Basin-scale topographic waves in the Gulf of Riga

Abstract

A two-dimensional circulation model has been used to test the hypothesis of whether the observed low-frequency current variations in the central Gulf of Riga, Baltic Sea, can be explained by basin-scale topographic wave response. A comparison of two-dimensional model results with measurements from a single current meter in the gulf showed good correlation. More sophisticated three-dimensional barotropic and baroclinic models provided only marginal improvement over the two-dimensional model. The model results indicate that wind-driven flow over variable bottom topography is the dominant process during moderate and strong winds. The double-gyre circulation pattern resembles the gravest basin-scale topographic wave. The free topographic wave propagates cyclonically around the basin, but does not complete a full cycle because of the shallowness of the Gulf of Riga. The evolution of the topographic wave under realistic wind conditions is analyzed using vorticity dynamics in a basin-scale sense. The topographic wave is reinforced by cyclonically rotating wind and can be destroyed most effectively by anticyclonically rotating wind. The topographic wave signature is more apparent in deep water and almost absent in shallow areas of the basin. During calm periods or under the influence of weak winds, the double-gyre circulation will evolve into predominantly cyclonic circulation.