Droughts usually evoke visions of cracked earth, withered crops, dried-up rivers and dust storms. But droughts can also form over oceans, and when they then move ashore they are often more intense and longer-lasting than purely land-born dry spells.

A Sept. 21 study published in the journal Water Resources Research found that, of all the droughts that affected land areas globally from 1981 to 2018, about 1 in 6 started over water and moved onto land, with a particularly high frequency along the West Coast of North America, said senior study author Noah Diffenbaugh, a Stanford climate researcher.

"Because they usually take a number of months to migrate onto land, there is a potential that tracking moisture deficits over the ocean could provide advance warning to help protect against at least some of the most severe droughts," he said, adding that the landfalling droughts, as those that move from the ocean to terra firma are known, grow three times as quickly as land-only droughts.

The research zoomed in on West Coast landfalling droughts and linked them with Pacific Ocean weather patterns that are changing in a warming world. Those dry spells happen when large areas of stable air persist much longer than normal off the West Coast.

The current Western drought could soon rise to a crisis level, with federal water managers warning that Lake Powell and Lake Mead, the two key Colorado River reservoirs, may drop to levels that could result in economically damaging cuts to water allocations in the Southwest and California.

But the climate processes leading to such deep dry spells are not fully understood, and that makes the landfalling drought study important, said Dim Coumou, a climate extremes researcher with the Potsdam Institute for Climate Impact Research.

Read the full article about droughts by Bob Berwyn at InsideClimate News.