Sensitivity of the overturning circulation of the Baltic Sea to climate change, a numerical experiment

the Danish Straits give the Baltic Sea typical sill estuarine dynamics. Although many dynamical features of the Baltic Sea are geostrophic, due to its size, it is still an estuary. As for any estuarine structure, the amount of freshwater delivered to the estuary generates an overturning circulation in the same sense as that described by Garvine and Whitney (2006). This means that the delivery of freshwater can be considered as a delivery of vorticity or energy (Hordoir et al. 2008). For this reason, one can define an overturning circulation in the Baltic Sea (Döös et al. 2004), which is much larger than the amount of runoff delivered to the estuary or coastal area. Of course any estuarine or freshwater driven circulation is influenced by many parameters, the most important ones being wind forcing (Fong and Geyer 2002; Hordoir et al. 2006) and tidal currents (Hordoir et al. 2006). In the case of the Baltic Sea, tides are negligible but wind forcing does play an important role in its overturning circulation. Wind forcing over the Baltic and North Seas, and especially wind forcing variability, drive Baltic Sea salt inflows and so-called “Major Baltic Inflows” (MBI hereafter) (Matthäus 2006). MBIs are the drivers of the haline conveyor belt defined by Döös et al. (2004). MBIs are spurious phenomena of which the haline conveyor belt is the long-term result, MBIs and the haline conveyor belt in the Baltic Sea are two different visions of the same thing. Many questions have arisen during the recent decades as the number of MBIs has decreased. After a stagnation period from 1983 to 1993, several MBIs have occurred but their frequency has been lower than during the 1960s and 1970s. Then MBIs occurred during three consecutive years (2014, 2015 and 2016), but it is yet too early to say if this latest series is part of a longer trend. The future of Baltic inflows is a subject of importance, and several studies have tried to identify whether changes of wind patterns related with a changing climate could be responsible for Abstract An ocean model covering the Baltic Sea area is forced by several climate scenarios for a period extending from 1961 to 2100. The Baltic Sea overturning circulation is then analyzed. The analysis shows that this circulation decreases between the end of the 20th century and the end of the 21st century, and that the decrease is amplified in the case of the strongest greenhouse gas emission scenarios, which corresponds with the highest warming cases. The reasons behind this decrease in overturning circulation are investigated. A strong increase of thermal stratification is noticed at the level of the Baltic Sea mixed layer. Based on buoyancy flux considerations, we demonstrate that the decrease in overturning circulation coincides with the increase of thermal stratification. Evidence shows that the underlying process is linked to a smaller erosion of the halocline due to a higher shielding, itself linked with a stronger and longer seasonal thermocline. This theory works if surface wind mixing is not taken into account directly in the computation of buoyancy fluxes.