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 "early warning signs" 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 early warning signs can include changes in the ocean surface temperature or in the position of major ocean currents such as the Gulf Stream.The best known role of the ocean for climate and weather is as a reservoir of heat and moisture. The ocean stores 1000 times more heat than the atmosphere. Heat stored in the ocean during summer moderates winter temperatures and in summer the large ocean heat capacity ensures that ocean temperatures rise less than on land areas, meaning that in summer the ocean cools the climate of surrounding land masses. This maritime effect is pronounced for the climate of the UK, Europe and Northwest America, with winters that are warmer and summers that are cooler than in other regions at similar latitudes. North Atlantic moisture is the source of a substantial fraction of the precipitation affecting Europe. A recent prominent example is the very unsettled spell of weather that led to widespread flooding in the UK in late 2013/early 2014. Together, the ocean and the atmosphere reduce the temperature difference between low and high latitudes by carrying heat from the tropics to high latitudes. In the Atlantic a circulation called 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.Over the mid-latitudes heat and moisture from the North Atlantic is carried towards Europe and well into Eurasia by the predominantly westerly winds (in particular the North Atlantic storm track). In ODYSEA we will investigate how variability in the ocean circulation modulates the atmospheric exchange between ocean and land. 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 efficiency and direction by which this heat 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 underlyingmechanisms 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.