Yes, it's raining more than usual — and climate change and El Niño are 2 reasons why
More than 120 million people from Atlanta to Philadelphia found themselves at risk of severe thunderstorms, including strong wind gusts, hail and heavy rain, on Monday. Some areas may even experience tornadoes, while the southern U.S. from Florida to Arizona continues to swelter under extreme heat.
Monday’s storms were the latest in a series of extreme rainfall events that have recently hit the eastern United States. Last month, Connecticut, Vermont and New York’s Hudson Valley endured dramatic flash floods caused by “1,000-year rains.” A week later, flash floods killed five people in Pennsylvania.
In the first half of the year, ending June 30, the precipitation total in the continental states was 15.70 inches, which is 0.39 of an inch above average.
Nationwide statistics aren’t yet available for July, but New England got drenched, with New Hampshire’s Mount Washington receiving 16.91 inches of precipitation — its wettest July ever. Last month, Boston got more than three times its usual rainfall, making it the city’s second-wettest July on record. Providence, R.I., and Worcester, Mass., also had their second-rainiest July on record.
Besides natural variability, the main factors contributing to this year’s wet weather are El Niño, a band of warm ocean water that develops in the Pacific Ocean, and climate change. Here’s how.
Climate change
As with the ongoing record-setting heat waves, heavier rainfall is a byproduct of climate change.
Warmer temperatures cause more water to evaporate, which dries out soil and exacerbates droughts, but also leads to more total rain and heavier individual rain events.
Studies have found that climate change is pushing the water cycle to extremes on both ends, with higher total rainfall.
“Since 1901, global precipitation has increased at an average rate of 0.04 inches per decade, while precipitation in the contiguous 48 states has increased at a rate of 0.20 inches per decade,” according to the Environmental Protection Agency.
These changes are not evenly distributed. While the dry Southwest has been locked in a multidecade megadrought, the already wet Northeast and Upper Midwest have gotten wetter. Between 1951 and 2017, the Great Lakes region’s average temperature increased 2.3 degrees Fahrenheit, its annual rainfall rose 17% and its heavy rain events went up by a whopping 35%.
Even in drought-stricken areas, rains can become more extreme when they do come. After its driest two decades in 1,200 years, California got inundated with near-record rain and snow this winter.
Studies have shown that climate change is weakening the jet stream, a current of eastward-blowing strong winds, causing storms like the one that recently dumped record amounts of rain on Vermont to stall out. The temperature difference between the Arctic and the tropics powers the jet stream, but the Arctic is warming faster than other regions, disrupting the flow of the jet stream.
“Human-caused warming from fossil fuel burning is impacting these events in several ways,” Michael Mann, a climate scientist at the University of Pennsylvania, told Yahoo News last month in an email. “A warmer atmosphere holds more moisture, so when it rains there’s potential for much more of it. And the pattern of warming is impacting the jet stream in such a way that we get more of these very stagnant or ‘stuck,’ wavy patterns which are associated with persistent weather extremes (both heat/drought/wildfire and flooding, depending on your location).”
El Niño
While El Niño generally increases temperatures across North America, its relationship to rain varies by region.
“The warmer waters cause the Pacific jet stream to move south of its neutral position,” the National Oceanic and Atmospheric Administration explains. “With this shift, areas in the northern U.S. and Canada are dryer and warmer than usual. But in the U.S. Gulf Coast and Southeast, these periods are wetter than usual and have increased flooding.”
But the overall effect of an El Niño year in the U.S. is more total rain, because, according to NASA, “the warm ocean surface warms the atmosphere, which allows moisture-rich air to rise and develop into rainstorms.”
The wettest three years on record — 1973, 1983 and 2015 — were all years that featured El Niño, according to the Fourth National Climate Assessment.
If you’re looking for relief from the high heat and heavy rain, don’t expect it to arrive very soon. The latest projections from the European Centre for Medium-Range Weather Forecasts show that El Niño is expected to reach peak strength in September and last until January.
