The Western Mediterranean summer variability and its feedbacks

The anomalous climatic variability of the Western Mediterranean in summer, its relationships with the large scale climatic teleconnection modes and its feedbacks from some of these modes are the targets of this study. The most important trait of this variability is the recurrence of warm and cold episodes, that take place at 2-4 year intervals, and which are monitored in the Western Mediterranean Index. We find that the Western Mediterranean events are part of a basin scale mode, and are related to the previous spring atmospheric anomalies. These anomalies are related mainly to the Pacific North America teleconnection pattern and the North Atlantic Oscillation, but also to a number of other climatic modes, connected with the previous two, as the Southern Oscillation, the Indian Core Monsoon and the Scandinavian teleconnection pattern. We identify the main spatial and temporal traits of the Western Mediterranean summer variability, the physical mechanisms at play in the generation of the events and their impacts. Considering the Atlantic Ocean, the Mediterranean events influence the sea surface temperature in the southeastern part of the North Atlantic Gyre. Additionally, they are significantly related to summer precipitation anomalies of the opposite sign in the Baltic basin (Central Germany and Poland) and near the Black Sea. We then estimate the mutual influence that the anomalous previous state of the Western Mediterranean, of the Pacific North America teleconnection pattern and of the North Atlantic Oscillation have on their summer conditions using a simple stochastic model. As the summer Western Mediterranean events have an influence on a part of the Baltic basin, we propose a second stochastic model in order to investigate if thereafter the Baltic basin variability will feedback on the Western Mediterranean sea surface temperature anomalies. Among the variables included in the second model are, in addition to the Western Mediterranean previous state, that of the Baltic Sea and of the Scandinavian teleconnection pattern. From each of the feedback matrices, a linear statistical analysis extracts spatial patterns whose evolution in time exhibits predictive capabilities for the Western Mediterranean evolution in summer and autumn that are above those of persistence, and that could be improved. © 2012 Springer-Verlag.