Baltic Sea Major Baltic Inflow: time/depth evolution S,T,O2 from Observations Reprocessing

'''DEFINITION'''

Major Baltic inflow time/depth evolution S,T,O2 ocean monitoring indicator was introduced in Copernicus Marine Service Ocean State Report, Issue 2 (Raudsepp et al, 2018) and is derived from in-situ observations product INSITU_BAL_PHYBGCWAV_DISCRETE_MYNRT_013_032. Major Baltic Inflows bring large volumes of saline and oxygen-rich water into the bottom layers of the deep basins of the central Baltic Sea, i.e. the Gotland Basin. These 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 (OMI_HEALTH_TEMPSALOXY_BALTIC_mbi_bottom_salinity_arkona_bornholm) and the time-depth plot of temperature, salinity and dissolved oxygen concentration in the Gotland Basin. Temperature, salinity and dissolved oxygen profiles in the Gotland Basin enable us to estimate the amount of the Major Baltic Inflow water that has reached central Baltic, the depth interval of which has been the most affected, and how much the oxygen conditions have been improved.

'''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). This implies that fresher water lies on top of water with higher salinity. The saline water inflows to the Baltic Sea through the Danish Straits, especially the Major Baltic Inflows, shape hydrophysical conditions in the Gotland Basin of the central Baltic Sea, which in turn have a substantial influence on marine ecology on different trophic levels (Bergen et al., 2018; Raudsepp et al.,2019). In the absence of the Major Baltic Inflows, oxygen in the deeper layers of the Gotland Basin is depleted and replaced by hydrogen sulphide (e.g., Savchuk, 2018). As the Baltic Sea is connected to the North Sea only through very narrow and shallow channels in the Danish Straits, inflows of high salinity and oxygenated water into the Baltic 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 overall weakening of the 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., 20225).

'''KEY FINDINGS'''

The Major Baltic Inflows of 1993, 2002, and 2014 (OMI_HEALTH_TEMPSALOXY_BALTIC_mbi_bottom_salinity_arkona_bornholm) present a distinct signal in the Gotland Basin, influencing water salinity, temperature, and dissolved oxygen up to a depth of 100 meters. Following each event, deep layer salinity in the Gotland Basin increases, reaching peak bottom salinities approximately 1.5 years later, with elevated salinity levels persisting for about three years. Post-2017, salinity below 150 meters has declined, while the halocline has risen, suggesting saline water movement to the Gotland Basin's intermediate layers. Typically, temperatures fall immediately after a Major Baltic Inflow, indicating the descent of cold water from nearby upstream regions to the Gotland Deep's bottom. From 1993 to 1997, deep water temperatures remained relatively low (below 6 °C). Since 1998, these waters have warmed, with even moderate inflows in 1997/98, 2006/07, and 2018/19 introducing warmer water to the Gotland Basin's bottom layer. From 2019 onwards, water warmer than 7 °C has filled the layer beneath 100 meters depth. The water temperature below the halocline has risen by approximately 2 °C since 1993, and the cold intermediate layer's temperature has also increased from 1993 to 2024. Oxygen levels begin to drop sharply after the temporary reoxygenation of the bottom waters. The decline in 2014 was attributed to a shortage of smaller inflows that could bring oxygen-rich water to the Gotland Basin (Neumann et al., 2017) and an increase in biological oxygen demand (Savchuk, 2018; Meier et al., 2018). Additionally, warmer water has accelerated oxygen consumption in the deep layer, leading to increased anoxia. By 2021, oxygen was completely depleted below the depth of 75 metres.

'''DOI (product):''' https://doi.org/10.48670/moi-00210