What controls the onset of winter stratification in a deep, dimictic lake?

Abstract

The transition from summer stratification to winter stratification is considered for deep, dimictic Lake Superior. The fall transition is dynamically distinct from the better‐studied spring transition; it is characterized by a wind‐driven collapse of weakening summer stratification, a surface‐cooling–driven period during which the water column is essentially isothermal, and eventual onset of inverse (cold upper layer) stratification, after which sub‐thermocline water temperatures are largely fixed for the rest of the season. Due to the small value of thermal expansivity near the temperature of maximum density, temperature gradients do not impart much stability to the water column, and it is difficult for winter stratification to form immediately upon dropping below the temperature of maximum density. Instead, the water column continues to cool until the thermal expansivity is sufficiently large in magnitude to allow temperature gradients to impart stability. This observation implies that the temperature locked into the deep water for the winter season will be a consequence of the specific meteorological forcing experienced in a given year; there may be long‐term climate change driven trends in the timing of the onset of stratification but not trends in winter deep‐water temperature. To address this hypothesis, 15 yr of moored temperature data collected in Lake Superior are examined, and a scaling argument and climatological data are applied to better understand interannual variability in the onset of winter stratification.