'Sunny day flooding' and 90-degree days during South's endless summer

Posted October 5, 2019 1:39 p.m. EDT

— For some American cities, this has been the summer that just won't end.

Atlanta has broken a record for 90-degree days this year -- there have been 91 of them, wildly surpassing the average of 37 per year.

Residents there aren't the only ones sweating. Many other cities have also set records for stretches of 90-degree days, including Nashville, Macon, Baltimore, and Wilmington, North Carolina.

This comes after more than a week of record-high temperatures across the US where nearly 200 record-high temperatures were broken.

Newly released data shows that last month tied for the hottest September on record globally. According to Copernicus ECMWF, a European weather agency, September of this year was about 0.57°C (1.02°F) above average, which ties with September of 2016. This comes after record-breaking summer temperatures globally. The hottest June on record? June of 2019. Ditto for July. And August 2019 was the second hottest August globally.

Here's why this is important (beyond your ridiculously-high air conditioning bill). Warmer air temperatures mean warmer ocean temperatures. Warmer oceans mean sea level rise in the form of melting sea ice, and thermal expansion. Thermal expansion is basically the idea that warmer water takes up more volume. Unfortunately, for coastal cities, sea level rise means more frequent coastal flooding events, often called "sunny day flooding."

Take Charleston, South Carolina, for example. On Friday, the National Weather Service office there announced that the Charleston Harbor tide gauge has observed 58 coastal flood events so far in 2019. This surpasses the previous record of 57 set back in 2015. Keep in mind that the number for 2019 may go up even more since it is only October.

More importantly, look at how sea-level rise has increased incidences of flooding just the past few decades:

• 1980s -- 9.3 events per year

• 1990s -- 18.8 events per year

• 2000s -- 21.4 events per year

• 2010s -- 39.1 events per year

Charleston is not alone. In Florida, both Miami and Fort Lauderdale are also at risk for "sunny day flooding." For years, Fort Lauderdale has implemented road improvements to help alleviate drainage concerns across the city. Miami Beach, for its part, even has an official Sea Level Mitigation Plan, which includes raising streets and installing more pipes and pumps to stop -- or at least limit -- the flooding.

Another factor for southeastern Florida is that the Gulf Stream slows down at the end of summer. It initiates in warm, tropical waters off of Florida. But when it slows down along Southeastern Florida, it backs up and creates a bulge of water along the coast.

This year, there have been problems along this same coastline due to strong Northeasterly winds, which are common in March, not September and October. On the surface, this may seem great because a Northeast wind provides a cooling sea breeze in Fort Lauderdale and Miami, plus relief from the summer heat. The problem is that the wind also pushes ocean water onshore, triggering coastal flooding, even though it is not raining.

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