"''DEFINITION'''

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 (BALTIC_OMI_WMHE_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).

'''CMEMS KEY FINDINGS'''

Major Baltic Inflows in 1993, 2002 and 2014 (BALTIC_OMI_WMHE_mbi_bottom_salinity_arkona_bornholm) show a very clear signal in the Gotland Basin, where Major Baltic Inflow events affect the water salinity, temperature and dissolved oxygen conditions up to 100-m depth. Each of the Major Baltic Inflows results in the increase of deep layer salinity in the Gotland Basin right after the event occurs, but maximum bottom salinities are detected about 1.5 years later. The periods with elevated salinity are rather long-lasting after the Major Baltic Inflows (about three years). Since 2017, the salinity below 150 m depth has decreased, but the halocline has pushed upwards, which indicates saline water transport to the intermediate layers of the Gotland Basin. Usually, temperature drops right after the Major Baltic Inflow occurs, which indicates that cold water from adjacent upstream areas submerges to the bottom in the Gotland Deep. During the period of 1993-1997, deep water temperature stayed relatively low (less than 6 °C). Starting from 1998, the deep water has become warmer. Even moderate inflows, like in 1997/98, 2006/07 and 2018/19 brought warmer water to the bottom layer of the Gotland Basin. Since 2019, warm water (more than 7 °C) has occupied the layer below 100-m depth. Compared to the year 1993, the water temperature below the halocline has increased about 2 °C. Also, the temperature of the cold intermediate layer has increased over the period 1993-2022. Oxygen concentrations start to decline quite rapidly after the temporary oxygenation of the bottom waters. In 2014, the reasons were the lack of smaller inflows after the Major Baltic Inflow that could supply more oxygenated water to the Gotland Basin (Neumann et al., 2017) and intensification of biological oxygen consumption (Savchuk, 2018; Meier et al., 2018). In addition, warm water has facilitated oxygen consumption in the deep layer and an enhancement of anoxia. In 2022, oxygen was completely consumed below the depth of 75 metres.

'''Figure caption'''

Profiles of salinity (a), temperature (b) and dissolved oxygen concentration (c) for the period of 1993-2022 in the Gotland Basin from the Copernicus Marine Service Baltic Sea in situ multiyear and near real time observations (INSITU_BAL_PHYBGCWAV_DISCRETE_MYNRT_013_032).

'''DOI (product):'''

https://doi.org/10.48670/moi-00210