No abstract

Intensification of coastal erosion caused by windstorms is an important problem in the coastal regions of Estonia. The objective of this study was to analyse relationships between storms observed at Vilsandi, the westernmost station in Estonia, and large-scale atmospheric circulation. Statistical analyses reveal close correlation between parameters of atmospheric circulation and the number of storms. Windstorms are related to intense zonal circulation, i.e. westerlies, while reduced storminess is associated with meridional circulation. The Arctic Oscillation (AO) index has the highest correlation (0.68) with the frequency of storms during the winter season (December to February). Even the correlation coefficients for the local zonal circulation index for Estonia do not exceed this value. These results show that large-scale circulation patterns determine circulation on the local scale. Results of the conditional Mann-Kendall test confirm that the increasing trend in winter storminess is induced by a positive trend in the intensity of westerlies, as seen in time series of the AO index and in the frequency of the circulation form W of Vangengeim and Girs. These changes were most substantial in February.

No abstract

Abstract. Salinity and stratification in the deep basins of the Baltic Sea are linked to the occurrence of Major Baltic Inflows (MBIs) of higher saline water of North Sea origin, which occur sporadically and transport higher saline and oxygenated water to deeper layers. Since the mid-1970s, the frequency and intensity of MBIs have decreased. They were completely absent between February 1983 and January 1993. However, in spite of the decreasing frequency of MBIs, there was no obvious decrease of larger Baltic Sea volume changes (LVCs). A LVC is defined by a total volume change of at least 100 km3. LVCs can be identified from the sea level changes at Landsort which is known to represent the mean sea level of the Baltic Sea very well. Strong inflows leading to LVCs are associated to a special sequence of atmospheric circulation patterns. Our analysis based on Jenkinson-Collison circulation (JCC) types confirms that most effective inflows occur if about a month before the main inflow period, eastern air flow with anticyclonic vorticity over the western Baltic prevails. These conditions reduce the mean sea level of the Baltic Sea and lead to an increased saline stratification in the Belt Sea area. An immediate period of strong to very strong westerly winds trigger the inflow and force LVCs/MBIs. The lack of MBIs coincide with a negative trend of eastern types and a parallel increase of western type JCCs.

Changes in the number of cyclones and cyclone trajectories in Central and Northern Europe during 1948–2000 are analysed using a database of cyclones. Two hypotheses are advanced. Firstly, the number of cyclones reaching Northern Europe has increased, causing a transition to a more maritime climate. Secondly, the trajectories of cyclones have moved northward, causing the advection of warm and moist air to Northern Europe and decreasing precipitation in Central Europe. These advances were confirmed by data analysis. A linear trend and its statistical significance (P<0.05) for the frequency of cyclones in the Atlantic–European sector (30°W–45°E, 35–75°N) were calculated. Circles with radii of 500, 1000, 1500 and 2000 km with centre coordinates 60°N and 22.5°E were generated. All the cyclones whose centres were located within these circles were counted. Also two meridians −5°E and 20°E – were selected and all the cyclones were counted whose centres crossed the meridians from west to east in the interval of 45–75°N. Changes in the frequency of long-term cyclones were analysed. The number of cyclones reaching Northern Europe has increased in the period 1948–2000. The number of cyclones over the Baltic Sea has increased, especially in the winter. In Central Europe, the number of cyclones has decreased, especially in the warm half-year. The number of long cyclones has increased over the Baltic Sea, especially in the cold half-year.