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The gradual warming of the North and tropical Atlantic Ocean is contributing to climate change in Antarctica, a team of New York University scientists has concluded. The findings, which rely on more than three decades of atmospheric data and appear in the journal Nature, show new ways in which distant regional conditions are contributing to Antarctic climate change.
"Our findings reveal a previously unknown—and surprising—force behind climate change that is occurring deep in our southern hemisphere: the Atlantic Ocean," says Xichen Li, a doctoral student in NYU's Courant Institute of Mathematical Sciences and the study's lead author. "Moreover, the study offers further confirmation that warming in one region can have far-reaching effects in another."
Over the past few decades, Antarctica has experienced dramatic climate change, with its peninsula exhibiting the strongest warming of any region on the planet. During its summer, Antarctic changes have been attributed to greenhouse gas increase and stratospheric ozone loss. However, less clear are the forces behind climate changes that occur during its winter. In addition, the effects of these changes during the cold season are complex, further stifling efforts to find the atmospheric culprit.
It has long been known that the region's climate is affected, in part, by changes in the distant Pacific Ocean climate. But the phenomena brought on by the Pacific have shorter-term influences—for instance, due to El Niño. Less understood are the longer-term forces that have produced warming along the Antarctic Peninsula or the sea-ice redistribution in the southern hemisphere's winter over many decades.
To address this question, the NYU researchers focused on a different candidate: the Atlantic Ocean, which has been overlooked as a force behind Antarctic climate change.
Specifically, the scientists studied the North and Tropical Atlantic's Sea Surface Temperature (SST) variability—changes in the ocean's surface temperature—focusing on the last three decades. This metric, the Atlantic Multidecadal Oscillation (AMO), had previously not been considered in seeking explanations for Antarctic climate change.
Using a time-series analysis, in which the scientists matched changes in the North and Tropical Atlantic's SST with subsequent changes in Antarctic climate, the researchers found strong correlations. Specifically, they observed that warming Atlantic waters were followed by changes in sea-level pressure in the Antarctic's Amundsen Sea. In addition, these warming patterns also preceded redistribution of sea ice between the Antarctic's Ross and Amundsen-Bellingshausen-Weddell Seas.
David Holland, co-author of the study, a professor at NYU's Courant Institute and past director of NYU's Center for Atmospheric Ocean Science, explained that the research consisted of two parts, which incorporated both the use of observational data and computer modeling.
The first part of the study, using the observational data, found a link, or correlation, between the Atlantic and Antarctic data sets. But a correlation means simply that two things appear to happen in conjunction and does not explain what may be causing a phenomenon.
The second used a global atmospheric model, which allowed the researchers to create a simulated warming of the North Atlantic. The model responded, as the researchers had suspected, by "changing" the climate in Antarctica.
"While our data analysis showed a correlation, it was the use of a state-of-the-art computer model that allowed us to see that North Atlantic warming was causing Antarctic climate change and not vice versa," he said.
The study's findings raise a number of deeper questions, such as, is Antarctic sea-ice change fundamentally different from the well-reported changes in the Arctic? In contrast to the sea-ice decline over the Arctic, Antarctic sea ice has not diminished. Rather, it has redistributed itself in ways that have perplexed scientists, with declines in some areas and increases in others.
Holland observes: "From this study, we are learning just how Antarctic sea-ice redistributes itself, and also finding that the underlying mechanisms controlling Antarctic sea ice are completely distinct from those in the Arctic."
Text of Nature press release:
The Atlantic Ocean has an important impact on Antarctic climate, according to a study in this week’s Nature. Changes in sea surface temperature in the Atlantic seem to contribute to the accelerated warming on the Antarctic Peninsula and to redistributions in Antarctic sea ice. Thus, projections of future climate change in Antarctica should consider the influence of the Atlantic.
The Antarctic Peninsula has seen some of the strongest warming of the past few decades. Additionally, in contrast to the overall sea ice decline over the Arctic, sea ice around Antarctica has increased in some areas and decreased in others. The causes of these phenomena had been unclear, but Xichen Li and colleagues suggest that the Atlantic Multidecadal Oscillation — a persistent driver of climate variability in the tropical and north Atlantic — may have a role. They show that sea level pressure changes in the Amundsen Sea, which affect temperature on the Peninsula and sea ice distributions, can be traced to sea surface temperature variations caused by the Atlantic Multidecadal Oscillation.
The authors propose that the tropical and north Atlantic have the potential to affect ocean circulation and sea-level change.
In recent decades, Antarctica has experienced pronounced climate changes. The Antarctic Peninsula exhibited the strongest warming of any region on the planet, causing rapid changes in land ice. Additionally, in contrast to the sea-ice decline over the Arctic, Antarctic sea ice has not declined, but has instead undergone a perplexing redistribution5, 6. Antarctic climate is influenced by, among other factors, changes in radiative forcing7 and remote Pacific climate variability but none explains the observed Antarctic Peninsula warming or the sea-ice redistribution in austral winter. However, in the north and tropical Atlantic Ocean, the Atlantic Multidecadal Oscillation (a leading mode of sea surface temperature variability) has been overlooked in this context. Here we show that sea surface warming related to the Atlantic Multidecadal Oscillation reduces the surface pressure in the Amundsen Sea and contributes to the observed dipole-like sea-ice redistribution between the Ross and Amundsen–Bellingshausen–Weddell seas and to the Antarctic Peninsula warming. Support for these findings comes from analysis of observational and reanalysis data, and independently from both comprehensive and idealized atmospheric model simulations. We suggest that the north and tropical Atlantic is important for projections of future climate change in Antarctica, and has the potential to affect the global thermohaline circulation and sea-level change.
Impacts of the north and tropical Atlantic Ocean on the Antarctic Peninsula and sea ice by Xichen Li, David M. Holland, Edwin P. Gerber & Changhyun Yoo published in Nature 505, 538–542 (23 January 2014) doi:10.1038/nature12945
Read the abstract and get the paper here.
News release from New York University issued via the EurekAlert! service of the AAAS here.
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