Extreme Storm Surges in the Gulf of Finland: Frequency-Spectral Properties and the Influence of Low-Frequency Sea Level Oscillations

An analysis of hourly tide gauge observation data yields time-frequency estimates of storm surges in the Gulf of Finland. Spectral analysis of long-term series made it possible to identify the main periods of natural oscillations in the Gulf of Finland. As a result of the spectral-temporal analysis of individual floods, the time-frequency spectra of storm surges were obtained. For the Gulf of Finland, dangerous storm surges have periods of 25–35 h. Thus, the period of sea level oscillations during the flood of January 1, 1984, was 26–34 h; in December 1986, 24–34 h; in mid-October 1994, 24–30 h; in November 1999, 27–34 h; and in January 2007, 25–30 h. In the case of floods in early October 1994, oscillations with longer periods prevailed: 48–80 h. For the flood on January 9, 2005, caused by passage of the Gudrun cyclone, sea level fluctuations had periods from 27 to 48 h. In some storm surge cases, an increase in high-frequency 8-h oscillations is observed. The most unfavorable conditions for the formation of extreme storm surges in the gulf are the cases of passage of a chain of atmospheric cyclones with a period of about 1–2 days. Such cases lead to the resonant “rocking” effect of natural oscillations in the Gulf of Finland with a period of 26–29 h. It is shown that background mean sea level variations with periods of more than 20 days contribute to flood formation in the Gulf of Finland. Four events were identified when the mean sea level exceeded 100 cm for several days, three of which were accompanied by floods.