Universität Aarhus: Walker Zirkulation im äquatorialen Pazifik variiert im Takt der Sonnenaktivität

Die deutsche Presse ist sich ganz sicher, dass die Sonne keinerlei Klimarelevanz besitzt. Wer das Gegenteil behauptet, wird prompt als verrückt dargestellt. Davon unbeeindruckt findet die internationale Wissenschaft immer weitere Hinweise darauf, dass die Sonne eine signifikante Rolle für das Erdklima spielt. Suchen Sie einfach mal nach dem Begriff „solar forcing“ auf Google Scholar. Hunderte von Arbeiten beschreiben das Phänomen, das es laut deutschen Meinungsführern nicht geben darf, weil politisch nicht genehm. Was treibt Mojib Latif und Sami Solanki dazu, die Zusammenhänge öffentlich zu leugnen?

Hier ein Beispiel. Eine aktuelle Arbeit zum Klimaeinfluss der Sonne wurde am 29. März 2019 auf phys.org besprochen (Pressemitteilung der der Universität Aarhus):

Solar variability weakens the Walker cell

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 eastward.

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 that did not survive proper statistical scrutiny in most cases. But besides statistical verification lies an even more challenging problem: How could miniscule changes in incoming solar radiation 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 the solar cycle barely reaches 0.1 K in a solar maximum—almost eight 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 showed much stronger anomalies in the Pacific. 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 accuracy of decadal predictions in that region.

Eins ist klar. Stergios Misio wird in Deutschland niemals eine Stelle bekommen. Er forscht an einem bei uns unerwünschten Thema. Hier der Abstract des Papers vom Misios et al. 2019, das in PNAS als „Direct Submision“ erschien:

Slowdown of the Walker circulation at solar cycle maximum
The Pacific Walker Circulation (PWC) fluctuates on interannual and multidecadal timescales under the influence of internal variability and external forcings. Here, we provide observational evidence that the 11-y solar cycle (SC) affects the PWC on decadal timescales. We observe a robust reduction of east–west sea-level pressure gradients over the Indo-Pacific Ocean during solar maxima and the following 1–2 y. This reduction is associated with westerly wind anomalies at the surface and throughout the equatorial troposphere in the western/central Pacific paired with an eastward shift of convective precipitation that brings more rainfall to the central Pacific. We show that this is initiated by a thermodynamical response of the global hydrological cycle to surface warming, further amplified by atmosphere–ocean coupling, leading to larger positive ocean temperature anomalies in the equatorial Pacific than expected from simple radiative forcing considerations. The observed solar modulation of the PWC is supported by a set of coupled ocean–atmosphere climate model simulations forced only by SC irradiance variations. We highlight the importance of a muted hydrology mechanism that acts to weaken the PWC. Demonstration of this mechanism acting on the 11-y SC timescale adds confidence in model predictions that the same mechanism also weakens the PWC under increasing greenhouse gas forcing.

Direct Submissions bei PNAS werden von unabhängigen Gutachtern geprüft. Allerdings gibt es auch „Contributed Papers“ bei PNAS, wobei man sich die Gutachter selber aussuchen kann. Das macht zum Beispiel Hans-Joachim Schellnhuber vom PIK sehr gerne. Darüber hat die Tagesschau noch nie berichtet. Seltsam, oder?

Aber zurück zur Walker Zirkulation. Dieses Windsystem im äquatorialen Pazifik hat sich kürzlich intensiviert. Sofort waren Alarmisten zu Stelle, die behaupteten, dass dies eine Folge des menschengemachten Klimawandels sei. Alles Quatsch, wie jetzt das Institute for Basic Science in einer Pressemitteilung am 1. April 2019 mitteilte. Natürliche Prozesse haben zur Intensivierung der Zirkulation geführt. Meldung auf EurekAlert!:

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 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).

Paper: Eui-Seok Chung, Axel Timmermann, Brian J. Soden, Kyung-Ja Ha, Lei Shi, Viju O. John. Reconciling opposing Walker circulation trends in observations and model projections. Nature Climate Change, 2019; DOI: 10.1038/s41558-019-0446-4