Low frequency response of coastal waters to wind forcing

• Main Author: Pietrzak, J. D.
• Published: Swansea University 1987
• Subjects: 551.46
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638531

The low frequency flow dynamics of the N.W. Aegean Sea, are investigated using a barotropic shelf circulation model, and by analysis of data from current meters deployed in Thermaikos Gulf. The performance of Groves (1955) and Lanczos-cosine filters is assessed to provide optimal low-pass filtering for the current meter records. The Lanczos-cosine filter is found to be the most effective in the present study. Time series and spectral analysis are applied to the filtered current meter records and to concurrent meteorological data. The results of these analyses show significant inputs of low frequency energy at periods of 2-4 days, 6-8 days and 20-26 days. An anomalous energy band exists that is uncorrelated with the wind and which is absent from one current meter record. The results from a depth-integrated momentum balance suggest that these bands are related to direct meteorological forcing. The 2-4 day band is related to 3-dimensional coastal water processes. The 6-8 day band is described in terms of internal Kelvin waves and the 20-26 day band is related to continental shelf waves. The anomalous band with 11-13 day period is associated with freshwater forcing from rivers. The low frequency shelf circulation of the Aegean Sea is modelled analytically. Solutions to the two-dimensional vorticity equation written in terms of stream functions are sought within a circular basin with exponential cross-shelf topography. The dispersion relations for the propagation of free waves is given. Introduction of the ageostrophic approximation allows solution to the wind-forced vorticity equation wherein, parameter separation leads to a wave equation soluble by the method of characteristics. The separation parameter is determined from the associated Sturm-Liouville equation. The damped wave problem is also solved. The model predicts the propagation of continental shelf waves, with periods of 20-26 days around the Aegean Sea.