Observed flow variability along the thalweg, and on the coastal slopes of the Gulf of Finland, Baltic Sea

Abstract Bottom-mounted ADCP measurements from 10 installations, collected between 2009 and 2014 and each lasting several months, are analysed in order to distinguish between different flow regimes, and to detect variability (a) along the thalwegof the elongated basin, with different regimes in summer and in winter, and (b) on the coastal slopes. In the deep thalwegarea the mean flow speed amounts to 6–13 cm s −1 , whereas the maximum speeds appear in winternear the bottom of the basin, and in summer within the halocline(around 70 m depth). The mean zonal flowcomponent reveals a nearly depth uniform inflowduring winter, and a layered inflow-outflow during summer. In years where up-estuary (W to SW) winds are stronger during the summer, inflowdominates in upper layers, and anti-estuarine outflow dominates in deeper layers. This causes the export of a salt wedge, and the weakening of haline stratification. Infra-low frequency zonal currents (i.e. excluding topographic waves etc. with periods of less than 10 days) have a structure which is uniform with depth for 53% of the time in winter; in summer, a layered structure is present 65% of the time. However, during both periods the reversed estuarine flow ( inflowin upper layers and outflow in the bottom layer) appears, on average, for 30% of the time. The deep flow zonal component is well correlated with westward winds during summer ( r = 0.84), and south-westward winds during winter( r = 0.77). On the coastal slopes, the speed of the currents are lower than in the thalwegregion, and they decay with depth. In the vertical the flow exhibits a layered structure in both the winterand summer seasons.