Wulff, C. Ole W. (2017) Summer Climate Variability in the North Atlantic-European region. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 73 pp.

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Abstract

The increasing number of climatically exceptional summers in the past two decades has drawn researchers’ interest to the characteristics of summer variability and its prediction. In this thesis, the atmospheric summer variability of the North Atlantic-European (NAE) sector is examined. Applying a Principal Component Analysis to reanalysis data of seasonal geopotential height anomalies at 500 hPa, two dominant modes are identified - the summer North Atlantic Oscillation (SNAO) and the summer East Atlantic (SEA) mode. The former is associated with a latitudinal shift of the jet stream but unlike its winter counterpart also impacts the meridional component of the flow. The latter is part of a zonal wave number 5 wave train and is associated with significant anomalies in meridional flow and meandering of the jet. Both modes are shown to have significant impacts on the surface climate of the NAE sector. The controls are mainly through cloud cover anomalies associated with the large-scale rising/sinking motions in their main centres of action and temperature and moisture advection by the anomalous geostrophic flow. In order to examine the potential for prediction of the NAE summer variability, seasonal hindcast experiments are carried out with an atmospheric model applying a relaxation technique in different regions of the atmosphere. The aim of these is to show from which parts of the climate system predictability of summer variability can arise. For the SNAO, no potential for an improved prediction of its interannual variability can be found from any of the forcing regions. Even though previous modelling and observational studies show that the long-term variability of the SNAO is controlled by the Atlantic multidecadal variability, the experiments do not confirm this link. For the SEA, the experiments indicate that no forcing is included in the relaxation regions but that predictability arises from the prescription of observed anomalous lower boundary conditions. Further investigation reveals that this is the result of diabatic heating anomalies in the Caribbean and the tropical Pacific driven by anomalous sea surface temperatures. As a consequence, the upper tropospheric flow is altered in the tropics but also further north where it can interact with the jet stream. This anomalous divergent flow acts as a source for Rossby waves in the eastern North Pacific which can propagate downstream in the jet stream wave guide. The resulting stationary wave train projects onto the SEA pattern in the NAE region. This tropical-extratropical teleconnection offers potential for enhanced predictability of the SEA mode. Since experiments with tropical relaxation quite accurately reproduce the diabatic forcing but cannot predict the SEA variability, it is clear that the application of the relaxation technique constitutes a problem. It is hypothesised that the failure at simulating the correct planetary wave propagation is attributable to altered dissipation properties in the relaxation zone.

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