Increased frequency of wintertime stratification collapse events in the Gulf of Finland since the 1990s Jüri Elken ⁎, Urmas Raudsepp, Jaan Laanemets, Jelena Passenko, Ilja Maljutenko, Ove Pärn, Sirje Keevallik Marine Systems Institute at Tallinn University of Technology, Akadeemia 15a, EE12618 Tallinn, Estonia abstract article info Article history: Received 10 January 2012 Received in revised form 23 April 2013 Accepted 24 April 2013 Available online 4 May 2013 Keywords: Estuary Wind-induced straining Wind mixing work Destratification Wind regime change Baltic Sea Gulf of Finland Since the 1990s, an increased frequency of stratification collapse events in the Gulf of Finland has been no- ticed, 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 stratifica- tion 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 cu- mulative westerly-southwesterly wind stress in December and January has increased from 1.7 N m -2 d dur- ing 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Kullenberg (1981), among others, has noted that the Gulf of Finland (Fig. 1) is a “true estuarine embayment” of the Baltic Sea multi-basin brackish water system. With its dimensions (length of about 400 km and width from 48 to 135 km over most of the length), low salinity at the entrance (from 6–7 g kg -1 /on the reference-composition salinity scale, IOC et al., 2010/at the surface to 8–11 g kg -1 in the bottom layers below 80–100 m), and almost absent tides, the gulf is, however, quite unique among the world estuaries (Hansen and Rattray, 1966; see also the reviews by Alenius et al., 1998; MacCready and Geyer, 2010). In the west, the gulf has a free connection, 60 km wide and about 90 m deep, to the Baltic Proper, the central basin of a system that undergoes in its northern part large variations of the stratification (e.g. Elken et al., 2006; Matthäus, 1984). River discharge is concen- trated in the eastern part of the gulf, where at the estuary head the Neva River drains an average of 2400 m 3 s -1 of freshwater, about two thirds of all of the freshwater imported to the gulf. Despite the large di- mensions, compared to the internal Rossby radius (typical scales from 2 to 4 km, Alenius et al., 2003) and variable cyclonic circulation with a number of loops, eddies, fronts, and upwelling events (Andrejev et al., 2004; Elken et al., 2011; Lehmann et al., 2002; Lips et al., 2009; Pavelson et al., 1997, Zhurbas et al., 2008), the along-basin salinity and density gradients are still very profound, especially when studied on the basis of temporally mean values over the seasons. Salinity and stratification of the Gulf of Finland undergo strong seasonal variations (Haapala and Alenius, 1994). In the period of highest thermal stratification in summer, after the spring maximum of freshwater discharge, the surface salinity is decreased from the winter values of about 6.5 g kg -1 down to about 5.5 g kg -1 in the cen- tral part of the gulf. At the same time, the deep salinity at around 90 m depth is increased from about 7.5 g kg -1 to about 10 g kg -1 , forming a kind of salt wedge. While a decrease of surface salinity dur- ing and after the period of high river discharge is a common feature of most of the estuaries (e.g. Hong et al., 2010; Kimbro et al., 2009; van Aken, 2008), a simultaneous increase of deep salinity is quite unique. The latter can be partly explained by the seasonal conditions of the ad- jacent larger sea basin, the Baltic Proper, where halocline goes deeper during the winter due to convection and entrainment from the layers above (e.g. Reissmann et al., 2009); in the Northern Baltic Proper deep salinity decrease in winter may exceed 1 g kg -1 below the halocline down to the bottom (e.g. Matthäus, 1984). Interannual changes of the oceanographic conditions of the Gulf of Finland reflect the variations in the large-scale forcing factors. Re- garding direct climate forcing, stronger zonal (westerly) winds have been identified in the 1990s and 2000s compared to the 1970s and 1980s. A number of climatic indices, including those at the regional level like the Baltic Sea Index (BSI, Lehmann et al., 2002, 2011) and the Baltic Winter Index (WIBIX, Hagen and Feistel, 2005) also reveal Journal of Marine Systems 129 (2014) 47–55 ⁎ Corresponding author. Tel.: +372 26204302; fax: +372 620 4301. E-mail address: juri.elken@msi.ttu.ee (J. Elken). 0924-7963/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmarsys.2013.04.015 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys