A three‐dimensional numerical model of deep ventilation in temperate lakes

Abstract

Because of the variation in the temperature of maximum density with depth, it is known that the upper (above 250 m) and lower water columns circulate separately in deep temperate lakes. While near‐surface water overturns twice per year, the deep waters often mix only partially during a complete annual cycle. It is believed that deep convection is triggered by storm surge, forcing of some of the relatively cold upper water column downward through its compensation depth, so that it becomes unstable and sinks. The scale and intensity of the resulting deep water forming plumes are studied numerically. A high‐resolution model based on the nonhydrostatic Boussinesq equations is used. Deep water formation is initiated by applying a statically unstable, initial temperature profile over various regions of the domain at depths below 300 m. The grid spacing is small enough to resolve individual plumes which carry surface water to the lake bottom. The domain of study is also large enough that the geostrophic Eady (1949) wave breakup of newly formed deep water can be observed. It is found that when the initial instability is applied in midbasin, remote from the solid sidewalls, relatively little fresh deep water is formed and the fluid quickly reaches a geostrophic and hydrostatic balance. Vigorous vertical mixing ceases after 4 days, and the resulting baroclinic fluid has a well‐defined and predictable scale. In contrast, when the initial instability is applied immediately adjacent to a solid boundary, vigorous plume motion continues for the duration of the numerical simulations (12 days), producing a much greater volume of fresh deep water. A parametric study investigates the scale and intensity of this boundary mixing. It is found that reduced surface water temperature, Coriolis acceleration, or higher horizontal diffusion coefficients increase the rate of deep water renewal.