Ocean Dynamics as Driver of Seasonal to Decadal European Atmospheric variability (ODYSEA)

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ODYSEA will assess how, when, and where the ocean affects atmospheric variability and weather in Europe and in particular in the UK on timescales up to a decade. Particular emphasis will be on the identification of oceanic precursors that indicate the development of unusually warm, cold, dry or wet conditions several months or years in advance, especially related to extreme weather events. Such precursors can include changes in the ocean surface temperature or in the position of major ocean currents such as the Gulf Stream.On a climatological level, the large heat capacity of the Atlantic ocean acts to moderate the seasonal cycle of temperature over Western Europe. The Atlantic also provides much of the moisture precipitating over Europe, including the recent widespread flooding in the UK in late 2013/early 2014. Together, the circulation of the ocean and the atmosphere act to reduce the temperature difference between low and high latitudes by carrying heat polewards from the tropics. In the Atlantic, the meridional overturning circulation (MOC) transports heat northward at a rate of more than 1000 Terawatts (TW) - equivalent to the energy produced by 1,000,000 average sized nuclear power stations. This heat transport leads to an additional warming of Western Europe that is present throughout the year and temperatures in Western Europe are on average higher than at similar latitudes in the maritime climate of Northwest America. Both theoretical arguments and GCM simulations suggest that ocean poleward heat transports are capable of having very profound impacts on the atmospheric storm tracks which are responsible for much of the day-to-day variability in European weather. Such changes may also influence the pathways of the proposed "teleconnections" that link atmospheric variability in remote regions to the European continent. In ODYSEA we will therefore investigate how variability in the ocean circulation modulates the atmospheric circulation and its impact on Europe. Research suggests that meanders of the Gulf Stream affect the atmosphere in a region that is key to the formation of North Atlantic Storms. The MOC has also been shown to be highly variable with likely impacts on ocean surface temperatures. This affects the amount of heat released to the atmosphere overlying the ocean, but also the atmospheric circulation through which oceanic heat and moisture is carried towards the continents. A recent study performed at NOC suggests that anomalies of surface ocean temperatures were key to the development of the atmospheric conditions that led to the extremely cold December of 2010. These anomalous ocean surface temperatures were preceded by a particularly weak MOC in 2009. In ODYSEA we will establish if similar oceanic impacts can be identified for previous weather extremes that have affected Europe and the UK (e.g. wet summers of 2005, 2007 and 2012, the heat waves in the summer of 2003 and of July 2006). Emphasis will be on acquiring a better understanding of the mechanisms through which the ocean can impact the atmosphere and therefore our weather and climate. Current knowledge strongly suggests that the ocean affects variability of European weather and climate on timescales of months to years, but the underlying mechanisms are far from fully understood. This hampers prediction and attribution of those events. ODYSEA will reduce this gap in our understanding of the variability of UK/European weather and climate by using cutting edge ocean and atmosphere models available in the UK as well as by analysing data from the latest seasonal to decadal forecasting systems run by the UK Met Office.