The droughts afflicting states in the Western U.S., as well as a general drying out that began with the turn of this century, has its roots even further west — in the winds of the Pacific Ocean, researchers say.
Those ocean winds have natural cycles of waxing and waning, they say, and the last 15 to 20 years have seen strong trade winds forcing heat down into the ocean depths.
This has, in turn been, forcing surface heat deep beneath the ocean, resulting in a slowdown — albeit a temporary one — in warming over surrounding land surfaces.
While many scientists and laymen argue of this warming hiatus and what it means — or doesn't — for climate change, other researchers have turned their attention to the impact the ocean wind patterns might be having on the ongoing severe droughts in the Western states.
Their conclusion is that the rainfall deficit driving drought conditions in the region beginning in the early 2000s is down to changes in ocean wind patterns in the Pacific region.
"We know there's a lot of natural variability in the (climate) system," says Tom Delworth, a climate modeler at the Geophysical Fluid Dynamics Laboratory in Princeton, N.J.
Delworth and his colleagues, writing in the Journal of Climate, suggest 92 percent of the drying conditions in the U.S. West can be linked to trade wind shifts in the Pacific, while the rest can be laid at the feet of humanity's greenhouse gas emissions.
Storms that would normally bring much-needed rain to the U.S. West have been deflected north into Canada by a resilient ridge created by ocean conditions, they say.
The study is just one of a number in recent years that have focused on the Pacific Ocean and its impact on both global warming and widespread drought.
Shang-Ping Xie, a scientist at the Scripps Institute for Oceanography who conducted one such study two years ago, says the new work offers "significant advances" in understanding those links.
"This new work, along with earlier studies, shows the importance of tropical Pacific conditions for U.S. droughts," he says. "It also illustrates the need for accurate and long-range prediction of tropical Pacific climate anomalies."
Such accurate forecasts could be helpful in preparing measures to deal with ongoing drought, the researchers say.
"We need water resource systems that are resilient to those extremes in systems due solely to natural variability," Delworth says. "Riding on top of that is this long-term warming trend that will also increase demand."
