6 The Gulf of Finland
197
the Lagrangian approach, by tracking floating indicators (drifters, buoys, dye spots).
Airborne tracking of floating drifters in the eastern gulf for different ‘phases’ (rise
and fall) of sea level oscillations showed that horizontal turbulence appears to be
less intense in coastal regions than in more open areas (Mikhailov 1981).
A series of experiments using drifters located in the uppermost 1 m layer and
drifting up to 8 days in summer and a few months in ice conditions have been performed in 2007 in the middle of the gulf, the western Estonian archipelago and in
the eastern sector of the Northern Gotland Basin (Kõuts et al. 2010; Verjovkina et al.
2010). Extensive experiments in summer and autumn 2010 with lightweight drifters
in the uppermost 1.5 m thick layer in several domains of the gulf (Soomere et al.
2011b) showed that the spreading rate of closely packed drifters reveals two distinct regimes depending on the distance and thus may require the use of different
coefficients of eddy diffusivity for applications with different horizontal resolution
(Chap. 8).
Recently new data have also been gained about vertical turbulence in the Gulf
of Finland, about which only very little information was available in the past (Alenius et al. 1998). Microstructure measurements were performed in three different
wind forcing regimes and three different background density stratification and current velocity shear situations at the entrance of the gulf (Lilover and Stips 2011).
It was remarkable that all three standard parameterizations of the eddy diffusivity
(using Richardson number, Brunt–Väisälä frequency and k–ε turbulence closure)
gave biased results in comparison to the measurements. A new parameterization is
thus necessary which would be able to account for the internal wave kinetic energy
of the super-inertial frequency band (Lilover and Stips 2011).
6.3 Meteorological Forcing
6.3.1 Wind Forcing
Meteorological forcing, the principal factor causing water movements, is extremely
complicated in the Gulf of Finland and not fully understood yet. The sparse observational network and the very limited amount of open sea data make the estimations of wind patterns very difficult. The gulf is characterized by a remarkable
wind anisotropy (Launiainen and Saarinen 1982; Soomere and Keevallik 2003; Keevallik and Soomere 2010). Complicated patterns of atmospheric temperature (that
develop due to the very variable surface roughness) modify local wind fields and
correspondingly air–sea interaction in and around the gulf, giving rise to the spatiotemporal variability of the momentum and heat exchange between the sea and the
atmosphere. The latitudes where the gulf is located experience a great variability in
the overall meteorological conditions and solar radiation throughout the year and
between different years, which in turn affects the circulation dynamics and stratification. It is thus not unexpected that the modelling of atmospheric forcing and
air–sea interaction in the Gulf of Finland area is a major challenge.
197
the Lagrangian approach, by tracking floating indicators (drifters, buoys, dye spots).
Airborne tracking of floating drifters in the eastern gulf for different ‘phases’ (rise
and fall) of sea level oscillations showed that horizontal turbulence appears to be
less intense in coastal regions than in more open areas (Mikhailov 1981).
A series of experiments using drifters located in the uppermost 1 m layer and
drifting up to 8 days in summer and a few months in ice conditions have been performed in 2007 in the middle of the gulf, the western Estonian archipelago and in
the eastern sector of the Northern Gotland Basin (Kõuts et al. 2010; Verjovkina et al.
2010). Extensive experiments in summer and autumn 2010 with lightweight drifters
in the uppermost 1.5 m thick layer in several domains of the gulf (Soomere et al.
2011b) showed that the spreading rate of closely packed drifters reveals two distinct regimes depending on the distance and thus may require the use of different
coefficients of eddy diffusivity for applications with different horizontal resolution
(Chap. 8).
Recently new data have also been gained about vertical turbulence in the Gulf
of Finland, about which only very little information was available in the past (Alenius et al. 1998). Microstructure measurements were performed in three different
wind forcing regimes and three different background density stratification and current velocity shear situations at the entrance of the gulf (Lilover and Stips 2011).
It was remarkable that all three standard parameterizations of the eddy diffusivity
(using Richardson number, Brunt–Väisälä frequency and k–ε turbulence closure)
gave biased results in comparison to the measurements. A new parameterization is
thus necessary which would be able to account for the internal wave kinetic energy
of the super-inertial frequency band (Lilover and Stips 2011).
6.3 Meteorological Forcing
6.3.1 Wind Forcing
Meteorological forcing, the principal factor causing water movements, is extremely
complicated in the Gulf of Finland and not fully understood yet. The sparse observational network and the very limited amount of open sea data make the estimations of wind patterns very difficult. The gulf is characterized by a remarkable
wind anisotropy (Launiainen and Saarinen 1982; Soomere and Keevallik 2003; Keevallik and Soomere 2010). Complicated patterns of atmospheric temperature (that
develop due to the very variable surface roughness) modify local wind fields and
correspondingly air–sea interaction in and around the gulf, giving rise to the spatiotemporal variability of the momentum and heat exchange between the sea and the
atmosphere. The latitudes where the gulf is located experience a great variability in
the overall meteorological conditions and solar radiation throughout the year and
between different years, which in turn affects the circulation dynamics and stratification. It is thus not unexpected that the modelling of atmospheric forcing and
air–sea interaction in the Gulf of Finland area is a major challenge.