Nav: Home

Natural climate processes overshadow recent human-induced Walker circulation trends

April 01, 2019

A new study, published this week in the journal Nature Climate Change, shows that the recent intensification of the equatorial Pacific wind system, known as Walker Circulation, is unrelated to human influences and can be explained by natural processes. This result ends a long-standing debate on the drivers of an unprecedented atmospheric trend, which contributed to a three-fold acceleration of sea-level rise in the western tropical Pacific, as well as to the global warming hiatus.

Driven by the east-west sea surface temperature difference across the equatorial Pacific, the Walker circulation is one of the key features of the global atmospheric circulation. It is characterized by ascending motion over the Western Pacific and descending motion in the eastern equatorial Pacific. At the surface trade winds blow from east to west, causing upwelling of cold water along the equator. From the early 1990s to about 2013, this circulation has intensified dramatically, cooling the eastern equatorial Pacific and triggering shifts in global winds and rainfall (see Figure 1). These conditions further contributed to drying in California, exacerbating mega-drought conditions and impacting agriculture, water resources and wild fires. Given these widespread impacts on ecosystems and society, the recent Walker circulation trends have become subject of intense research.

In contrast to the observed strengthening, the majority of climate computer models simulates a gradual weakening of the Walker Circulation when forced by increasing greenhouse gas concentrations (see Figure 1). "The discrepancy between climate model projections and observed trends has led to speculations about the fidelity of the current generation of climate models and their representation of tropical climate processes", said Eui-Seok Chung, researcher from the Center for Climate Physics, Institute for Basic Science, South Korea, and lead-author of the study.

To determine whether the observed changes in the tropical atmospheric circulation are due to natural climate processes or caused by human-induced climate change, scientists from South Korea, the United States and Germany came together to conduct one of the most comprehensive big-data analyses of recent atmospheric trends to date. "Using satellite data, improved surface observations and a large ensemble of climate model simulations, our results demonstrate that natural variability, rather than anthropogenic effects, were responsible for the recent strengthening of the Walker circulation", said Prof. Axel Timmermann, Director of the IBS Center for Climate Physics at Pusan National University and co-author of this study.

In their integrated analysis, the researchers found that the satellite-inferred strengthening of the Walker circulation is substantially weaker than implied by other surface observations used in previous studies. "Putting surface observations in context with latest satellite products was a key element of our study", said co-author Dr. Lei Shi from NOAA's National Centers for Environmental Information in the United States.

Analyzing 61 different computer model simulations forced with increasing greenhouse gas concentrations, the authors showed that, although the average response is a Walker circulation weakening, there are substantial discrepancies amongst the individual model experiments, in particular when considering shorter-term trends. "We found that some models are even consistent with the observed changes in the tropical Pacific, in stark contrast to other computer experiments that exhibit more persistent weakening of the Walker circulation during the observational period", said co-author Dr. Viju John from EUMETSAT in Germany. The authors were then able to tease apart what caused the spread in the computer model simulations.

Co-author Prof. Kyung-Ja Ha from the IBS Center for Climate Physics and Pusan National University explains "Natural climate variability, associated for instance with the El Niño-Southern Oscillation or the Interdecadal Pacific Oscillation can account for a large part of diversity in simulated tropical climate trends".

"The observed trends are not that unusual. In climate model simulations we can always find shorter-term periods of several decades that show similar trends to those inferred from the satellite data. However, in most cases, and when considering the century-scale response to global warming, these trends reverse their sign eventually", said co-author Prof. Brian Soden from the Rosenstiel School of Marine and Atmospheric Science, at the University of Miami, United States.

The study concludes that the observed strengthening of the Walker circulation from about 1990-2013 and its impact on western Pacific sea level, eastern Pacific cooling, drought in the Southwestern United States, was a naturally occurring phenomenon, which does not stand in contrast to the notion of projected anthropogenic climate change. Given the high levels of natural decadal variability in the tropical Pacific, it would take at least two more decades to detect unequivocally the human imprint on the Pacific Walker Circulation (see Figure 1, right panel).

Institute for Basic Science

Related Global Warming Articles:

Global warming will accelerate water cycle over global land monsoon regions
A new study provides a broader understanding on the redistribution of freshwater resources across the globe induced by future changes in the monsoon system.
Comparison of global climatologies confirms warming of the global ocean
A report describes the main features of the recently published World Ocean Experiment-Argo Global Hydrographic Climatology.
Six feet under, a new approach to global warming
A Washington State University researcher has found that one-fourth of the carbon held by soil is bound to minerals as far as six feet below the surface.
Can we limit global warming to 1.5 °C?
Efforts to combat climate change tend to focus on supply-side changes, such as shifting to renewable or cleaner energy.
Global warming: Worrying lessons from the past
56 million years ago, the Earth experienced an exceptional episode of global warming.
More Global Warming News and Global Warming Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...