We refer to their approach as the AV2010 conceptual model. Cyclone Erwin (or Gudrun) crossed the Baltic Sea on 8–9 January 2005, giving rise to the highest historical sea levels at nearly all northern Baltic coastal stations (Suursaar et al. 2006). The temporal

variability of single storm surges and their correlations with local wind forcing and large-scale atmospheric circulation have been analysed on the basis of model simulations and data over past decades (Suursaar et al., 2003, Suursaar et al., 2010 and Suursaar et al., 2011). One of the general conclusions from the aforementioned works is that extreme storm surges in Estonian coastal waters occurred there because the centre of an learn more intense, fast-moving cyclone was propagating northwards from the Scandinavian Peninsula over the Gulf of Finland. The corresponding local wind pattern was SW winds over the central Baltic

veering west, pushing water first towards the northern Baltic and then into the Gulf of Finland and Gulf of Riga. Storm surges are the main cause of coastal flooding in the Baltic Sea, although as historical data show, a single storm is not enough to cause extreme sea levels: a series of cyclones are needed (Suursaar et al. 2006). Hydrodynamically, extreme storm surges have been thoroughly studied and their different aspects well simulated by models, ranging from conceptual and semi-empirical ones (Suursaar et al. 2002) to operational 3D numerical simulations (Lagemaa et al. 2011). Although sea levels around the average signaling pathway are well represented and validated, extreme sea levels are frequently captured with much poorer accuracy (Raudsepp et al. 2007). This problem could be addressed using an ensemble modelling approach, which gives a measure of uncertainty to estimated sea level extremes; probably, however, this still does not improve the physical understanding of the occurrence of extremes. We find that the real trigger of these extreme events comes from atmospheric conditions, which give rise to a situation where cyclones with

similar tracks and the deepest phase location are clustered in time: it is this periodicity that is the true driver Resveratrol of sea level extremes. These atmospheric factors of such events have not yet been described in great detail. This brings us to the aim of our paper, which is primarily to study the statistics of the physical properties of single cyclones and their tracks that have caused 40 high storm surges on the Estonian coast, measured at Pärnu and Tallinn, and to show how variable the key properties are for dangerous cyclones, as pointed out by the AV2010 model. To that end, we use the characteristics of cyclones from the database of Northern Hemispheric cyclones in Gulev et al. (2001). The second task of the paper is to test the hypothesis that a series of cyclones is needed to force extreme sea levels on northern Baltic Sea coasts.