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After a Brief Respite, Climate Change Enhanced Drought is Returning to the U.S.

Unseasonable warmth across the American West and overall dry conditions across the South is causing drought to expand throughout many parts of the United States.

According to the U.S. Drought Monitor, most of the southern half of the United States is presently experiencing abnormally dry or drought conditions. Meanwhile, an intense drought that has remained in place over the Dakotas and Montana for multiple months continues to persist.

Severe drought conditions are now present in the south-central U.S. with exceptional and extreme drought expanding through Arkansas, Oklahoma, Texas, Louisiana and Missouri. Deepening drought in California and Texas are notable due to the fact that Southeast Texas recently experienced record rainfall due to Hurricane Harvey and California experienced a very wet winter and spring period from 2016 to 2017. Somewhat milder drought is also spreading through the Southeast.

Re-expanding Southern California drought is also enhancing record wildfire activity in that state.

Much Warmer than Normal Temperatures

A strengthening La Nina in the Equatorial Pacific is helping to generate a drought tendency for the Southern U.S. However, various climate change related features including above normal temperatures and a persistent high pressure ridge in the West are lending intensity to the rising drought regime.

(U.S. 30 day average shows much warmer than normal conditions for the lower 48 with extreme warmth prevalent over the American West. Image source: Global and Regional Climate Anomalies.)

Over the past 30 days, temperatures for the U.S. as a whole have been 1.52 C above average (see image above). Much of this excess heat has been concentrated over the West, with mountain and Pacific regions seeing between 4 and 5 C above average temperatures.

Excess heat of this kind helps to speed the drying of soils and vegetation by increasing the rate of evaporation. A condition that can lead to flash drought — whose incidence has been expanding in lock-step with the human-forced warming of the globe.

A Ridiculous Ridge

Linked to the western heat and drought is a strong and persistent high pressure ridge. One that has hit a very intense 1041 hPa pressure as of Monday afternoon over the U.S. Mountain West.

(Very intense high pressure ridge over the U.S. west is presently locking in both warmer than normal and drier than normal conditions. Image source: Earth Nullschool.)

Persistent ridging of this kind was a key feature of the recent 2012 through 2017 California drought. Some climate studies have identified a tendency of these kinds of strong western ridges to form as Arctic sea ice recedes. And during the past decade, strong high pressure ridges have been a rather consistent and significant climate feature for the U.S. West. It is also notable that formation of more powerful ridging features during the fall and winter help to strengthen the Santa Ana winds — which fan California wildfires.

Present drought is nowhere near as intense as it has been during recent years. Especially in California which during 2017 has experienced a bit of a respite. However, with La Nina gaining traction in the Pacific, with global temperatures now in a range between 1.1 and 1.2 C above 1880s averages, and with persistent ridging again taking hold over the U.S. West, the risk of a return to intense drought — especially for the Southwest — is increasing.

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From Record Floods to Drought in Three Months: Unusually Hot, Dry Conditions Blanket South

Back during late August of 2017, Hurricane Harvey dumped as much as 60.48 inches of rain over southeast Texas. Harvey was the wettest tropical cyclone on record ever to strike the U.S. — burying Houston and the surrounding region under multiple feet of water, resulting in the loss of 91 souls, and inflicting more than 198 billion dollars in damages.

Harvey was the costliest natural disaster ever to strike the U.S. Its tropical rains were the heaviest ever seen since we started keeping a record. But strangely, almost inexplicably, just a little more than three months later, the region of southeast Texas is now facing moderate drought conditions.

(Just three months after Harvey’s record rains, Southeast Texas is experiencing drought. No, this is not quite normal despite a mild La Nina exerting a drying influence. Image source: U.S. Drought Monitor. Hat tip to Eric Holthaus.)

How did this happen? How did so much water disappear so soon? How could an instance of one of the most severe floods due to rainfall the U.S. has ever experienced turn so hard back to drought in so short a time?

In a sentence — climate change appears to be amplifying a natural switch to warmer, drier weather conditions associated with La Nina.

Climate change, by adding heat to the Earth’s atmosphere and oceans fundamentally changes the flow of moisture between the air, the ocean and the land. It increases the intensity of both evaporation and precipitation. But this increase isn’t even. It is more likely to come about in extreme events. In other words, climate change increases the likelihood of both more extreme drought and more extreme rainfall.

Of course, climate change does not exist in a vacuum. Base weather and climate conditions influence climate change’s impact. At present, with La Nina emerging in the Pacific, the tendency for the southern U.S. would be to experience warmer and drier conditions. But in a normal climate, these conditions would tend to be milder. In the present climate — warmed up by fossil fuel burning — the tendency is, moreso, to turn toward an extreme. In this case, an extreme on the hot and dry end of the climate spectrum.

For the region of Southeast Texas flooded so recently by Harvey’s record rains, it means that a turn from far too wet to rather too dry took just a little more than 3 months.

(Both temperature and moisture took a very hard turn over the past 30 days. Such extremely warm and dry conditions increase the likelihood of flash drought. A climate feature that has become far more frequent as the Earth has warmed. Image source: NOAA.)

South Texas, however, is just one pin in the map of a larger trend toward drought that is now blanketing the South. Over the past month, precipitation levels were less than 50 percent of normal amounts in most locations with a broad region over the south and west experiencing less than 10 percent of the normal allotment of moisture. Meanwhile, 90-day precipitation averages are also much lower than normal across the South.

Precipitation is a primary factor determining drought. But temperature can mitigate or worsen drought conditions. Higher temperatures cause swifter evaporation — driving moisture out of soils at a faster rate. And average temperatures across the south have been quite warm recently. With one month averages ranging from 1 C above normal over most of the south to a whopping 8 C above normal over parts of New Mexico. As with lower than normal precipitation, higher than normal temperatures have also extended into the past 90 day period across most of the South.

 

(Moderate drought conditions are widespread as severe to extreme drought is starting to crop up in the South-Central U.S. With La Nina likely to continue through winter and with global temperatures in the range of 1.1 to 1.2 C above pre-industrial averages, there is risk that conditions will intensify. Image source: U.S. Drought Monitor.)

The upshot is that moderate drought is taking hold, not just in southeast Texas, but across the southwest, the southeast, and south-central U.S. Severe to extreme drought has also already blossomed from northern Texas and Louisiana through Oklahoma, Arkansas and Missouri. This is relatively early to see such a sharp turn, especially considering the fact that La Nina conditions have only lasted for a short while and have, so far, been rather mild on the scale of that particular climate event.

Furthermore, like Texas, many of these drying regions experienced extreme rainfall events during spring and summer. Such events, however, were not enough to stave off a hard shift to drought in a world in which human-caused climate change is now driving both droughts and more extreme rainfall events to rising intensity.

(Predicted temperature and precipitation variance from normal over next three months. Climate change is likely to enhance this variability related feature. Image source: NOAA.)

With La Nina likely to remain in place throughout winter, the typical climate tendency would be for continued above average temperatures across the south and continued below average rainfall for the same region. Present human-caused global warming through fossil fuel burning in the range of 1.1 to 1.2 C above pre-industrial averages will tend to continue to amplify this warm, dry end of the natural variability cycle (for the southern U.S.).

In other words, there is not insignificant risk that the hard turn away from record wet conditions in the South will continue and that severe to very severe drought conditions will tend to spring up and expand.

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