Increased frequency of wintertime stratification collapse events in the Gulf of Finland since the 1990s

Since the 1990s, an increased frequency of stratification collapse events in the Gulf of Finland has been noticed, when the density difference between near-bottom and surface waters fell below 0.5 kg m^{- 3}. Such stratification crashes occur in the winter months, from October-November to March-April, when saline and thermal stratification decrease compared to the summer period according to the well-known seasonal cycle. The stratification decay process is forced primarily by (1) the westerly-southwesterly wind stress, which causes anti-estuarine straining, and (2) direct wind mixing proportional to the wind speed cubed. The potential energy anomaly (PEA) is occasionally reduced from the average winter level of 70 J m^{- 3} (per unit volume; 4.9 kJ m^{- 2} per unit area of 70-m water column) to nearly zero, manifesting the stratification collapse, when the current-straining work and wind-mixing work significantly exceed their average levels. Increased collapse frequency is caused by the shift of wind forcing. Namely, the average bimonthly cumulative westerly-southwesterly wind stress in December and January has increased from 1.7 N m^{- 2} d during 1962-1988 to 3.7 N m^{- 2} d during 1989-2007, yielding a reduction in PEA during these two winter months of about 4.4 kJ m^{- 2} between the periods. The other component of the reduction in PEA, wind mixing work per unit surface area, has also increased by 4.6 kJ m^{- 2} since 1999 for these two months.