Erwärmung im Indischen Ozean intensiviert Golfstrom im Atlantik

Der Golfstrom bringt warmes Wasser aus Amerika nach Europa. Einige Klimaforscher unken, dass der Golfstrom durch den Klimawandel zum Erliegen kommen könnte, andere sehen hierfür bislang keinen Grund. Wir haben bereits mehrfach darüber an dieser Stelle berichtet. Was gibt es Neues in der Literatur zum Golfstrom, der in der Fachsprache auch Atlantic Meridional Overturning Circulation (AMOC) genannt wird.

Am 16.9.2019 berichtete die Yale University, dass der Golfstrom wohl auch vom Indischen Ozean mitgesteuert wird. Wenn sich der Indik erwärmt, intensiviert sich über Niederschlagsverschiebungen auch der Golfstrom:

Atlantic Ocean may get a jump start from the other side of the world

A key question for climate scientists in recent years has been whether the Atlantic Ocean’s main circulation system is slowing down, a development that could have dramatic consequences for Europe and other parts of the Atlantic rim. But a new study suggests help may be on the way from an unexpected source — the Indian Ocean.

Think of it as ocean-to-ocean altruism in the age of climate change. The new study, from Shineng Hu of the Scripps Institution of Oceanography at the University of California-San Diego and Alexey Fedorov of Yale University, appears Sept. 16 in the journal Nature Climate Change. It is the latest in a growing body of research that explores how global warming may alter global climate components such as the Atlantic meridional overturning circulation (AMOC).

AMOC is one of the planet’s largest water circulation systems. It operates like a liquid escalator, delivering warm water to the North Atlantic via an upper limb and sending colder water south via a deeper limb. Although AMOC has been stable for thousands of years, data from the past 15 years, as well as computer model projections, have given some scientists cause for concern. AMOC has showed signs of slowing during that period, but whether it is a result of global warming or only a short-term anomaly related to natural ocean variability is not known.

“There is no consensus yet,” Fedorov said, “but I think the issue of AMOC stability should not be ignored. The mere possibility that the AMOC could collapse should be a strong reason for concern in an era when human activity is forcing significant changes to the Earth’s systems.

We know that the last time AMOC weakened substantially was 15,000 to 17,000 years ago, and it had global impacts,” Fedorov added. “We would be talking about harsh winters in Europe, with more storms or a drier Sahel in Africa due to the downward shift of the tropical rain belt, for example.”

Much of Fedorov and Hu’s work focuses on specific climate mechanisms and features that may be shifting due to global warming. Using a combination of observational data and sophisticated computer modeling, they plot out what effect such shifts might have over time. For example, Fedorov has looked previously at the role melting Arctic sea ice might have on AMOC.

For the new study, they looked at warming in the Indian Ocean. “The Indian Ocean is one of the fingerprints of global warming,” said Hu, who is first author of the new work. “Warming of the Indian Ocean is considered one of the most robust aspects of global warming.” The researchers said their modeling indicates a series of cascading effects that stretch from the Indian Ocean all way over to the Atlantic: As the Indian Ocean warms faster and faster, it generates additional precipitation. This, in turn, draws more air from other parts of the world to the Indian Ocean, including the Atlantic.

With so much precipitation in the Indian Ocean, there will be less precipitation in the Atlantic Ocean, the researchers said. Less precipitation will lead to higher salinity in the waters of the tropical portion of the Atlantic — because there won’t be as much rainwater to dilute it. This saltier water in the Atlantic, as it comes north via AMOC, will get cold much quicker than usual and sink faster. “This would act as a jump start for AMOC, intensifying the circulation,” Fedorov said. “On the other hand, we don’t know how long this enhanced Indian Ocean warming will continue. If other tropical oceans’ warming, especially the Pacific, catches up with the Indian Ocean, the advantage for AMOC will stop.”

The researchers said this latest finding exemplifies the intricate, interconnected nature of global climate. As scientists try to understand the unfolding effects of climate change, they must attempt to identify all of the climate variables and mechanisms that are likely to play a role. “There are undoubtedly many other connections that we don’t know about yet,” Fedorov said. “Which mechanisms are most dominant? We’re interested in that interplay.”

The National Science Foundation, the ARCHANGE project of the Make Our Planet Great Again initiative in France, the Guggenheim fellowship, and the Scripps Institution postdoctoral fellowship helped to support the research.

Das dazugehörige Paper: Shineng Hu, Alexey V. Fedorov. Indian Ocean warming can strengthen the Atlantic meridional overturning circulation. Nature Climate Change, 2019; DOI: 10.1038/s41558-019-0566-x

Eine Studie von Joyce et al. 2019 zeigt, dass sich der Punkt, an dem der Golfstrom die nordamerikanische Küste verlässt und nach Europa abbiegt, verändert. Jen nach Abbiegepunkt führt dies zu mehr oder weniger blockierten Wetterlagen. Abstract:

Meridional Gulf Stream Shifts Can Influence Wintertime Variability in the North Atlantic Storm Track and Greenland Blocking

After leaving the U.S. East Coast, the northward flowing Gulf Stream (GS) becomes a zonal jet and carries along its frontal characteristics of strong flow and sea surface temperature gradients into the North Atlantic at midlatitudes. The separation location where it leaves the coast is also an anchor point for the wintertime synoptic storm track across North America to continue to develop and head across the ocean. We examine the meridional variability of the separated GS path on interannual to decadal time scales as an agent for similar changes in the storm track and blocking variability at midtroposphere from 1979 to 2012. We find that periods of northerly (southerly) GS path are associated with increased (suppressed) excursions of the synoptic storm track to the northeast over the Labrador Sea and reduced (enhanced) Greenland blocking. In both instances, GS shifts lead those in the midtroposphere by a few months.

Weijer et al. 2019 haben eine Review-Arbeit zur Stabilität des Golfstroms veröffentlicht. Allerdings kommen die Autoren wenig überraschend zum Schluss, dass sie weitere Forschung benötigen, um hier weiterzukommen:

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

The notion that the Atlantic Meridional Overturning Circulation (AMOC) can have more than one stable equilibrium emerged in the 1980s as a powerful hypothesis to explain rapid climate variability during the Pleistocene. Ever since, the idea that a temporary perturbation of the AMOC—or a permanent change in its forcing—could trigger an irreversible collapse has remained a reason for concern. Here we review literature on the equilibrium stability of the AMOC and present a synthesis that puts our understanding of past and future AMOC behavior in a unifying framework. This framework is based on concepts from Dynamical Systems Theory, which has proven to be an important tool in interpreting a wide range of model behavior. We conclude that it cannot be ruled out that the AMOC in our current climate is in, or close to, a regime of multiple equilibria. But there is considerable uncertainty in the location of stability thresholds with respect to our current climate state, so we have no credible indications of where our present‐day AMOC is located with respect to thresholds. We conclude by identifying gaps in our knowledge and proposing possible ways forward to address these gaps.

Schließlich wollen wir Ihnen noch eine historische Perspektive zur Entdeckung des Golfstroms durch Walfänger empfehlen, die von Richardson & Adams 2018 stammt.