Baltic Sea Major Baltic Inflow: bottom salinity from Reanalysis

'''DEFINITION'''

Major Baltic inflow bottom salinity ocean monitoring indicator was introduced in Copernicus Marine Service Ocean State Report, Issue 2 (Raudsepp et al, 2018) and is derived from regional reanalysis product BALTICSEA_MULTIYEAR_PHY_003_011. Major Baltic Inflows bring large volumes of saline and oxygen-rich water into the bottom layers of the deep basins of the Baltic Sea- Bornholm basin, Gdansk basin and Gotland basin. The Major Baltic Inflows occur seldom, sometimes many years apart (Mohrholz, 2018). The Major Baltic Inflow OMI consists of the time series of the bottom layer salinity in the Arkona basin and in the Bornholm basin and the time-depth plot of temperature, salinity and dissolved oxygen concentration in the Gotland basin (OMI_HEALTH_TEMPSALOXY_BALTIC_mbi_sto2tz_gotland). Bottom salinity increase in the Arkona basin is the first indication of the saline water inflow, but not necessarily Major Baltic Inflow. Abrupt increase of bottom salinity of 2-3 units in the more downstream Bornholm basin is a solid indicator that Major Baltic Inflow has occurred.

'''CONTEXT'''

The Baltic Sea is a huge brackish water basin in Northern Europe whose salinity is controlled by its freshwater budget and by the water exchange with the North Sea (e.g. Neumann et al., 2017). The saline and oxygenated water inflows to the Baltic Sea through the Danish straits, especially the Major Baltic Inflows, occur only intermittently (e.g. Mohrholz, 2018). Long-lasting periods of oxygen depletion in the deep layers of the central Baltic Sea accompanied by a salinity decline and the overall weakening of vertical stratification are referred to as stagnation periods. Extensive stagnation periods occurred in the 1920s/1930s, in the 1950s/1960s and in the 1980s/beginning of 1990s Lehmann et al., 2022). Bottom salinity variations in the Arkona Basin represent water exchange between the Baltic Sea and Skagerrak-Kattegat area. The increasing salinity signal in that area does not indicate that a Major Baltic Inflow has occurred. The mean sea level of the Baltic Sea derived from satellite altimetry data can be used as a proxy for the detection of saline water inflows to the Baltic Sea from the North Sea (Raudsepp et al., 2018). The medium and strong inflow events increase oxygen concentration in the near-bottom layer of the Bornholm Basin while some medium size inflows have no impact on deep water salinity (Mohrholz, 2018). 

'''KEY FINDINGS'''

Time series data of bottom salinity variations in the Arkona Basin are instrumental for monitoring the sporadic nature of water inflow and outflow events. The bottom salinity in the Arkona Basin fluctuates between 11 and 25 g/kg. The highest recorded bottom salinity value is associated with the Major Baltic Inflow of 2014, while other significant salinity peaks align with the Major Baltic Inflows of 1993 and 2002. Low salinity episodes in the Arkona Basin mark the occasions of barotropic outflows of brackish water from the Baltic Sea. In the Bornholm Basin, the bottom salinity record indicates three Major Baltic Inflow events: the first in 1993, followed by 2002, and the most recent in 2014. Following the last Major Baltic Inflow, the bottom salinity in the Bornholm Basin rose to 20 g/kg. Over the subsequent nine years, it has declined to 16 g/kg. The winter of 2024/25 did not experience a Major Baltic Inflow.

'''DOI (product):'''
https://doi.org/10.48670/moi-00209