Warming of Baltic Sea water masses since 1850

The Baltic Sea is among the fastest-warming seas globally in recent decades affecting biogeochemical conditions such as euxinic areas but also pelagic and benthic marine ecosystems. It is therefore crucial to understand how this heat gain is distributed vertically. We used reconstructed atmospheric forcing fields for 1850–2008 to perform an ocean climate simulation that adequately captures climatogical temperature and salinity profiles. Then, a water mass classification distinguishes three water masses corresponding to the classical view, warm and fresh surface waters, cold and fresh intermediate waters, and cold and salty bottom waters, and two transition water masses. The temperature trends show a similar three layers pattern with fast warming at the surface (~ 0.06 K decade − 1 ) and bottom (> 0.04 K decade − 1 ) and slow in the intermediate layers (< 0.04 K decade − 1 ). The slow warming in the intermediate layer is explained by both weakly warmed water winter convection and the summer surface thermocline isolating the intermediate layers. The warming in the deep layers is related to warm surface inflow from the North Sea and Baltic proper in the southern and northern Baltic Sea respectively. Furthermore, sensitivity experiments show that the warming magnitude is controlled by rising air temperature while the vertical distribution of heat gain is related to surface wind conditions. Finally, the North Atlantic Oscillation and Atlantic Multidecadal Oscillation are well correlated with the temperature minimum and thus modulate the magnitude of warming in the intermediate layers on shorter time scales. This study provides a new picture of the Baltic Sea’s warming and suggests that this complexity is essential for understanding the influence of climate change on marine ecosystems.