Revisiting the Role of Convective Deep Water Formation in Northern Baltic Sea Bottom Water Renewal

Abstract

Deep water renewal and ventilation of the Baltic Sea is commonly considered to occur solely via saltwater inflows from the North Sea. However, recent analysis of geophysical and sediment proxy data suggests convective deep water formation during wintertime as a second process that has contributed to the ventilation of the northern and central Baltic Sea bottom waters during cold climate periods. Here, we investigate the role of deep water formation in the northern Baltic Sea in a regional ocean circulation model. Selecting the particularly cold winter 1986/1987 as a reference period, we perform sensitivity experiments in which the atmospheric temperature forcing is changed and/or localized brine rejection on subgrid scales is accounted for through a parameterization. We study the sinking and circulation of water masses via passive tracers introduced into the model. Generally, our model results support the established view that convective deep water formation does not play a major role in Baltic Sea deep water renewal. While a reduction of air temperatures does not qualitatively change the sinking and circulation of water masses in our experiments, brine rejection could potentially lead to localized deep water formation. However, the impact is too weak to possibly change the large-scale deep water exchange between the Baltic proper and the northern Baltic sub-basins. Although being in line with established knowledge, through consideration of the model limitations, our results provide insights on how and under which circumstances convective deep water formation could potentially have occurred during cold climate periods. Plain Language Summary The transport of oxygen-rich water into the deep parts of the Baltic Sea is of crucial importance for the ecosystem, as under low-oxygen conditions (hypoxia), most organisms cannot survive and so-called dead zones are formed. The ventilation of Baltic Sea bottom waters is commonly considered to occur solely through inflows of dense, oxygenated water from the North Sea. However, recent analysis of sediment cores and geophysical data suggests the contribution of a second process during cold climate periods: convective deep water formation. This process is initiated through surface cooling or an increase of surface water salinity during sea ice formation (brine rejection) in the winter season. Here, we investigate the role of deep water formation in the northern Baltic Sea in a regional ocean circulation model. We test the influence of reduced air temperatures and consideration of strong, localized brine rejection by performing sensitivity experiments. Generally, our model results do not support the hypothesis of convective deep water formation contributing to the renewal of Baltic Sea bottom waters, but are in line with the established view. Nevertheless, our results provide insights on how and under which circumstances convective deep water formation could potentially have occurred during cold climate periods.

Investigating Hypoxic and Euxinic Area Changes Based on Various Datasets From the Baltic Sea

“…Proportionally less seasonal variations were seen in the more distant and deeper basins, which would account for their permanent large-scale hypoxia and anoxia. The northern Gotland Basin is exposed to deep water inflows from the Gulf of Finland in the east (Giesse et al, 2020). Due to the long water residence times of the northern and eastern basins, their seasonality is minimal and their reactions to the influences of winter inflows clearly delayed.…”

Deoxygenation of the Baltic Sea during the last millennium

“…Thus, both the onset and the ending of periods of deepwater anoxia in the Baltic Sea may depend on extreme sequences of saltwater inflow. Still, identifying a physical mechanism that drove the increase in hypoxia during the last millennium has been challenging so far (Schimanke et al, 2012;Giesse et al, 2020;Moros et al, 2020).…”