Latvia is located on the eastern coast of the Baltic Proper, and around the southern part of the Gulf of Riga. The latter is separated from the Baltic Proper by Irbe and Muhu straits. The hydrological regime (currents, waves, temperature, salinity, oxygen regime, ice conditions etc.) is quite different in Baltic Sea, Gulf of Riga, and Irbe strait. The hydrological conditions in these basins are of key importance for fisheries, navigation and off-shore recreation.
The episodic observations of the hydrological parameters in open marine basins are available since 1920s, while regular seasonal series are available since 1960s (for the Baltic Proper) or 1970s (for the Gulf of Riga).
The Gulf of Riga is semi-enclosed water body. It has two connections with the Baltic Proper. The mean depth of the Gulf is 26 m, maximum depth reaches 60 m. Mean river runoff to the Gulf from its drainage basin (area 134000 square kilometers) is 32 cubic kilometers per annum, what is comparable to the volume of the Gulf ~424 cubic kilometers. The water exchange with the Baltic Proper is evaluated by salt budget; on average it is ca. 110 cubic kilometers per year.
Typical average water salinity of the Gulf is 5 to 6 PSU. Small water depth ensures full mixing of the water column by negative buoyancy and wind-induced turbulence during autumn-winter. Seasonal pycnocline develops during spring; it resides at ca. 20-m depth throughout the summer. Typical temperature difference is +18 oC/+20 oC (above/below pycnocline). The erosive deepening of thermocline starts in late summer/early autumn; it reaches bottom in November-December.
The calculation of the circulation of the Gulf of Riga by shallow water approach is possible for autumn-winter period.
Calculated steady-state discharge and elevation distribution in the Gulf of Riga
See examples of steady-state flow field. The investigations of the flow patterns by mathematical modelling have shown that the current field in the Gulf of Riga is wind driven whilst the water level adjusts to the overall level changes in the Baltic; flow patterns consist of series of vortexes; two connections with the Baltic Proper may cause transit flow through the northern part of the Gulf; the calculation results are in reasonable agreement with water level observations.
The seasonal cycle of buoyancy flux (heat exchange on sea/atmosphere interface and freshwater inflow), and wind regime produce non-linear response of the time-development of the vertical temperature-salinity structure. The model of dynamics of this structure includes wind generated turbulent mixing, saltwater inflow in deeper layers, ice formation/melting etc.