July 8, 2015 | As a key driver of winter weather patterns across Europe, the North Atlantic Oscillation (NAO) has long sparked the interest of atmospheric researchers. Now a new study sheds light on the history of the NAO over the past 1,000 years, potentially moving scientists closer to the goal of projecting weather patterns for Europe months to years in advance.

The NAO is a seesaw in atmospheric pressure that helps direct the flow of winter storms across the Atlantic. When the contrast between high pressure over the Azores and low pressure in the far north Atlantic is stronger than normal, the NAO is said to be in a positive phase. This usually drives mild Atlantic storms into northern Europe but keeps the Mediterranean region comparatively dry.

But when the contrast is lower than normal, the NAO becomes negative and opens the door to cold, dry Arctic intrusions into northern Europe and wet, slow-moving systems across the south.

If researchers can determine what causes the NAO to change between positive and negative phases, they can potentially lay the groundwork for predicting the NAO and associated winter weather patterns.

When the NAO is in positive phase (left), the contrast between high pressure over the Azores and low pressure in the far north Atlantic is stronger than normal, which leads to mild storms over northern Europe and drying over the Mediterranean. But when the contrast is lower than normal, the NAO enters negative phase (right), leading to cold, dry weather in northern Europe and wet conditions across the south. (Image by Pablo Ortega. This image is freely available for media & nonprofit use.)

As a step in that direction, a team of scientists has reconstructed annual changes in the NAO over the past millennium. The team was led by Pablo Ortega at France’s LSCE/IPSL (Climate and Environment Sciences Laboratory/Institute Pierre Simon Laplace), with co-author Flavio Lehner, now at NCAR. Their findings appeared last week in Nature.

To create the reconstruction, the scientists first analyzed 48 sources of proxy climate information distributed around the Atlantic Ocean, including tree rings, speleothems (such as stalagmites or stalactites), ice cores, and lake sediments. This gave them a rough approximation of past NAO changes. They then turned to six climate models to evaluate the relationship between conditions at all the proxy locations and phases of the NAO. Using the information from the models, the authors created a reconstruction of annual NAO changes by selecting only the proxies most consistent with the model simulations.

Among the findings:

• The NAO was in a largely positive phase during a period of unusually warm temperatures in northern Europe from about 1150 to 1400. Contrary to an earlier analysis, however, the NAO did not become persistently positive beginning about 1000.
• In the following four centuries until the early 1800s, the NAO became predominantly negative.
• The NAO appears to enter a positive phase about two years after major volcanic eruptions, based on its response to the 11 strongest volcanic eruptions during the past 1,000 years. This may help explain why the NAO was generally positive during medieval times, when there was considerable volcanic activity. It also provides additional evidence that the NAO can be predicted months to years in advance.

Using models to validate and select the proxy records generated a more robust picture of the NAO than relying on the proxies alone, the authors concluded. Lehner said the technique could be used for other research looking into past climate patterns.

“The key point here is that bringing in climate models enabled us to do a more thorough job of reconstructing the NAO,” he said. “Traditionally, scientists have relied on the short observational record to validate and determine the usefulness of a given proxy for climate reconstruction. We show that models can provide additional information, and we hope our approach will become a new standard for reconstructing past climate.”

About the article

Pablo Ortega, Flavio Lehner, Didier Swingedouw, Valerie Masson-Delmotte, Christoph C. Raible, Mathieu Casado, and Pascal Yiou, A model-tested North Atlantic Oscillation reconstruction for the past millennium, Nature, doi:10.1038/nature14518

Writer/contact
David Hosansky

Collaborating organizations
LSCE/IPSL (Climate and Environment Sciences Laboratory/Institute Pierre Simon Laplace), France
University of Bern, Switzerland
University of Bordeaux, France
University of Reading, United Kingdom

Funders
French National Research Agency
Swiss National Science Foundation
Swedish Research Council

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