Thermobaric Processes Both Drive and Constrain Seasonal Ventilation in Deep Great Slave Lake, Canada

Abstract

AbstractThe contrasting roles of seasonal stratification and seasonal ventilation are among the most important attributes of lake systems in defining their geochemistry, biology and response to climate forcing. The physical processes that regulate these processes are especially significant in very deep lakes, where the joint effect of temperature and pressure on water density becomes important. Here we report on a year‐long time series (2019–2020) of temperature from a mooring deployed in seasonally ice covered and dimictic Great Slave Lake (614 m depth). Because the temperature of maximum density (TMD) decreases with increasing pressure, once ∼4°C water begins to sink, it is no longer at its depth‐specific maximum density and additional processes are required to drive convection and convective mixing. The key physical mechanism governing deep‐water renewal is the conditional thermobaric instability. We show here that (a) different temperature dynamics control the quantity and timing of convective renewal in the upper ∼200 m versus deep‐water renewal below; (b) fall and spring deep‐water renewal events are asymmetric and linked to weather during the brief time of ice forming (melting) and surface temperatures cooling (warming) through 4°C; (c) short term variability (days to weeks) related to the length of time between surface waters passing through TMD and ice formation (fall) and melt (spring) shapes the subsequent thermal structure; and (d) the penetration of solar radiation through ice in early spring drives penetrative convection in the upper layer, deepening the mixed‐layer, and this likely affects the timing of spring phytoplankton production.