Wednesday, August 6, 2014

A team of scientists from Australia and US has found a solution for a challenging problem in climate research. Climate models predicted more greenhouse gases would weaken the equatorial Pacific trade winds.However, over the past two decades, observations showed this Walker circulation was getting stronger, accelerating sea level rise in the western Pacific, and consequent changes in global climate.The researcher team reports, “The answer to the puzzle is that recent rapid Atlantic Ocean warming has affected climate in the Pacific”.Their study, “Recent Walker circulation strengthening and Pacific cooling amplified by Atlantic warming”, was published on Sunday in Nature Climate Change.

While previous research supposed natural variability alone accounted for cooling in the eastern Pacific, this study highlights a previously overlooked climate feedback: as the Atlantic warms, it alters the winds over the Pacific, depressing the ocean temperature there.As coauthor Shayne McGregor of the University of New South Wales explains, “the main cause of the Pacific wind, temperature, and sea level trends over the past 20 years lies in the Atlantic Ocean […] We saw that the rapid Atlantic surface warming observed since the early 1990s, induced partly by greenhouse gases, has generated unusually low sea level pressure over the tropical Atlantic. This, in turn, produces an upward motion of the overlying air parcels. These parcels move westward aloft and then sink again in the eastern equatorial Pacific, where their sinking creates a high pressure system. The resulting Atlantic–Pacific pressure difference strengthens the Pacific trade winds.”

Coauthor Malte Stuecker of the University of Hawaii Meteorology Department reports that “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 [three times faster than the global average] in the western Pacific, eastern Pacific surface cooling, the global warming hiatus, and even the massive droughts in California”. His colleague cauthor Fei-Fei Jin adds, “We are just starting to grasp the scope of the impacts of this global atmospheric reorganization and of the out-of phase temperature trends in the Atlantic and Pacific regions”.Work earlier this year by coauthor Matthew England, University of New South Wales, showed the stronger winds have churned up the waters of the Western Pacific Ocean, so more heat flows from the winds into the water. This appears to explain why global surface temperatures have recently risen more slowly.

Coauthor Axel Timmermann of the University of Hawaii notes a further amplifying effect: “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 […] 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.”As Professor England said, “It will be difficult to predict when the Pacific cooling trend and its contribution to the global hiatus in surface temperatures will come to an end. […] However, a large El Niño event is one candidate that has the potential to drive the system back to a more synchronized Atlantic/Pacific warming situation.”