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El Niño Expected to Increase Nuisance Coastal Flooding in U.S.

By: Jeff Masters 7:03 PM GMT on September 09, 2015

In the waters of the Eastern Pacific, strong westerly winds have pushed a massive amount of warm water against the coasts of the Americas, resulting in one of the strongest El Niño events ever observed. Not only does El Niño impact atmospheric patterns, changing storm tracks and suppressing Atlantic hurricane frequency, it also typically resulting in an increase in coastal “nuisance” flooding at high tide along the U.S. West Coast and mid-Atlantic coasts. Nuisance flooding is expensive, causing frequent road closures, overwhelmed storm water systems, and damage to infrastructure. According to a September 9 press release from NOAA, some cities along the mid-Atlantic coast can expect record amounts of “nuisance” flooding at high tide during the coming winter—at Sandy Hook, NJ, Lewes DE, Washington D.C. and Norfolk, VA.

Figure 1. Nuisance street flooding in Norfolk, Virginia on October 9, 2013, during passage of a front at high tide. In the background: on the right side is a church built in 1902 and now for sale largely due to frequent flooding, and in the middle behind the trees is the Chrysler Museum of Art that just completed renovation that include improvements to reduce the potential damage from flooding. Image credit: Tal Ezer, Old Dominion University.

How El Niño events cause more coastal flooding
Along the U.S. West Coast, El Niño wind patterns drive ocean currents that pile up water along the coast, raising sea level for many months. Recent satellite data show that ocean levels are elevated by 7 - 18 cm (3 - 7”) along the coast of North America, from California to Mexico. Along the mid‐Atlantic coast, atmospheric patterns during El Niño typically favor more and stronger winter storms along the coast that drive a higher frequency of storm surges. Both conditions have historically led to flooding during periods of seasonally high tides in the non-summer months. Along the U.S. West Coast, the nuisance flooding resulting from the El Niño-driven boost to the base water level is not a big deal during high tide--if it's a nice sunny day. However, the higher base sea level means that if a whopper storm does hit the coast during El Niño, the resulting storm surge could potentially do a lot more damage. Along the mid‐Atlantic coast, the storm surges from El Niño-related extratropical storms will not be riding up on top of an elevated base sea level like occurs on the West Coast.

Figure 2. Street flooding in Norfolk, Virginia has increased dramatically in recent years, largely due to global-warming induced sea level rise. However, part of the steep increase in Norfolk's flooding events may be due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and its upper branch, the Gulf Stream. See the 2013 paper Ezer et al., Gulf Stream's induced sea level rise and variability along the U.S. mid-Atlantic coast. The top years for flooding in Norfolk, 1998 and 2009, both featured El Niño events, and a series of significant coastal storms (twin major Nor'easters in 1998, and the Veteran's Day Nor'easter of 2009.) Norfolk experienced eight nuisance flood days during the 2014 meteorological year (May 2014 through April 2015), and the new NOAA report predicts that the city may experience 18 days in meteorological year 2015 (May 2015 - April 2016), due to El Niño—125 percent above average. Image credit: Larry Atkinson.

“We know that nuisance flooding is happening more often because of rising sea levels, but it is important to recognize that weather and ocean patterns brought on by El Niño can compound this trend,” said co-author of the report, William Sweet. “By using the historic data that NOAA has collected from tide gauges for more than 50 years, we can better understand and anticipate how the weather patterns may affect nuisance flooding in these communities.”

AP news story on the report by Seth Borenstein
Sea Level Rise Making Floods Routine for Coastal Cities, October 2014 story from Climate Central

Today (September 9) is the 50th anniversary of Category 3 Hurricane Betsy hitting Louisiana. Storm Surge expert Hal Needham has a blog post analyzing Betsy's storm surge.

Wunderblogger Steve Gregory has a new Wednesday afternoon post, California Heat Wave While Much Cooler Temps overspread the East.

Jeff Masters

Flood Sea level rise

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.