Observations of near-bottom currents in the Gulf of Riga, Baltic Sea

Abstract

Near-bottom currents and temperature were measured at 2 and 4 m above bottom (m.a.b.) in the Gulf of Riga, a semi-enclosed sub basin of the Baltic Sea. Measurements were taken during three periods of different vertical stratification and weather conditions. Water depth was 54 meters. The studied area comprised the outer region of the bottom boundary layer, as judged by the observed Ekman veering of the current vector.¶Local wind was the primary driving force of near-bottom currents in the deepest area of the Gulf of Riga. Imposed wind stress induced surface-layer flow to the right, while compensating return flow developed in the bottom boundary layer. The response of current field was dominated by low-frequency variability with a clear indication of topographic steering and minor variability due to near-inertial frequency processes. The near-inertial frequency processes were generated also by interaction of low-frequency waves with the topography. The low-frequency and near-inertial frequency processes were of equal value in producing vertical shear of horizontal velocity in the near-bottom layer. The quasi-steady flow in autumn was forced by strong net inflow to the Gulf of Riga through the Irbe Strait. Lateral advection of water is important for the deep-water renewal that is due to dominating low-frequency currents.