Wind-driven, coastal-trapped waves off the island of Gotland, Baltic Sea

An analysis is presented of repeated density and current profiles and recor ding current meter (RCM) data collected off the northwest coast of Gotland Island (Baltic Sea) during the late summer and fail of 1977. Large, low-fre quency fluctuations were found in alongshore flow and isopycnal displacemen ts and were significantly correlated with winds observed at Gotland's west coast. The best correlations were found with winds from the island's southe rn lip 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), w hereby how at 90 m led flow at 70 m. An EOF analysis of profile observation s showed a 7-day, baroclinic wave with a two- and a three-layer current str ucture "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 sc attering) was applied to the Gotland west coast and model output was compar ed with data. This application assumed zero CTW amplitude at the island's s outhern tip, the starting point for the forced-wave integration, and used t wo coastal segments, a late summer and a winter stratification, and winds f rom three coastal sites. A very simple model version (first two CTW modes o nly, no bottom friction or scattering, winds from the island's southern tip only) was able to reproduce reasonably well structure, amplitudes, and pha ses of observed alongshore current fluctuations in the lower part of the wa ter column. Observed upward phase propagation was explained by lagged super position of the two CTW modes. In the strongly stratified Baltic Sea, buoya ncy forces may act to reduce the effects of bottom friction on low-frequenc y flow. A combination of island length scale and synoptic wind scales act t o "filter out" higher CTW modes. The effects of scattering were greatest fo r weaker, winter stratification.