Natural climate processes overshadow recent human-induced Walker circulation trends

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 Nino-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).

related report
Winds of change...Solar variability weakens the Walker cell
Aarhus C, Denmark (SPX) Apr 03 - An international team of researchers from United Kingdom, Denmark, and Germany has found robust evidence for signatures of the 11-year sunspot cycle in the tropical Pacific. They analyzed historical time series of pressure, surface winds, and precipitation with specific focus on the Walker Circulation - a vast system of atmospheric flow in the tropical Pacific region that affects patterns of tropical rainfall. They have revealed that during periods of increased solar irradiance, the trade winds weaken and the Walker circulation shifts eastwards.

Stergios Misios, a postdoctoral researcher at the University of Oxford, said "We deal with a very short record of observations in the tropical Pacific and we must be very careful with how we filter out other interannual fluctuations. After a careful treatment of the data covering the last 60 years, we detected a robust slowdown of the Walker cell during years associated with solar-cycle maxima".

The analysis shows that, in tandem with changes in the wind anomalies, the dominant patterns of tropical precipitation shift to the central Pacific during solar-cycle maxima. As a result, rainfall decreases over Indonesia and in the western Pacific, and increases over the central Pacific Ocean.

Simple mechanisms amplify the solar signal
The issue of solar influences on climate is long and controversial as there have been numerous claims which in most of the cases did not survive proper statistical scrutiny. But besides statistical verification lies an even more challenging problem: how miniscule changes in incoming solar radiation could produce significant climate signatures?

"Soon enough, we realized that the magnitude of the wind anomalies that we detected in observations simply could not be explained by radiative considerations alone. We thought that if it comes from the Sun, there must be another mechanism that amplifies the weakening of the Walker circulation." said Prof. Lesley Gray of University of Oxford. With the aid of a global climate model, this mechanism was found in the dynamical coupling between the atmosphere and ocean circulation in the tropical Pacific.

Averaged over the globe the surface temperature imprint of solar cycle barely reaches 0.1 K in a solar maximum - almost 8 times weaker than the global warming trends observed in the 20th century. Yet, even such a weak surface warming influences the Walker circulation through changes in global hydrology.

As the surface warms, water vapor in the atmosphere increases at a higher rate than is lost by precipitation, necessitating a weakening of the Walker cell. This is a well-tested mechanism in model simulations of increased CO2 concentrations but it turns out that is operating under the 11-year solar cycle, too.

S. Misios, said "Our model showed westerly wind anomalies in the Pacific region even when we considered only changes in global hydrology, but the magnitude was far too weak. We hypothesized that atmosphere-ocean coupling, essentially the Bjerknes feedback, can amplify the solar signal".

Using a climate model forced by the 11-year solar cycle alone, researchers found the evidence to support their hypothesis. Their model showed much stronger wind anomalies in the Pacific of magnitude as observed.

They proposed that changes in global hydrology and the Bjerknes feedback mediate solar cycle influences on the Tropical Pacific. The researchers now hope that if the interplay between those mechanisms is properly represented by other climate models, it could give potential to improve the skill of decadal predictions in that region.

Research paper and Research paper

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