Simulation of observed changes in the North Atlantic Oscillation and surface climate in the latter half of the 20th Century.

The North Atlantic Oscillation (NAO) is the dominant pattern of winter variability in the climate of the Atlantic region. The NAO also shows a large positive trend between the 1960s and 1990s which is manifested as a rapid increase in winter surface temperature and rainfall in Northern Europe. Despite this, the cause of the trend remains a source of much debate and it is not reproduced in simulations of the 20th century with global climate models. Constraining lower boundary conditions using historical sea-surface temperature and sea-ice distributions allows models to reproduce a weak trend in the NAO and land surface temperature. However, we find that the full observed trend can be reproduced when conditions in the lower stratosphere are relaxed towards observations. This result indicates that upper level circulation changes and the strength of the downward coupling to the surface are strong enough to explain a large fraction of the recent change in North European and North American surface climate. It also suggests that upper level variability should be reproduced in models to fully simulate surface climate variations, and that the response of the Atlantic region to anthropogenic forcing is strongly influenced by natural variability on decadal timescales.