon which lifts the normal diurnal high tide by about 1 foot.

However, there is another factor at play here. The ocean surface is somewhat warmer than normal off the Mid-Atlantic coast (Fig. 2). Warmer ocean surface also means higher sea level height anomalies and any long duration onshore wind event drives that piling water westward into the Mid-Atlantic coast.

Fig. 2: Weekly SSTA analysis provided by NOAA/NHC identifying the warmer-than-normal waters off the Mid-Atlantic coast.

The tendency of warmer than normal surface water off the Mid-Atlantic (and Northeast U.S.) coastline has accelerated since 2013-2014. At that time the general decadal warming of the North Atlantic (part of the Atlantic multi-decadal oscillation cycle) developed an area of cooling to the south of Greenland (Fig. 3). The cooling is due to increased fresh water melt of the Greenland Ice Sheet. The region of cool water in an otherwise warm North Atlantic basin has been named the “North Atlantic Warm Hole” or NAWH by the climate scientist community.

The NAWH is causing the warm northeast flowing Gulf Stream to accumulate warm and piling waters off the U.S. East Coast in recent years leaving East Coast locations increasingly susceptible to ocean rise and attendant flooding when strong or/and long duration onshore wind episodes occur.

Fig. 3: Daily global sea surface temperature anomaly analysis provided by NCDC/PSD identifying the North Atlantic Warm Hole to the south of Greenland.

The NAWH was initially discovered in a 2014 study by the University of Reading and suggested a possible slowing of the Atlantic Ocean Meridional Circulation (AMOC). We’ve been in the warm phase since the 1990’s which helps to explain in-part the decay of the polar ice cap. Further investigation by Pennsylvania State University and Potsdam Institute for Climate Impact Research has increasingly confirmed the initial research.

Climate Impact Company has noted the tendency of this cool pool of water south of Greenland to cause a persistent upper trough of low pressure to persist in the same vicinity which during the warm season has caused downstream upper ridge patterns to form over Europe causing drought while upstream high pressure ridge across over east/northeast North America during summer have caused slowdowns of tropical cyclones once reaching the U.S. coastline such as Harvey (2017) and Florence (2018).