Variability of pycnoclines in a three-layer, large estuary: the Gulf of Finland

Thirty-five surveys were conducted across the Gulf of Finland from 2006 to 2013 to study the characteristics and variability of the two pycnoclines in this stratified estuary. Easterly winds caused a shallower position of the upper pycnocline near the southern coast and the deeper pycnocline along the northern coast. Westerly winds had an opposite effect. Surfacing of the upper pycnocline (upwelling) was observed several times while the upper mixed layer was deeper than 40 m in the case of downwelling. The deeper pycnocline weakened and deepened significantly during the southwesterly wind bursts while it was observed at shallower depths than 50 m several times during the periods of easterly winds. The upper pycnocline was on average 3 m shallower near the southern coast while the deeper pycnocline was leveled. The inclination of the upper pycnocline corresponding to the average local wind forcing agrees well with the general cyclonic circulation of the upper layer in the Gulf of Finland and the water exchange estimates between the gulf and the Baltic Proper. An average seasonal inclination of the pycnoclines showed high inter-annual variability depending on the prevailing wind forcing. Easterly winds caused stronger vertical stratification and horizontal gradients across the gulf while in the case of westerly winds, horizontal and vertical gradients were weaker.