Harsh Droughts Can Actually Start over Oceans

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Droughts conjure images of vast expanses of hard, cracked soil and parched plants, but new research suggests that disastrous dry spells can develop over the wettest place of all: the ocean. Low-moisture air masses sometimes form and migrate thousands of kilometers over the sea, similar to the way hurricanes behave. These dry-air regions are less coherent, changing shape as they develop, however, and they move much slower. Some take more than half a year before they hit land, where they can destroy crops and threaten water security. Yet the long travel time means forecasters might be able to predict when this newly recognized type of drought will impact key regions, such as the western U.S.

Thinking about droughts as a dynamic hazard is a new idea,” says environmental engineer Julio Herrera Estrada, who helped discover the phenomenon. He and his Stanford University postdoctoral advisor Noah Diffenbaugh describe what they have dubbed “landfalling droughts” in a study published this fall in Water Resources Research.

Herrera Estrada and Diffenbaugh made their discovery by retroactively following areas of relatively low atmospheric moisture worldwide, over both land and sea, from meteorological records between 1981 and 2018. “We watched them change shape from month to month and tracked how they moved in space and time,” says Herrera Estrada, who now focuses on sustainability as an applied scientist for Descartes Labs, which is based in Santa Fe, N.M. The researchers found that most areas of drought began and ended entirely over either the ocean or land. But one in six of the droughts afflicting continents turned out to have started over the ocean. “It’s not an obvious thing to wrap your head around. It’s a little counterintuitive to think about droughts over the ocean, because it’s wet,” Herrera Estrada says. “But there can still be lower rainfall over the ocean.”

News Date: 

Monday, November 23, 2020