Tropical Indo-Pacific climate shifts to a more El Nino-like state, research finds

Nov 14, 2012
This shows a convection over the tropical Pacific. Credit: Image courtesy Yuji Kashino, RIGC/JAMSTEC

Climate models predict a slowdown of the Walker circulation with global warming. Atmospheric models, however, have failed to reproduce the slowdown already observed over the last 60 years, casting doubt on their ability to simulate slow climate change. Now a study published in this week's issue of Nature has succeeded in simulating the slowdown and shows that changes in the sea surface temperature pattern across the Indo-Pacific are the cause.

The Walker circulation determines much of the tropical Indo-Pacific and has a global impact as seen in the floods and droughts spawned by the El Niño-. Meteorological observations over the last 60 years show this has slowed: the trade winds have weakened and rainfall has shifted eastward toward the central Pacific.

The immediate cause of this slowdown has puzzled . They could not reproduce it in their atmospheric models, questioning the ability of 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 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 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 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 , 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."

Explore further: Remnants of Tropical Depression Peipah still raining on Philippines

More information: H. Tokinaga, S.-P. Xie, Clara Deser, Yu Kosaka, Yuko Okumura: Slowdown of the Walker Circulation Driven by Tropical Indo-Pacific Warming, Nature, November 15, 2012.

Related Stories

Forecasters say El Nino may be developing

Jun 08, 2009

(AP) -- A new El Nino could be approaching. Sea-surface temperatures have been warming in the tropical Pacific Ocean, suggesting the potential for the development of the El Nino climate phenomenon this summer, according ...

Ocean height indicator of climate

Oct 07, 2005

U.S. and Canadian authors say an index of sea surface heights gathered by satellites could be a useful indicator of long-term climate cycles.

Climate change may alter natural climate cycles of Pacific

Oct 17, 2010

While it's still hotly debated among scientists whether climate change causes a shift from the traditional form of El Nino to one known as El Nino Modoki, online in the journal Nature Geoscience, scientists now say that E ...

La Nina caused global sea level drop

Oct 29, 2012

The 2011 La Niña was so strong that it caused global mean sea level to drop by 5 millimeters (0.2 inches), a new study shows. Since the early 1990s, sea level has been rising by about 3 millimeters (0.1 inches) per year, ...

Recommended for you

Warm US West, cold East: A 4,000-year pattern

1 hour ago

Last winter's curvy jet stream pattern brought mild temperatures to western North America and harsh cold to the East. A University of Utah-led study shows that pattern became more pronounced 4,000 years ago, ...

New study outlines 'water world' theory of life's origins

3 hours ago

(Phys.org) —Life took root more than four billion years ago on our nascent Earth, a wetter and harsher place than now, bathed in sizzling ultraviolet rays. What started out as simple cells ultimately transformed ...

Agriculture's growing effects on rain

Apr 15, 2014

(Phys.org) —Increased agricultural activity is a rain taker, not a rain maker, according to researchers at Pacific Northwest National Laboratory and their collaborators at the University of California Los ...

User comments : 0

More news stories

Warm US West, cold East: A 4,000-year pattern

Last winter's curvy jet stream pattern brought mild temperatures to western North America and harsh cold to the East. A University of Utah-led study shows that pattern became more pronounced 4,000 years ago, ...

UN weather agency warns of 'El Nino' this year

The UN weather agency Tuesday warned there was a good chance of an "El Nino" climate phenomenon in the Pacific Ocean this year, bringing droughts and heavy rainfall to the rest of the world.

ESO image: A study in scarlet

This new image from ESO's La Silla Observatory in Chile reveals a cloud of hydrogen called Gum 41. In the middle of this little-known nebula, brilliant hot young stars are giving off energetic radiation that ...

First direct observations of excitons in motion achieved

A quasiparticle called an exciton—responsible for the transfer of energy within devices such as solar cells, LEDs, and semiconductor circuits—has been understood theoretically for decades. But exciton movement within ...

Patent talk: Google sharpens contact lens vision

(Phys.org) —A report from Patent Bolt brings us one step closer to what Google may have in mind in developing smart contact lenses. According to the discussion Google is interested in the concept of contact ...

Tech giants look to skies to spread Internet

The shortest path to the Internet for some remote corners of the world may be through the skies. That is the message from US tech giants seeking to spread the online gospel to hard-to-reach regions.