Abstract. Circulation plays an essential role in the creation of physical and biogeochemical fluxes in the Baltic Sea. The main aim of the work was to study the quasi-steady circulation patterns under prevailing forcing conditions. Six months of continuous vertical profiling and fixed-point measurements of currents, two monthly underwater glider surveys, and numerical modelling were applied in the central Baltic Sea. The vertical structure of currents was strongly linked to the location of the two pycnoclines: the seasonal thermocline and the halocline. The vertical movements of pycnoclines and velocity shear maxima were synchronous. The quasi-steady circulation patterns were in geostrophic balance and high-persistent. The persistent patterns included circulation features such as upwelling, downwelling, boundary current, and sub-halocline gravity current. The patterns had a prevailing zonal scale of 5–60 km and considerably higher magnitude and different direction than the long-term mean circulation pattern. Northward (southward) geostrophic boundary current in the upper layer was observed along the eastern coast of the central Baltic in the case of southwesterly (northerly) wind. The geostrophic current at the boundary was often a consequence of wind-driven, across-shore advection. The sub-halocline quasi-permanent gravity current with a width of 10–30 km from the Gotland Deep to the north over the narrow sill separating the Farö Deep and Northern Deep was detected in the simulation, and it was confirmed by an Argo float trajectory. According to the simulation, a strong flow, mostly to the north, with a zonal scale of 5 km occurred at the sill. This current is an important deeper limb of the overturning circulation of the Baltic Sea. The current is stronger with northerly winds and restricted by the southwesterly winds. The circulation regime has an annual cycle due to seasonality in the forcing. Boundary currents are stronger and more frequently northward during the winter period. The sub-halocline current towards the north is strongest in March–May and weakest in November–December.

Abstract. Circulation plays an essential role in the creation of physical and biogeochemical fluxes in the Baltic Sea. The main aim of the work was to study the quasi-steady circulation patterns under prevailing forcing conditions. A total of 6 months of continuous vertical profiling and fixed-point measurements of currents, two month-long underwater glider surveys, and numerical modeling were applied in the central Baltic Sea. The vertical structure of currents was strongly linked to the location of the two pycnoclines: the seasonal thermocline and the halocline. The vertical movements of pycnoclines and velocity shear maxima were synchronous. The quasi-steady circulation patterns were in geostrophic balance and highly persistent. The persistent patterns included circulation features such as upwelling, downwelling, and boundary currents, as well as a sub-halocline gravity current. The patterns had a prevailing zonal scale of 5–60 km as well as considerably higher magnitude and different direction than the long-term mean circulation pattern. A northward (southward) geostrophic boundary current in the upper layer was observed along the eastern coast of the central Baltic in the case of southwesterly (northerly) wind. The geostrophic current at the boundary was often a consequence of wind-driven, across-shore advection. The sub-halocline quasi-permanent gravity current with a width of 10–30 km from the Gotland Deep to the north over the narrow sill separating the Fårö Deep and Nothern Deep was detected in the simulation, and it was confirmed by an Argo float trajectory. According to the simulation, a strong flow, mostly to the north, with a zonal scale of 5 km occurred at the sill. This current is an important deeper limb of the overturning circulation of the Baltic Sea. The current was stronger with northerly winds and restricted by the southwesterly winds. The circulation regime had an annual cycle due to seasonality in the forcing. The boundary current was stronger and more frequent northward during the winter period. The sub-halocline current towards the north was strongest in March–May and weakest in November–December.

Modern research methods enable unfolding the structure of the water column with higher resolution than ever, revealing the importance of submesoscale. Submesoscale processes have intermediate space and time scales of <5 km and a few days in the Baltic Sea. A glider mission was conducted in the Gulf of Finland in May 2018. The appearance of a mesoscale front as a response to the persisting NE–E winds was observed. Within the front, smaller scale features at a lateral scale of a km were apparent. The tracer patterns indicated the presence of two adjacent motions – cold (warm) water penetrating upward (downward) on the lighter (denser) side of the front. We suggest they were traces of ageostrophic secondary circulation emerging while the loss of the upwelling-favorable forcing arrested the strengthening of the front. The analysis showed favorable conditions for the baroclinic and wind-driven instability. Such circulations could work to equalize the differences in cross-front direction, affecting the stratification and acting against the persistence of the mesoscale front. The spatial spectra of isopycnal tracer variance revealed the depth-dependence of the spectral slopes at the lateral scales of 1–10 km in the upper part of the water column. The differing of the slopes in the density layers associated with the mesoscale front indicates that frontal dynamics contribute to the energy cascade.

Modern research methods enable unfolding the structure of the water column with higher resolution than ever revealing the importance of submesoscale. Submesoscale processes have intermediate space and time scales of <5 km and a few days in the Baltic Sea. A glider mission was conducted in the Gulf of Finland in May 2018. During the formation of the seasonal thermocline characterized by the increase of the maximum temperature gradient from 0.5 to 3.1 °C dbar -1 , the structure of the water column changed constantly. A horizontal buoyancy gradient was captured and the vertical flows at a lateral scale of a km were apparent in isotherms intersecting with isopycnals. We suggest that frontal submesoscale processes manifested as smaller-scale tracer patterns on the mesoscale background demonstrate an ageostrophic secondary circulation. While the front appeared because of the persisting NE-E winds, the decrease in wind stress promoted the non-forced dynamics that, in turn, appeared to enhance the development of the thermocline. The spatial spectra of isopycnal tracer variance revealed the slopes a bit gentler than -2 between the lateral scales 2-10 km in the upper part of the water column. The discrepancy from the deeper layers where the slope closed to -1 suggests the contribution of the ageostrophic frontal effects in the energy cascade.

&lt;p&gt;Eutrophication and consequent increase in biomass production and sedimentation of organic material cause oxygen depletion of the deep layers and an increase in hypoxic bottom areas in the Baltic Sea.&lt;/p&gt;&lt;p&gt;The Baltic Sea &amp;#8211; a semi-enclosed brackish sea &amp;#8211; has restricted water exchange with the North Sea. High fresh water runoff and sporadic inflows of saline water through the Danish Straits maintain stratification. Seasonal thermocline and quasi-permanent halocline, their vertical location, shape and strength are sensitive to atmospheric forcing and influence the oxygen depletion in the near-bottom layer. Physical processes altering deoxygenation in the three sub-basins of the Baltic Sea (Baltic Proper, Gulf of Finland and Gulf of Riga) are under scope of the present overview. Permanent halocline is present in the deep Baltic Proper, while in the Gulf of Finland, it occasionally vanishes during winter. Complete mixing occurs in each winter in the shallow Gulf of Riga separated from the Baltic Proper by the sill. We show that the bathymetry, combined with physical drivers, causes distinct spatial and temporal patterns of oxygen depletion in the basins. The results presented here are a summary of in-situ measurement campaigns conducted by the research vessel, underwater glider, autonomous vertical profiler and bottom moorings in 2011&amp;#8211;2020.&lt;/p&gt;&lt;p&gt;Large barotropic inflows from the North Sea temporarily ventilate the deep layer of the Central Baltic Proper, but rather intensify hypoxia in the Northern Baltic Proper and the Gulf of Finland. Wind-driven estuarine circulation alterations shape the hypoxic area and volume in the Gulf of Finland considerably. Seaward winds support estuarine circulation and the advection of hypoxic saltier water of the Northern Baltic Proper into the gulf deep layer. The landward wind can reverse estuarine circulation, the collapse of stratification and mixing of the whole water column in winter (when the seasonal thermocline is absent), thus, temporarily improving oxygen conditions in the deep layer of the gulf. Intrusion of cold saltier water of the Baltic Proper over the sill into the Gulf of Riga deep layer strengthens water column stratification and supports hypoxia formation in summer. Such a water exchange regime is related to the northerly wind forced upwelling along the eastern coast of the Baltic Proper. The role of submesoscale processes on vertical mixing and deep layer ventilation is still unclear, and the data of high-resolution in situ measurements in the Baltic Sea is limited yet. Preliminary results from the dedicated underwater glider surveys conducted at the coastal slope of Eastern Baltic Proper in 2019-2020 will be presented.&lt;/p&gt;