Wind-Driven, Coastal-Trapped Waves off the Island of Gotland, Baltic Sea

Abstract

An analysis is presented of repeated density and current profiles and recording current meter (RCM) data collected off the northwest coast of Gotland Island (Baltic Sea) during the late summer and fall of 1977. Large, low-frequency fluctuations were found in alongshore flow and isopycnal displacements and were significantly correlated with winds observed at Gotland?s west coast. The best correlations were found with winds from the island?s southern tip rather than with local winds. Coherence was high between RCM current fluctuations at 70 and at 90 m (100 m water depth, 4 km from the coast), whereby flow at 90 m led flow at 70 m. An EOF analysis of profile observations showed a 7-day, baroclinic wave with a two- and a three-layer current structure ?trapped? in the coastal zone. The time series of the (weaker) three-layer mode was found to lag that of the (stronger) two-layer mode by about one day. Wind-forced, coastal-trapped wave (CTW) theory (with bottom friction and scattering) was applied to the Gotland west coast and model output was compared with data. This application assumed zero CTW amplitude at the island?s southern tip, the starting point for the forced-wave integration, and used two coastal segments, a late summer and a winter stratification, and winds from three coastal sites. A very simple model version (first two CTW modes only, no bottom friction or scattering, winds from the island?s southern tip only) was able to reproduce reasonably well structure, amplitudes, and phases of observed alongshore current fluctuations in the lower part of the water column. Observed upward phase propagation was explained by lagged superposition of the two CTW modes. In the strongly stratified Baltic Sea, buoyancy forces may act to reduce the effects of bottom friction on low-frequency flow. A combination of island length scale and synoptic wind scales act to?filter out? higher CTW modes. The effects of scattering were greatest for weaker, winter stratification.