Signs of climate change circulating

NEW YORK — An important wind-circulation pattern over the Pacific Ocean has begun to weaken because of global warming caused by human activity, perhaps altering climate and the marine food chain in the region, new research suggests.

It's not clear what climate changes might arise, but the long-term effect might resemble El Niņo, albeit a weak one.

El Niņos boost rainfall in the southern United States and western South America and bring dry weather or even drought to Indonesia, Malaysia and elsewhere in the western Pacific. In the Pacific Northwest, El Niņos can contribute to drier, warmer winters.

As for the Pacific food chain near the equator, the slowdown may be reducing populations of tiny plants and animals on which fish feed, because of reduced nutrition welling up from the deep, said Gabriel Vecchi, author of the study.

Such reductions in upwelling also have been observed in the Northwest and may play a role in fish die-offs and dramatic drops in the population of some bird species. But it's not clear how much of the reduction in ocean nutrients is the result of natural ocean cycles.

Results of the study by Vecchi, a visiting scientist at a National Oceanic and Atmospheric Administration lab in Princeton, N.J., and colleagues are in today's issue of Nature.

The weakening of wind circulation was detected in shipboard and land-based data going back to the mid-1800s. It matches an effect predicted by computer climate simulations that trace global warming to a buildup of heat-trapping greenhouse gases, the researchers report. But simulations that consider only natural influences fail to produce the same slowdown, Vecchi said.

So, it appears the circulation slowdown is due to the buildup of greenhouse gases caused by human activity, the researchers concluded.

The study focused on what scientists call the Walker circulation, a wind pattern that covers almost half the circumference of Earth. The pattern traces a huge loop. Trade winds blow across the Pacific from east to west. The air rises in the western Pacific and returns eastward at an altitude of a few miles. It then sinks to the surface and starts the loop again.

Dennis Hartmann, a professor of atmospheric sciences at the University of Washington, said the study makes a strong case that the Walker circulation has slowed. While such an effect had been predicted as a result of global warming, he said, "it's not been demonstrated before as clearly as they've done here."

The new study is based on barometric-pressure readings, since differences in air pressure drive winds near the equator. Results suggest the average wind speed in the Walker circulation has weakened by about 3.5 percent since the mid-1800s. It has weakened faster since World War II than since the mid-1800s, Vecchi said.

Computer simulations say the circulation might weaken an additional 10 percent by 2100, Vecchi said. But the natural variability in ocean conditions makes it difficult to predict how much of the potential effects, such as reduced runs of salmon, will be linked directly to the change.

Seattle Times staff reporter Craig Welch contributed to this report.