| Summary: | The aim of this thesis is to provide a more comprehensive understanding of the role played by the North Atlantic Ocean in influencing North Atlantic and European atmospheric circulation and surface temperatures using climate models and observations. In this thesis the pattern of occurrences of re-emergence of sea surface temperature anomalies and positive and negative North Atlantic Oscillation (NAO) events over the last 140 years (1871-2011) from historical observations are examined to understand the historical relationship between the ocean and atmosphere. The findings are compared with CMIP5 historical ensemble model output (1850-2005). The aim is to understand how these models which have been used to simulate changes in the Earth’s climate through the twentieth and early twenty-first centuries perform in terms of their ability to simulate the observed links between re-emergence and the state of the NAO. Finally, in an ocean-atmosphere model simulation, the role played by North Atlantic sea surface temperatures (SSTs) on shorter (monthly) timescales is investigated in two recent European cold weather events that both coincided with similarly low NAO values: the winter of 2009/2010 and the early winter of 2010/2011. The evidence from the simulation study of the recent cold winters indicates that the NAO was influenced by the pattern of ocean surface temperatures occurring in October to December 2010 and re-emergence of SST anomalies in the North Atlantic contributed towards the development of an SST anomaly pattern, which favoured the persistence of a negative NAO resulting in the cold weather anomaly of December 2010 in Northern Europe. Observations show a link between NAO strength and re-emergence after negative NAO winters. The occurrence of a reemergence event increases the chance of predicting the atmospheric state in the second winter. The analysis of the CMIP5 model output suggests that the majority of the models do not correctly represent re-emergence processes in the North Atlantic and are limited in their ability to reproduce the variability in oceanic and atmospheric conditions seen in observations. Historical observations show a link between the NAO strength and re-emergence, but potential re-emergence events cannot be predicted from the atmospheric state alone. Whilst this thesis has identified factors which point to when these events are likely to occur there still remains considerable uncertainty in our ability to predict them.
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