The wind effects are directly related to the pressure distribution over an area. However, as shown by tide gauge records, true sea level surges and falls can be several times higher than the values resulting from the action of tangential wind stress upon a fluid surface (Wiśniewski & Holec 1983). Suursaar et al. (2003) pointed out that the highest surge events on the west Estonian coast are associated with deep depressions producing strong SW and W winds in suitably oriented bays such as Pärnu Bay. An example is the mid-latitude

depression Gudrun, which occurred in January 2005 and caused the heaviest storm surge along the coasts of the Gulf of Riga (Suursaar et al. 2006). The sea level at Pärnu was 2.75 m higher selleck chemicals than the mean level there. In the Gulf of Finland, record increases in sea level were measured

as well, e.g. at Helsinki (1.51 m). Skriptunov & Gorelits (2001) NVP-LDE225 showed that significant wind-induced variations in the water level near the River Neva as well as their magnitude and duration result from the wind regime and the morphology of the near-mouth offshore zone. Averkiev and Klevanny, 2007 and Averkiev and Klevanny, 2010 analysed the effects of atmospheric pressure as well as wind direction and speed on the sea level in the Gulf of Finland. They showed the low pressure system trajectory to be potentially important in generating storm surges particularly damaging for St. Petersburg (Russia). The problem of sea surface deformation by concentric, mesoscale, fast-moving deep low-pressure systems was addressed by Lisowski, 1960, Lisowski, 1961 and Lisowski, 1963, Wiśniewski, 1996, Wiśniewski, 1997 and Wiśniewski, 2003, Wiśniewski & Holec (1983), Wiśniewski & Kowalewska-Kalkowska (2007) and Wiśniewski and Wolski, 2009a and Wiśniewski and Wolski, 2011. It seems, however, that this Rapamycin factor has been generally underestimated, even downright ignored, in the literature, a situation that has been detrimental to attempts at explaining mechanisms

of such extreme phenomena as coastal floods or low sea levels that adversely affect navigation safety, stability of hydraulic engineering structures, etc. It is true that a lowered atmospheric pressure system (a tropical cyclone or a concentric low pressure system) overlies a water cushion, moving together with the pressure system at the sea surface. Wave height depends on the pressure decrease in the centre of the system. A pressure drop of Δp = 1 hPa results in a static sea level rise of ΔHs = 1 cm under a stationary low ( Figure 1a, formula  (3)). When the depression moves over the sea surface, the latter becomes dynamically deformed (ΔHd). The sea level deformation shows positive wave elevations in the centre and negative elevations on the flanks of the deformation ( Figure 1b, formula  (4)).