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K. Myrberg and T. Soomere
frequently transformed into topographically-driven baroclinic mesoscale motions
characterized by large isopycnal displacements (more than 20 m within a distance
of 10–20 km) and high intra-halocline current speeds (>20 cm/s).
Similarly to the observations, also in models the Finnish side is primarily dominated by offshore outflow down to the depth of 40–50 m and with a mean velocity
of 8 cm/s (Meier 1999; Andrejev et al. 2004a). This outflow can be interpreted as a
persistent buoyancy current stabilized and amplified by certain subtle mechanisms.
Generally, baroclinic instability leads to disintegration of the flow but a relatively
gently sloping bottom at the Finnish coast may cause an internal baroclinic adjustment of the flow to a quasi-stable state (Stipa 2004).
Alternatively, the outflow can be interpreted as an analogue to large-scale nearly
zonal ocean currents where the impact of the beta-effect is mimicked by an equivalent impact of the sloping bottom (Cushman-Roisin and Beckers 2011). Such
baroclinic currents are generally more pronounced than barotropic ones (Soomere
1995). Somewhat counter-intuitively, a comparatively intense transport (speed about
8 cm/s; apparently an Ekman-type drift) goes into the Gulf of Finland on the Finnish
side in the uppermost model layer (0–2.5 m) (Andrejev et al. 2004a). This feature
once more confirms that the surface layer and the layer below may have clearly
different dynamics.
Lehmann and Hinrichsen (2000) were the first to numerically analyse the water
exchange between the Gulf of Finland and the Northern Gotland Basin. The net
outflow was the same, around 130 km 3 /yr, as in Witting (1910) but the volumes
of in- and outflow were much higher. The large deviations are caused by the complexity of water dynamics at the entrance of the gulf, which makes some of the
volumes strongly dependent on the temporal resolution of the processes (Andrejev
et al. 2004a). Leaving aside the small-scale dynamics, which is at present accurately reflected neither by measurements nor in numerical simulations, processes at
two basic scales contribute to the water budget.
The estimates of the net in- and outflow in Witting (1910) reflect the amount of
water entering the interior of the gulf, remaining there for a long time, or ultimately
being transported out of it. Relatively persistent or quasi-periodic, mesoscale features (local jets, synoptic eddies, inertial oscillations) induce comparatively shortterm transport of water across the entrance line. The water is usually not transported far from its original location and does not affect the interior of the gulf. These
motions were not accounted for by Witting (1910). When the water exchange was
summed every 30 minutes (accounting for the full dynamics of the mesoscale circulation), the average in- and outflows were 3154 and 3273 km 3 /yr for 1987–1992
(Andrejev et al. 2004a). A similar estimate from the average velocities for the 5-year
period (only accounting for the quasi-stationary circulation pattern) gave 1417 and
1532 km 3 /yr, respectively. The net outflow (119 or 115 km 3 /yr) was close to the
total river runoff to the Gulf of Finland.
K. Myrberg and T. Soomere
frequently transformed into topographically-driven baroclinic mesoscale motions
characterized by large isopycnal displacements (more than 20 m within a distance
of 10–20 km) and high intra-halocline current speeds (>20 cm/s).
Similarly to the observations, also in models the Finnish side is primarily dominated by offshore outflow down to the depth of 40–50 m and with a mean velocity
of 8 cm/s (Meier 1999; Andrejev et al. 2004a). This outflow can be interpreted as a
persistent buoyancy current stabilized and amplified by certain subtle mechanisms.
Generally, baroclinic instability leads to disintegration of the flow but a relatively
gently sloping bottom at the Finnish coast may cause an internal baroclinic adjustment of the flow to a quasi-stable state (Stipa 2004).
Alternatively, the outflow can be interpreted as an analogue to large-scale nearly
zonal ocean currents where the impact of the beta-effect is mimicked by an equivalent impact of the sloping bottom (Cushman-Roisin and Beckers 2011). Such
baroclinic currents are generally more pronounced than barotropic ones (Soomere
1995). Somewhat counter-intuitively, a comparatively intense transport (speed about
8 cm/s; apparently an Ekman-type drift) goes into the Gulf of Finland on the Finnish
side in the uppermost model layer (0–2.5 m) (Andrejev et al. 2004a). This feature
once more confirms that the surface layer and the layer below may have clearly
different dynamics.
Lehmann and Hinrichsen (2000) were the first to numerically analyse the water
exchange between the Gulf of Finland and the Northern Gotland Basin. The net
outflow was the same, around 130 km 3 /yr, as in Witting (1910) but the volumes
of in- and outflow were much higher. The large deviations are caused by the complexity of water dynamics at the entrance of the gulf, which makes some of the
volumes strongly dependent on the temporal resolution of the processes (Andrejev
et al. 2004a). Leaving aside the small-scale dynamics, which is at present accurately reflected neither by measurements nor in numerical simulations, processes at
two basic scales contribute to the water budget.
The estimates of the net in- and outflow in Witting (1910) reflect the amount of
water entering the interior of the gulf, remaining there for a long time, or ultimately
being transported out of it. Relatively persistent or quasi-periodic, mesoscale features (local jets, synoptic eddies, inertial oscillations) induce comparatively shortterm transport of water across the entrance line. The water is usually not transported far from its original location and does not affect the interior of the gulf. These
motions were not accounted for by Witting (1910). When the water exchange was
summed every 30 minutes (accounting for the full dynamics of the mesoscale circulation), the average in- and outflows were 3154 and 3273 km 3 /yr for 1987–1992
(Andrejev et al. 2004a). A similar estimate from the average velocities for the 5-year
period (only accounting for the quasi-stationary circulation pattern) gave 1417 and
1532 km 3 /yr, respectively. The net outflow (119 or 115 km 3 /yr) was close to the
total river runoff to the Gulf of Finland.