Monday, August 4, 2014

Pacific tradewinds enhanced by Atlantic warming

In our mid-Pacific isolation, it can be hard to understand how globally connected things are.
Case in point: Our tradewinds have increased in strength thanks to the warming of the Atlantic Ocean.

An August 3, 2014,  paper in the journal Nature Climate Change, by a team of Hawai`i and Australia researchers, says there are lots of other interconnections, too.

That Atlantic warming is also associated with drought in California, sea level rise in the Western Pacific, and cooling of parts of the Pacific.

"We were surprised to find that the main cause of the Pacific wind, temperature, and sea level trends over the past 20 years lies in the Atlantic Ocean," said University of New South Wales researcher Shayne McGregor.

Here’s how it works. The Atlantic ocean has warmed at the surface, partly due to global climate change and greenhouse gas. The warm water causes the air above it to warm and rise. That creates low pressure in the tropical Atlantic atmosphere. All that rising air comes down again as it cools, but this time over the eastern equatorial Pacific, where it creates high pressure. The difference between the low and high pressure systems creates stronger tradewinds.

Co-author Axel Timmermann, of the University of Hawai`i International Pacific Research Center expands: 

"Stronger trade winds in the equatorial Pacific also increase the upwelling of cold waters to the surface. The resulting near-surface cooling in the eastern Pacific amplifies the Atlantic–Pacific pressure seesaw, thus further intensifying the trade winds." 

That new cooling in the Pacific has likely caused a kind of pause in rising global temperatures, something that was previously unexplained in climate models, Timmermann said. 

"It turns out that the current generation of climate models underestimates the extent of the Atlantic–Pacific coupling, which means that they cannot properly capture the observed eastern Pacific cooling, which has contributed significantly to the leveling off, or the hiatus, in global warming."

"Our study documents that some of the largest tropical and subtropical climate trends of the past 20 years are all linked: Strengthening of the Pacific trade winds, acceleration of sea level rise in the western Pacific, eastern Pacific surface cooling, the global warming hiatus, and even the massive droughts in California," said co-author Malte Stuecker from the University of Hawaii Meteorology Department.

But it is unlikely to last. The researchers said an El Nino, like the one now developing in the Pacific, could reset the system.

Citation: Recent Walker circulation strengthening and Pacific cooling amplified by Atlantic warming by Shayne McGregor, Axel Timmermann, Malte F. Stuecker, Matthew H. England, Mark Merrifield, Fei-Fei Jin & Yoshimitsu Chikamoto published in Nature Climate Change doi:10.1038/nclimate2330

© Jan TenBruggencate 2014

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