Low pressure paths and their relation to the variability of extreme wind waves in the Southern Baltic

Over the Baltic Sea, a semi-closed sea with a complex shoreline, direction of wind and its rapid changes within short time period can have significant influence on the resulting hydrodynamic processes. Highest air pressure gradient and therefore the strongest wind speeds over Baltic are usually the result of low pressure systems moving across the sea or in its vicinity. These events are strongly connected to a large scale atmospheric circulation in Northern Hemisphere and usually are generated over north-western Atlantic. In this study we examine the meteorological conditions associated with high waves in the Gulf of Gdask (southern Baltic Sea). We selected 34 extreme storm events in five distinctly different locations within the area of interest based on the significant wave height (SWH) for years 19582001. The analysis of these events will be presented in a separate publication. Based on the 1-hour atmospheric pressure fields over north-eastern Europe, we traced the trajectories of low pressure centres for each storm event. These trajectories were subsequently classified based on their common characteristics and the impact they exert on the wind wave field in the region. We identify and analyse four cyclone paths: two representing the most common trajectories (P1 and P2) associated with extreme wind wave conditions in the Gulf of Gdask, and two unique trajectories (P3 and P4) that occurred only once during the analysed period. Trajectories P3 and P4 are highly atypical for storm events in the area. Path P1 is characterised by a low pressure centre moving from west to east across Scandinavia and through the Baltic, a pattern arising from zonal circulation that dominates the region. Storms generated by lows travelling along this path are among the most severe in the Gulf of Gdask, with SWH reaching nearly 9 m. Path P2 typically originates over the Norwegian Sea, following a NW-SE trajectory across the Baltic. This path has been infrequently considered in relation to wind waves in the southeastern Baltic, more commonly being associated to storm surges along the eastern Polish coast. Path P3 features a low pressure system moving northward in the Atlantic, along the western coast of the Scandinavian Peninsula. During this particular storm (1214 January 1984) the lowest SWH was recorded out of all 34 analysed events. The low pressure system following path P4 (9-12 April 1986) moved south of the Baltic Sea along the W-E trajectory. Its relative position to the Gulf of Gdask resulted in different wind directions compared to those in storms following path P1. At one of our selected analysis points, northeastern winds have the longest fetch. Therefore it is not surprising that this storm generated the highest significant wave height at this location for the entire study period. This study utilised modelled wind wave and atmospheric data resulting from project HIPOCAS. The meteorological dataset was produced using the REMO atmospheric model (Jacob and Podzun 1997), based on NCEP reanalysis data. The wind wave dataset was created with the WAM wave model (Cielikiewicz Papliska-Swerpel 2008).