Warm saltwater inflows strengthen oxygen depletion in the western Baltic Sea

The Baltic Sea is one of the coastal seas worldwide that suffer most strongly from hypoxia. With its pronounced haline stratification, the Baltic Sea is naturally prone to oxygen deficiency, but the strong growth of hypoxic and anoxic regions since the middle of the 20th century was mainly driven by eutrophication due to large nutrient inputs from anthropogenic sources. Since the oxygen solubility is lower in warmer water and oxygen consumption rates are higher, increasing water temperatures due to climate change are expected to worsen oxygen conditions in the future. Today, the effects of warming are still considered minor compared to those of the high nutrient loads. However, we show, by analyzing 159-years long hindcast simulations of the Baltic Sea with three different models, that exceptionally high temperatures in certain parts of the western Baltic Sea additionally deteriorated the oxygen conditions during the course of the 20th century on an interannual scale. These high temperatures were not mainly caused by global warming but by a shift in the seasonality of saltwater inflows from the North Sea, namely an increase in warm summer and early autumn inflows and a decrease in colder winter inflows. This we can conclude from comparing the reference simulations with a sensitivity experiment that excludes global warming. Hence, we identify a new driver of hypoxia in the western Baltic Sea.