Understanding Climate Change in the North Atlantic Region

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Many of the impacts of anthropogenic greenhouse gas emissions on the global average climate are now well-known. However much uncertainty remains over aspects of the regional impact, especially in a region as complex as the North Atlantic. The western edge of the Atlantic is one of the world's premier growth regions for mid-latitude storms. These are then steered along the jet stream, forming the North Atlantic storm track and bringing precipitation and milder temperatures to Europe. Variability associated with the strength and location of the Atlantic jet has caused dramatic variations in European winter climate over recent decades, from the bitterly cold winters of the 1960s to much milder and wetter conditions in the 1990s. This variability is described by the North Atlantic Oscillation, or NAO, which is essentially a measure of the average strength of the westerly winds across the North Atlantic. If the westerly winds are stronger than usual, Europe is affected more by the mild maritime influence of the Atlantic, and the NAO is said to be in a positive phase, whereas if they are weaker than normal the influence is more from the colder Eurasian continent (the negative phase). The NAO is widely recognised as the dominant pattern of natural climate variability affecting Europe. Much remains to be understood of the mechanisms behind changes in the NAO. There is no consensus on whether the changes over the last few decades are a result of anthropogenic climate change or simply due to natural variability. Modern climate models also disagree over the response of the NAO to climate change. Several models predict more positive conditions in the future, but other models do not. Climate models are only just beginning to converge on the response of the storm tracks to climate change. In the Pacific most current models now predict that the storm track will move polewards, but in the Atlantic there is less agreement. Here the situation is more complicated because different models predict different changes in the NAO, and also in the Atlantic Meridional Overturning Circulation in the ocean, which transports heat towards the far north. These disagreements between models are a key source of uncertainty in the regional effects of climate change over Europe. This work aims to improve our understanding of the reasons behind the disagreements, so that this uncertainty can be reduced. Recent work at the University of Reading has suggested that the long-term variations in the NAO are largely due to variations in the occurrence of a particular kind of weather system in the Atlantic. These systems result from the breaking of large scale waves in the upper atmosphere, and are very similar to the blocking high pressure systems often seen over Europe. Periods during which these events occur frequently will exhibit weak westerly winds on average, and so will be classed as negative NAO periods. In this project, data from state of the art climate models contributing to the latest IPCC report will be studied, using this new perspective in an attempt to understand the reasons behind the NAO responses they predict. If we can understand how changes in the mean climate of a model will affect the likelihood, or preferred location, of wave-breaking, we can then understand its NAO response. It is likely that the disagreement over North Atlantic storm track changes in climate models arises at least partly because there are several competing mechanisms at work. For example, the pattern of atmospheric warming would act to push the storm track polewards, as seen in other ocean basins, but sea surface temperature changes associated with a weakening of the Meridional Overturning Circulation are likely to lead to an equatorwards shift. Here we plan to use a new technique recently developed at the Met Office to separate out these different effects, and quantify the relative importance of each one.