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PUBLIC RELEASE DATE:
1-Apr-2014

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Contact: Michael Bishop
michael.bishop@iop.org
01-179-301-032
Institute of Physics
@PhysicsNews

Warm North Atlantic Ocean promotes extreme winters in US and Europe

The extreme cold weather observed across Europe and the east coast of the US in recent winters could be partly down to natural, long-term variations in sea surface temperatures, according to a new study published today.

Researchers from the University of California Irvine have shown that a phenomenon known as the Atlantic Multidecadal Oscillation (AMO)--a natural pattern of variation in North Atlantic sea surface temperatures that switches between a positive and negative phase every 60-70 years--can affect an atmospheric circulation pattern, known as the North Atlantic Oscillation (NAO), that influences the temperature and precipitation over the Northern Hemisphere in winter.

When the AMO is in its positive phase and the sea surface temperatures are warmer, the study has shown that the main effect in winter is to promote the negative phase of the NAO which leads to "blocking" episodes over the North Atlantic sector, allowing cold weather systems to exist over the eastern US and Europe.

The results have been published today, Wednesday 2 April, in IOP Publishing's journal Environmental Research Letters.

To arrive at their results, the researchers combined observations from the past century with climate simulations of the atmospheric response to the AMO.

According to their observations, sea surface temperatures in the Atlantic can be up to 1.5 °C warmer in the Gulf Stream region during the positive phase of the AMO compared to the negative, colder phase. The climate simulations suggest that these specific anomalies in sea surface temperatures can play a predominant role in promoting the change in the NAO.

Lead authors of the study Yannick Peings and Gudrun Magnusdottir said: "Our results indicate that the main effect of the positive AMO in winter is to promote the occurrence of the negative phase of the NAO. A negative NAO in winter usually goes hand-in-hand with cold weather in the eastern US and north-western Europe."

The observations also suggest that it takes around 10-15 years before the positive phase of AMO has any significant effect on the NAO. The reason for this lag is unknown; however, an explanation might be that AMO phases take time to develop fully.

As the AMO has been in a positive phase since the early 1990s, it may have contributed to the extreme winters that both the US and Europe have experienced in recent years.

The researchers warn, however, that the future evolution of the AMO remains uncertain, with many factors potentially affecting how it interacts with atmospheric circulation patterns, such as Arctic sea ice loss, changes in solar radiation, volcanic eruptions and concentrations of greenhouse gases in the atmosphere.

The AMO also shows strong variability from one year to the next in addition to the changes seen every 60󈞲 years, which makes it difficult to attribute specific extreme winters to the AMO's effects.

Responding to the extreme weather that gripped the eastern coast of the US this winter, Yannick Peings continued: "Unlike the 2012/2013 winter, this winter had rather low values of the AMO index and the pattern of sea surface temperature anomalies was not consistent with the typical positive AMO pattern. Moreover, the NAO was mostly positive with a relatively mild winter over Europe".

"Therefore it is unlikely that the positive AMO played a defining role on the east coast of the US, although further work is necessary to answer this question. Such an event is consistent with the large internal variability of the atmosphere, and other external forcings may have played a role.

"Our future studies will look to compare the role of the AMO compared to Arctic sea ice anomalies, which have also been shown to affect atmospheric circulation patterns and promote colder, more extreme winters."

From Wednesday 2 April, this paper can be downloaded from http://iopscience.iop.org/1748-9326/9/3/034016/article

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Notes to Editors

Contact

1. For further information, a full draft of the journal paper or contact with one of the researchers, contact IOP Press Officer, Michael Bishop: Tel: 0117 930 1032 E-mail: michael.bishop@iop.org For more information on how to use the embargoed material above, please refer to our embargo policy.

IOP Publishing Journalist Area

2. The IOP Publishing Journalist Area gives journalists access to embargoed press releases, advanced copies of papers, supplementary images and videos. In addition to this, a weekly news digest is uploaded into the Journalist Area every Friday, highlighting a selection of newsworthy papers set to be published in the following week. Login details also give free access to IOPscience, IOP Publishing's journal platform. To apply for a free subscription to this service, please email Michael Bishop, IOP Press Officer, michael.bishop@iop.org, with your name, organisation, address and a preferred username.

Forcing of wintertime atmospheric circulation by the multidecadal fluctuations of the North Atlantic ocean

3. The published version of the paper 'Forcing of wintertime atmospheric circulation by the multidecadal fluctuations of the North Atlantic ocean' (Yannick Peings and Gudrun Magnusdottir 2014 Environ. Res. Lett. 9 034018) will be freely available online from 2 April. It will be available at http://iopscience.iop.org/1748-9326/9/3/034018/article.

Environmental Research Letters

4. Environmental Research Letters is an open access journal that covers all of environmental science, providing a coherent and integrated approach including research articles, perspectives and editorials.

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5. IOP Publishing provides a range of journals, magazines, websites and services that enable researchers and research organisations to reach the widest possible audience for their research. We combine the culture of a learned society with global reach and highly efficient and effective publishing systems and processes. With offices in the UK, US, Germany, China and Japan, and staff in many other locations including Mexico and Russia, we serve researchers in the physical and related sciences in all parts of the world. IOP Publishing is a wholly owned subsidiary of the Institute of Physics. The Institute is a leading scientific society promoting physics and bringing physicists together for the benefit of all. Any profits generated by IOP Publishing are used by the Institute to support science and scientists in both the developed and developing world. Go to ioppublishing.org.

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