Scheef, Helga (2014) Blocking : sensitivity to index definition and model bias. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, x, 85 pp.

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Abstract

Atmospheric blocking is a persistent pressure pattern, which blocks the westerly flow in the mid-latitudes and strongly impacts weather both in its immediate vicinity and in regions upstream and downstream of it. Extreme events cause droughts in summer ( e.g. Russian heat wave 2010) and cold spells in winter ( e.g. European winter 1962\63). Blockings are also connected to Sudden Stratospheric Warmings (SSWs). In winter 1962\ 63 a blocking occured both before and after an SSW. However, blocking dynamics are not well understood and the phenomenon is not well-represented in climate models. To detect blockings, different definitions were derived, but deduced blocking frequencies are dependent on the blocking index. In the first part of this study, climatologies are derived from reanalysis data, to detect differences between index definitions. The indices by Lejenas and 0kland (1983) (LO*), Tibaldi and Molteni (1990) (TM) Pelly and Hoskins (2003) (PH) detect patterns, which show a reversal of the climatological flow. Anomalous high pressure systems are detectable with the index of Sausen et al. (1995) (SA). All indices show frequency maxima over the Euro-Atlantic region and the Pacific. The highest frequencies occur in winter. The TM and LO* indices show further maxima in spring and the PH index shows an additional frequency maximum in summer. Short blockings are more evenly distributed in location and season than long-lasting blocking episodes, which are accumulated over the ocean basins in winter. The second aim of the study is the investigation of the insufficient representation of blocking in climate models. Hence, climatologies are derived for CESM(WACCM). The model underestimates blockings, especially over the Euro-Atlantic region in winter. The frequency changes over the Pacific are dependent on the index. Discrepancies between the sectors are probably caused by the misrepresentation of transient eddies in the Atlantic region, which are neccessary for the formation and persistence of blockings. It is suggested that the model resolution is not high enough to simulate transient eddies, but further investigation is needed. A realistic mean flow in the model is also required. Mean flow biases could cause the underestimation over the Pacific. To investigate influences of external forcings, different model experiments
were used. Increasing greenhouse gases could lead to a decrease in blocking frequency; El Nifio Southern Oscillation events probably cause a decrease over the Pacific. But these results have to be handled with care as the misrepresentation of blocking frequencies predominates differences between model runs.

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