The Walker circulation determines much of the tropical Indo-Pacific climate and has a global impact as seen in the floods and droughts spawned by the El Niño-Southern Oscillation. Meteorological observations over the last 60 years show this atmospheric circulation has slowed: the trade winds have weakened and rainfall has shifted eastward toward the central Pacific.
The immediate cause of this slowdown has puzzled climate scientists. They could not reproduce it in their atmospheric models, questioning the ability of climate models to simulate gradual climate change.
A breakthrough is now reported in a study spearheaded by Hiroki Tokinaga, associate researcher at the International Pacific Research Center, University of Hawaii at Manoa, and published in the November 15, 2012, issue of Nature.
At the root of the models' failure, Tokinaga suspected, was the lack of precise sea surface temperature data used to drive the models. Slight differences in this temperature across the tropical Indo-Pacific Ocean can greatly affect wind and rainfall.
Over the 60-year period the methods used to measure ocean temperature have evolved enormously. Until satellites came along in the 1980s, such measures were taken by ships. This makes it difficult to have one continuous, unbiased record that goes back for so long.
Tokinaga, who is an expert in understanding old, archived data sets and at correcting their biases, found two measures that have been taken by ships over the whole period: the bucket technique, in which the temperature is taken of sea water scooped up in a bucket lowered from a ship, and night time marine air temperature.
"Removing observational biases from the measurements was still challenging, but we saw that these quite different ways of measuring sea surface temperature turned out to agree very well over the 60-year span from 1950 – 2009, and were supported by subsurface ocean temperature observations," explains Tokinaga. "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."
Using this unbiased, reconstructed surface temperature data set in four widely used 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.
"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," explains Tokinaga.
"Short-term fluctuations in the strength of the Walker circulation happen every few years: during La Niña the circulation strengthens, during El Niño it weakens," says co-author Shang-Ping Xie, meteorology professor at the International Pacific Research Center. "The Walker circulation affects tropical convection, and the global impacts of a temporary slowdown during an El Niño are well known, resulting in extreme floods or droughts in North America and other regions of the world. How the gradual slowdown observed in this study impacts global climate still needs to be investigated."
University of Hawaii ‑ SOEST:
This press release was posted to serve as a topic for discussion. Please comment below. We try our best to only post press releases that are associated with peer reviewed scientific literature. Critical discussions of the research are appreciated. If you need help finding a link to the original article, please contact us on twitter or via e-mail.
The UK's wind farms generated more power than its nuclear power stations on Tuesday, the National Grid says.
A charming and poetic account of apiculture in Mark Winston's Bee Time reminds us why an ancient partnership between humans and bees needs saving
Use of a cattle drug that has devastated vulture populations in India is in decline, offering hope of recovery – but vultures in Europe may now be at risk
The winners of the Society of Biology's third annual photography contest include amazing images of a haunting leopard, an otherworldly spider, and Yellowstone National Park's Grand Prismatic Spring.
Bee colonies in Brisbane are waging war for months on end, according to a new study, and the victorious swarms are taking over the hives of rival species.
Hundreds of thousands of cranes stop in Germany on their way to warmer climates
May, June, August and September have all been record-breaking months
Host Audie Cornish talks with Drew Gronewold, a hydrologist at the National Oceanic and Atmospheric Administration, about why water levels in lakes Superior, Michigan and Huron are rising.
Tornadoes in the United States are increasingly coming in swarms rather than as isolated twisters, according to a study by U.S. government meteorologists published on Thursday that illustrates another trend toward extreme weather emerging in recent years.
Just a few years ago, authors were predicting production would soon hit a peak and then decline. But since then, supplies have surged. So are the forecasters now slapping themselves in the head?