Decadal predictability of North Atlantic blocking and the NAO


Panos J. Athanasiadis, Stephen Yeager, Young-Oh Kwon, Alessio Bellucci, David W. Smith, Stefano Tibaldi

npj Climate and Atmospheric Science
Work Package 1



North Atlantic atmospheric circulation can impact weather patterns over Europe, leading to locked-in dry spells or successive storms, amongst other phenomena. We can accurately predict trends in these patterns up to a few months ahead, but multi-year prediction poses additional problems. This study uses a large, 40-member climate model ensemble to successfully predict North Atlantic blocking and variability in the North Atlantic Oscillation up to 10 years ahead. These predictable patterns are connected to low-frequency oceanic variability, also correctly simulated by the model ensemble. The ability to forecast trends in North Atlantic atmospheric circulation years in advance will allow policymakers, as well as industries vulnerable to extreme weather, to put measures in place to adapt to and mitigate the impact of extreme events in the future.

Large-scale atmospheric circulation in the North Atlantic takes many forms including the North Atlantic Oscillation (NAO), a standing pattern of air pressure differences between the Azores and Iceland, and atmospheric blocking, in which meanders of the jet stream allow high-pressure systems to stay in one area, blocking the normal movement of weather systems. Phenomena like these, and the weather they influence, have major impacts on people and society yet it is only in recent years that we have been able to predict them with confidence. We can now predict these forms of climate variability in the mid-latitudes on a seasonal scale, that is a few months ahead, with significant skill, but predicting these patterns on a multi-annual or decadal timescale has so far proven difficult. This paper attempts to address this issue and improve the predictability of climate variability in the mid-latitudes out to a decade ahead.

The team found that their model ensemble was able to simulate the NAO and wintertime blocking frequency in the North Atlantic on a multi-annual timescale with remarkable skill. This is partly due to the large number of simulations used in their ensemble; once these various simulations are averaged out, any signals that only appear in a few are lost, while signals that remain occur in many of the simulations. This suggests they originate in some common, predictable component of the variation. The predictable atmospheric anomalies appear to be a response to low-frequency oceanic variability very similar to the Atlantic Multi-decadal Variability, a regular pattern of oscillating sea-surface temperatures that the models are also able to predict.

This study made use of the Community Earth System Model – Decadal Prediction Large Ensemble (CESM-DPLE), run by the National Center for Atmospheric Research (NCAR) in the USA. This is an ensemble of climate model simulations comprising 40 members; large ensembles such as this are required for multi-annual prediction of atmospheric circulation. This ensemble simulated atmospheric circulation in the North Atlantic using simulations of past climate, begun every November between 1954 and 2015 and run for 10 years. Variations between the simulations were created by slightly altering the initial conditions within the bounds of measurement uncertainty. The results of these simulations were then compared with observational records from NCAR Reanalysis and the Met Office HadISST dataset to see how well the simulations had performed.

Policy relevance
The large-scale North Atlantic atmospheric circulation this study addresses has a great influence on the weather, particularly in Europe. Blocking can cause prolonged spells of hot or dry weather, adversely affecting agriculture, while changes in the NAO affect the track of potentially damaging storms. Multi-year prediction of trends in these events can allow policymakers to take measures to protect communities or industries vulnerable to these types of extreme weather, allowing us to adapt to it and mitigate its impact.


Can multi-annual variations in the frequency of North Atlantic atmospheric blocking and mid-latitude circulation regimes be skilfully predicted? Recent advances in seasonal forecasting have shown that mid-latitude climate variability does exhibit significant predictability. However, atmospheric predictability has generally been found to be quite limited on multi-annual timescales. New decadal prediction experiments from NCAR are found to exhibit 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. This is partly due to the large ensemble size that allows the predictable component of the atmospheric variability to emerge from the background chaotic component. The predictable atmospheric anomalies represent a forced response to oceanic low-frequency variability that strongly resembles the Atlantic Multi-decadal Variability (AMV), correctly reproduced in the decadal hindcasts thanks to realistic ocean initialization and ocean dynamics. 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. 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.