During the last decades, the Baltic Sea has been among the fastest warming seas in the world. The warming is mainly driven by increasing air temperatures but deeper water layers can also be warmed by lateral advection of heat. By analyzing a 159 years long (1850–2008) hindcast simulation of the Baltic Sea, we link the exceptionally strong bottom water warming in the western Baltic Sea to a shift in the seasonality of saltwater inflows from the North Sea to the Baltic Sea. Over the model period, warm summer and early autumn inflows have increased while cold winter inflows have decreased. Sensitivity experiments reveal that these changes were partly driven by a shift in river runoff seasonality. The strong warming could lead to faster oxygen depletion in the affected layers and thus have ecological consequences.
N2 - Plain Language Summary: The Baltic Sea is home to various marine and freshwater species and an important economic factor for the surrounding countries. Like other seas, the Baltic Sea is getting warmer due to climate change. The water at the surface warms especially fast because it takes up heat from the warming atmosphere. After some time, temperatures also increase in deeper layers. However, some deep parts in the western Baltic Sea are warming even faster than the sea surface. In our study, we investigate if the exceptional warming can be explained by an increase in warm saltwater inflows from the North Sea. Hence, we use a model simulation of the Baltic Sea for over 150 years to compare long time series of warm inflows and the temperatures in the deep layers of the western Baltic Sea. We find a strong correlation. Thus, we can link the exceptional warming in the deep layers of the western Baltic Sea during the last decades to an increase in warm inflows. The warming has ecological consequences since in warmer water, the oxygen is consumed faster and the deep water layers of the Baltic Sea are suffering from low oxygen concentrations. N2 - Key Points: Summer and early autumn salt import into the Baltic Sea increased significantly since 1851 compared to the annual salt import.Salt import between June and October is highly correlated with the annual sub‐thermocline temperature maximum in the western Baltic Sea.
The shift in inflow seasonality was partly caused by seasonal changes in river runoff.