Dying tradewinds? It's the water temperature.

University of Hawai`i researchers report that changes in ocean temperatures may be responsible for the dying of the trade winds over the past 60 years.

The scientist team used an amazingly low-tech data set to help them reach that conclusion—buckets hauled aboard ships to test ocean water temperature over decades.

The study, published in the Nov. 15, 2012, issue of Nature, was led by Hiroki Tokinaga, associate researcher at the International Pacific Research Center at the University of Hawaiʻi at Mānoa. 

It argues that warmer waters are linked to weakening tradewinds and an eastward shift in oceanic rainfall, toward the Central Pacific. That’s associated with a slowing of what climate scientists call the Walker Circulation, in which a regional wind pattern is created by warm, moist air rising over warm waters.

RaisingIslands.com covered the dying trades last month. 

Climate scientists have been baffled by the significant change in wind and rain patterns, because their climate models couldn’t explain them. Tokinaga felt that might simply be because the models didn’t have precise enough water temperature information.   

He tracked down archived sets of old data collected over the entire 60-year period, in which ships kept track of night time marine air temperatures as well as ocean water temperature—determined by putting thermometers into buckets of water pulled from the sea as the ships crossed the Pacific.

“To our surprise both measures showed that the surface temperature across the Indo-Pacific did not rise evenly with global warming, but that the east-west temperature contrast has actually decreased by 0.3-0.4°C, similar to what happens during an El Niño,” Tokinaga said.

When they plugged to reconstructed temperature data into four separate computerized atmospheric models, “the scientists were able to reproduce quite closely the observed patterns of climate change seen over the 60-year period in the tropical Indo-Pacific and the slowdown of the Walker circulation.”

Here is the University of Hawai`i press release on the study. An abstract of the actual paper is available here.  A nice piece on Walker Circulation from NOAA and NASA is here. 

 "Our experiments show that the main driver of the change in the Walker circulation is the gradual change that has taken place in the surface temperature pattern toward a more El Niño-like state. We don't have enough data yet to say to what degree the slowdown over the last 60 years is due to a rise in man-made greenhouse gases or to natural cycles in the climate," Tokinaga said.

© Jan TenBruggencate 2012