188
K. Myrberg and T. Soomere
and the winds and atmospheric pressure gradients are weak. At that time of the year
this ‘juvenile fresh water’ (Stigebrandt 2001) flows out of the Gulf of Finland in
the surface layer. This feature may substantially modify the transport patterns in the
surface layer during the spring season. As it only occurs in a quite thin upper layer,
ocean models of a modest vertical resolution do not always resolve this transport. It
can also be easily overlooked in oil and pollution propagation models. In summer
and autumn the fresh water input is smaller and the wind-induced circulation takes
over.
Evaporation E and precipitationP more or less balance each other in the annual
mean over the whole Baltic Sea area (Ehlin 1981; Leppäranta and Myrberg 2009).
However, the P –E budget strongly depends on the place and time. In the northern
regions of the Baltic Sea, incl. the Gulf of Finland, precipitation exceeds evaporation in the annual average but both components have pronounced seasonal and interannual variations (HELCOM 1986; Omstedt et al. 1997). The existing estimates
are not very accurate and definitely need further studies (Leppäranta and Myrberg
2009). The water budget of this basin furthermore depends on the water exchange
with the open Baltic, the water from which substantially affects the stratification and
circulation in the gulf.
6.2.3 Horizontal and Vertical Structure of Salinity
The stratification of the Gulf of Finland has several features that are characteristic
for large estuaries. The entire gulf can be treated as a transition zone from almost
fresh water in the Neva Bay and its vicinity to the typical brackish waters of the
Gotland Sea. As there is no sill between the gulf and the Gotland Sea, no specific
topographically-isolated water masses exist in the deeper part of the gulf. The characteristic features are (i) a fresh water inflow at the landward end, (ii) a saltier wedge
of water penetrating into the estuary along the bottom from the seaward end combined with (iii) strong mixing (in a part of the gulf it is strongly reduced due to the
halocline) and (iv) large gradients of salinity, temperature and density in the whole
water body (Alenius et al. 1998; Myrberg 1998; Soomere et al. 2008). The seasonal
and interannual variability of salinity and temperature in both horizontal and vertical directions is pronounced due to a large natural variability in wind forcing, fresh
water input and energy budget. A specific feature of the gulf is that horizontal gradients of salinity and temperature can occasionally be extremely large as a result of
local upwelling.
In the Gulf of Finland, like in the entire Baltic Sea but unlike most of the World
Ocean, salinity mostly determines the stratification of water masses. In other words,
it affects density much more than temperature. The distribution of salinity in the
Baltic Sea develops in a specific manner as described in Chaps. 2 and 5. The water
in the bottom layers of the Baltic Sea is formed from relatively saline waters of the
North Sea that at times enter the Baltic Sea and move further to inner parts of the
sea. The entering waters are denser than the ambient surface waters and thus sink
K. Myrberg and T. Soomere
and the winds and atmospheric pressure gradients are weak. At that time of the year
this ‘juvenile fresh water’ (Stigebrandt 2001) flows out of the Gulf of Finland in
the surface layer. This feature may substantially modify the transport patterns in the
surface layer during the spring season. As it only occurs in a quite thin upper layer,
ocean models of a modest vertical resolution do not always resolve this transport. It
can also be easily overlooked in oil and pollution propagation models. In summer
and autumn the fresh water input is smaller and the wind-induced circulation takes
over.
Evaporation E and precipitationP more or less balance each other in the annual
mean over the whole Baltic Sea area (Ehlin 1981; Leppäranta and Myrberg 2009).
However, the P –E budget strongly depends on the place and time. In the northern
regions of the Baltic Sea, incl. the Gulf of Finland, precipitation exceeds evaporation in the annual average but both components have pronounced seasonal and interannual variations (HELCOM 1986; Omstedt et al. 1997). The existing estimates
are not very accurate and definitely need further studies (Leppäranta and Myrberg
2009). The water budget of this basin furthermore depends on the water exchange
with the open Baltic, the water from which substantially affects the stratification and
circulation in the gulf.
6.2.3 Horizontal and Vertical Structure of Salinity
The stratification of the Gulf of Finland has several features that are characteristic
for large estuaries. The entire gulf can be treated as a transition zone from almost
fresh water in the Neva Bay and its vicinity to the typical brackish waters of the
Gotland Sea. As there is no sill between the gulf and the Gotland Sea, no specific
topographically-isolated water masses exist in the deeper part of the gulf. The characteristic features are (i) a fresh water inflow at the landward end, (ii) a saltier wedge
of water penetrating into the estuary along the bottom from the seaward end combined with (iii) strong mixing (in a part of the gulf it is strongly reduced due to the
halocline) and (iv) large gradients of salinity, temperature and density in the whole
water body (Alenius et al. 1998; Myrberg 1998; Soomere et al. 2008). The seasonal
and interannual variability of salinity and temperature in both horizontal and vertical directions is pronounced due to a large natural variability in wind forcing, fresh
water input and energy budget. A specific feature of the gulf is that horizontal gradients of salinity and temperature can occasionally be extremely large as a result of
local upwelling.
In the Gulf of Finland, like in the entire Baltic Sea but unlike most of the World
Ocean, salinity mostly determines the stratification of water masses. In other words,
it affects density much more than temperature. The distribution of salinity in the
Baltic Sea develops in a specific manner as described in Chaps. 2 and 5. The water
in the bottom layers of the Baltic Sea is formed from relatively saline waters of the
North Sea that at times enter the Baltic Sea and move further to inner parts of the
sea. The entering waters are denser than the ambient surface waters and thus sink