Observations of the Vertical Structure of the Keweenaw Current, Lake Superior

Abstract

The vertical structure of the Keweenaw Current, on the south shore of Lake Superior, is studied using data from an autonomously profiling current meter moored 2 km offshore (depth = 100 m). Vertical profiles of temperature, current speed, and current direction show current direction strongly aligned east-west with the topographic contours, with currents below 50 m depth most frequently flowing eastward at speeds of 6-12 cm/s, and those above 50 m depth flowing sometimes eastward and sometimes westward at speeds of 10-20 cm/s. Principal component analysis on profiles of eastward velocity reveals two vertical modes which have approximately the barotropic and first baroclinic modal form. The dominant “barotropic” mode contains 90% and 82%, respectively, of the eastward velocity and temperature variances. Strong occurrences of this mode (full-water-column eastward flows with speeds up to 37 cm/s) are related to strong increases in the temporal integral of longshore wind stress, consistent with coastal jet theory. The “baroclinic” mode, containing 6% of the eastward velocity variance, is dominant only during periods of weak longshore wind stress. Its 4-day period is comparable with a typical interval between atmospheric frontal disturbances.