Periodic variability of currents induced by topographically trapped waves in the coastal zone in the Gulf of Finland

The aim of this paper is to examine the water exchange regime between the bays of northern Estonia (Pakri Bay, Ihasalu Bay and Muuga Bay) and the open part of the Gulf of Finland. To this end, the current measurements and CTD-castings performed at the border of the bays and the open part of the Gulf of Finland in summer 1994, 1995–96 and 1997 are analysed. All the current measurements displayed one feature in common: the existence of periodic variability with a current amplitude of between 5 and 25 cm s−1 and a variability period of 3–4 days (68 hours in Pakri Bay, 72 hours in Muuga Bay and 78 hours in Ihasalu Bay). The amplitudes of this variability differed during different time periods of the experiment and in different parts of the southern Gulf of Finland. The hypothesis was propounded that this variability is the result of bottom-trapped waves, as had been found in many other regions of the Baltic Sea (Aitsam & Talpsepp 1982, Talpsepp 1983). To interpret the results of the measurements, a model of bottom-trapped waves for this region was used. This was the short-wave version of Huthnance’s (1978) numerical model of coastal-trapped waves, according to which the wave parameters for the experimental regions were calculated. Comparison of the model and the measurements implies that coastal-trapped waves do exist off the southern coast of the Gulf of Finland.