Transverse Internal Seiches in Large Oblong Lakes and Marginal Seas

Abstract

The generation by unsteady winds of standing oscillations on the pycnocline of lakes and enclosed seas, which are much longer than they are wide, is studied on the assumption that end effects are negligible. The approach is valid for a limited period only, and then only to the exclusion of end zones. Regardless of wind direction, only odd modes are excited by a suddenly imposed, horizontally uniform wind stress. If the lake is no wider than a few times the radius of deformation in the internal mode, the excitation is heavily concentrated on the uninodal transverse seiche. In much wider lakes the excitation is evenly spread out over the first few modes having 1, 3, 5, ? nodes across the basin. Observations on Lakes Michigan and Ontario (which are ?wide? in the above sense) indeed show such multinodal internal seiches to be prominently present, in contrast with smaller lakes in which uninodal internal seiches are usually dominant. Also the amplitudes of observed internal seiches in the Great Lakes are in qualitative accord with the simple theory of their generation.