Stalled global warming linked to North American drought

PHOENIX — The extreme winds blamed for putting the brakes on global warming may also have contributed to the record-setting drought currently parching the southwestern United States, suggests new research presented January 5 at the American Meteorological Society’s annual meeting.

Unusually strong winds blowing east-to-west over the tropical Pacific draw up cold water from the deep ocean along North America’s west coast, a process that scientists believe is partially responsible for the recent stall in global warming. This cold water causes atmospheric changes that push rainstorms away from the region, proposes Thomas Delworth, a climate scientist at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J. The winds boosted the likelihood of a dry decade in the region to nearly 50 percent, Delworth estimates.

“It all depends on the winds,” he said. “If these winds persist, the drought will probably persist as well.”

The average annual global surface temperature has plateaued at around 14.5° Celsius since the late 1990s. Because amassing greenhouse gases continue to trap heat, scientists wondered where an expected one-third of a degree of total warming had gone.

In 2014 climate scientists pointed to abnormally strong trade winds that drag over the tropical Pacific Ocean and push surface water westward. The shifting water creates a conveyer belt that pumps cold water from the ocean depths near North America to the surface (SN: 3/22/14, p. 12).

This process accounts for roughly half of the missing one-third of a degree of global warming, proposed climate scientist Clara Deser of the National Center for Atmospheric Research in Boulder, Colo., in a separate presentation.

The cold surface water pulls heat from the nearby atmosphere, changing atmospheric conditions worldwide, Delworth said. To uncover the local effects of this change, Delworth and colleagues ran climate simulations with and without the atypically potent winds.

They found that the atmospheric changes caused by the winds create a high-pressure zone over the western United States. When water-laden clouds traveling from the west meet the high-pressure air, the clouds deflect northward, away from the drought-stricken Southwest. Delworth estimates that without the boosted trade winds, the odds of a decade with more than 15 percent less precipitation than average were about 3 percent. With the boosted trade winds, the chances of a parched decade rose to roughly 46 percent. The resulting rainfall reduction, he said, combined with record-high temperatures caused the most severe drought on record for the region (SN: 1/10/15, p. 16).

“The drying over western North America during the last decade looks like natural variability in the climate system and does not appear to be due to global warming,” he said. “If the winds go back to their long-term average, it’s likely we would end this drought.”

The cause of the forceful gusts remains unclear, though the winds seem linked to the El Niño and La Niña cycles and are expected to slacken as the planet warms (SN: 9/6/14, p. 11). Delworth predicts this wind weakening will reduce the chances of a similar cooling event, and such severe droughts, happening again in the future.

Delworth’s findings should help researchers estimate the odds of future droughts in the western United States and recognize the effects of the climate pause elsewhere around the Pacific, Deser said. “It’s important to diagnose the impacts of the warming hiatus beyond just the global mean temperature record and look at regional effects as well,” she said.

Editor’s note: This story was updated on January 8, 2015, to correct that water-laden storm clouds coming from the west, not the east, are deflected northward.