How to better predict North Atlantic’s atmospheric pressure patterns

European and North American winter weather is dominated by variations in the North Atlantic Oscillation (NAO), an irregular fluctuation of atmospheric pressure over the North Atlantic Ocean. The NAO can take place on a yearly basis, or the fluctuations can happen decades apart. These variations are referred to as oscillation because of the back and forth switching between two prevailing patterns: the positive and negative modes. In the case of the former, a strong subtropical high is located over the Azores islands in the central North Atlantic, with a strong low pressure system centred over Iceland. In the latter mode, weaker high and low pressure systems alternate over the same locations. NAO-related impacts on winter climate extend from Florida to Greenland, and from north-western Africa through Europe far into northern Asia.

The phase of the oscillation – whether the NAO is in the positive or negative mode – and the difference in pressure between high and low systems is measured by the NAO index. In recent decades, the phase of the NAO has been shifting from mostly negative to mostly positive index values, where it seems likely that human activities are playing an important role. To better assess long-term climate projections and forecast short-term anomalies, it’s crucial to make decadal predictions, particularly for the NAO and the blocking in the North Atlantic that refers to a class of weather systems in the middle to high latitudes.Partially supported by the EU-funded Blue-Action and EUCP projects, a team of researchers has achieved a breakthrough in such decadal predictions. The researchers “made use of a large ensemble of decadal predictions and found remarkable skill in reproducing the observed multi-annual variations of wintertime blocking frequency over the North Atlantic and of the North Atlantic Oscillation (NAO) itself,” as noted in a ‘EurekAlert!’ news release. Their study was published in the journal ‘npj Climate and Atmospheric Science’. “The occurrence of blocking in certain areas of the Euro-Atlantic domain determines the concurrent circulation regime and the phase of known teleconnections, such as the NAO, consequently affecting the stormtrack and the frequency and intensity of extreme weather events.” The researchers conclude: “Therefore, skilfully predicting the decadal fluctuations of blocking frequency and the NAO may be used in statistical predictions of near-term climate anomalies, and it provides a strong indication that impactful climate anomalies may also be predictable with improved dynamical models.”

The Blue-Action (Arctic Impact on Weather and Climate) project that supported the study was set up to demonstrate the role of the Arctic in increasing the predictive capability of the Northern Hemisphere’s weather and climate. The project factsheet states: “Modeling and prediction capabilities will be enhanced by targeting firstly, lower latitude oceanic and atmospheric drivers of regional Arctic changes and secondly, Arctic impacts on Northern Hemisphere climate and weather extremes.”

The EUCP (European Climate Prediction system) project that also contributed to the study focuses on developing “an innovative ensemble climate prediction system based on high-resolution climate models for Europe for the near-term (~1-40years),” as noted in the project factsheet. This system will be utilised “to produce consistent, authoritative and actionable climate information … to support climate-related risk assessments and climate change adaptation programmes,” the factsheet adds. The system will also look at high-impact extreme weather events of the near past and near future.