Current structure, circulation and transport in the Central Baltic Sea observed by array of moorings and gliders

Abstract. An array of moored current meters and gliders were deployed across the Central Baltic Sea. Weak and low persistent currents driven by the wind-driven Ekman transport were observed in the upper layer in the offshore area. Stronger currents with higher persistency were observed near the coast, presumably driven by the local sea level gradient. The kinetic energy was largely observed to be in the low-frequency band (timescales >36 h), but higher kinetic energy on shorter timescales was observed in the areas, where the halocline was close to the seafloor. A strong and highly persistent gravity current was observed in the near bottom layer of the Fårö sill. Circulation pattern, current structure, and meridional transport varied on synoptic and seasonal timescale due to changes in local forcing and caused changes in the water column habitats. A low persistent, cyclonic gyre in the upper layer and meridional transport to the north in the deep layer prevailed in the seasonal (3–4 months) timescale. Circulation patterns with higher persistency formed under stable forcing. A cyclonic circulation in the upper layer was supported by southerly winds and reversed by northerly winds. Northward transport in the deep layer was intensified by northerly winds and reversed by southerlies. Due to the seasonality in the meridional wind component, the pattern associated with northerly (southerly) wind is more common in spring and summer (autumn and winter).

The deep-water transport is approximately 400 km³ y^-1 towards the Northern Baltic Proper, which is almost 1/3 of the total deep-water volume in the Northern Baltic Proper, Gulf of Finland, and Western Gotland Basin. This transport mostly occurs during spring and summer and brings denser, oxygen-depleted water to the deep layer of these areas.