Surface Normal Modes of Lake Michigan: Calculations Compared with Spectra of Observed Water Level Fluctuations

Abstract

Periods and structures of several normal modes of Lake Michigan (including Green Bay) are calculated theoretically, taking into account the Lake's topography and the earth's rotation. The calculations are based on a Galerkin method developed by Rao and Schwab (1976). Even though the calculations give both rotational and gravitational modes, attention is focused primarily on the latter. The calculations show that there are several modes dominant in the main basin of Lake Michigan and some dominant in Green Bay. The lowest Lake Michigan mode has a period of 9.27 h. Green Bay exhibits a (co-oscillating or Hemlholtz) mode with a period 10.35 h. For the modes dominant in the main basin, the periods and structures obtained from theoretical calculations are compared to those deduced from spectral analyses of water level data from various stations around the Lake. The agreement is found satisfactory for several of the lowest modes.