Circulation patterns in the Gulf of Finland derived from the EOF analysis of model results

For the Gulf of Finland, we carried out the EOF (empirical orthogonal functions) analysis of hourly forecasts from the Baltic Sea operational HIROMB-SMHI model for the period 2006-2008. It is possible to distinguish two regions with a specific regime of circulation variability. The western region behaves like a wide channel. Dominant EOF modes at different sections have similar patterns and their time-dependent amplitudes are well correlated. A prevailing mode of currents (23%-42% of the variance) is barotropic (unidirectional over the whole section) and its oscillation (spectral peak at 24 h) is related to the water storage variation of the Gulf. A two-layer flow pattern (surface Ekman transport with deeper compensation flow, 19%-22%) reveals both inertial and lower frequencies. Highest outflow of surface waters occurs during north-easterly winds. The eastern wider region has more complex flow dynamics and only patterns that are nearly uniform over the whole Gulf were detected here. On the sea surface, quasi-uniform drift currents are deflected on the average by 40 to the right from the wind direction and they cover 60% of the circulation variance. Sea level variability is heavily (98%) dominated by nearly uniform changes which are caused by the water storage variations of the Gulf. Sea level gradients contain the main axis (23%) and transverse (17%) components, forced by winds of the same direction. The flows below the surface are decomposed also into the main axis (24%-40%) and transverse (13%-16%) components that are correlated with the sea level gradients according to the geostrophic relations.