High-frequency water level fluctuations in Lake Michigan

During certain meteorological events, water level oscillations up to 1.5 m with periods of less than 2 It have been observed in the Great Lakes. The squall line events of 7-11 March 1998, 29 May-2 June 1998, and 8-12 November 1998 in Lake Michigan, are analyzed by spectral and analytical methods. Dominant periods of less than 2 h are identified in the spectra of water level fluctuations and coherencies among the spectral peaks of water levels of different station pairs are calculated to determine whether the oscillations are localized or basin wide. Explicit numerical calculations of normal mode periods and structures using a Lanczos procedure show that the dominant periods in the observed data are consistent with the structures and periods of some of the calculated modes. The March 1998 and the November 1998 episodes show higher surges with a gradual rise of water level, while the episode of May 1998 shows an abrupt rise in the water level at Calumet Harbor and about ten times higher spectral signature than the former two. Many of the high-frequency modes have large amplitude at or near Calumet Harbor and the periods are close to the periods of edge waves that would be generated by a squall line similar to the May 1998 squall line. The trapping of energy and localized higher modes in a water body can work together to excite edge waves and localized seiches causing abrupt water level fluctuations.