Wind‐induced internal seiches in Lake Zurich observed and modeled1

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Abstract
During August and September 1978, 31 current meters and 120 temperature sensors were deployed to record every 10 or 20 min at various depths at 12 moorings (with wind speed and direction at three moorings) in Lake Zurich. We explore here the baroclinic (internal seiche) response to wind impulses, observed as fluctuations in isotherm depth and current speeds. Those fluctuations and their energy spectra are compared with the predictions of two models fitted to basin topography and to the observed average thermal structure: a two‐layered variable‐depth (TVD) model developed by D. J. Schwab, fitted to basin topography and incorporating a two‐dimensional horizontal grid, and C. H. Mortimer’s two‐layered modification of a simpler procedure originally developed for surface seiche calculations by A. Defant.The dominant responses to wind impulses were internal seiches of the first longitudinal mode (average period 44 h). Weaker signals from the second (24 h) and third (17 h) modes were also seen in spectra of temperature and current fluctuations. The models displayed patterns of thermocline displacement and current which, in periodicity and structure, were closely similar to those observed. Predictions of the Defant model were less precise, particularly for current.Founded on linear theory and neglecting the effects of rotation, the models were unable to reproduce two features occasionally seen in the lake motions: clockwise or anticlockwise rotation of current direction; and internal surges arising when storms induced large‐amplitude downstrokes of the thermocline at one basin end or the other. The lake’s internal response was principally dependent on the timing, strength, and duration of the wind impulse, relative to and interacting with internal seiche motions already in progress.