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

The 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 (T-MD) decreases with increasing pressure, once similar to 4 degrees 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 similar to 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 degrees C; (c) short term variability (days to weeks) related to the length of time between surface waters passing through T-MD 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. Plain Language Summary Seasonal water stratification and exchange of heat, water and gases with the overlying atmosphere are important attributes of lakes in defining their geochemistry, biology and response to climate change. The physical processes that regulate these are especially important in very deep lakes, like Great Slave Lake (GSL) (614 m depth). Here we use data from a yearlong mooring of thermistors in GSL (2019-2020) to investigate the physical processes that are important and show that these processes likely affects the timing of spring phytoplankton production in the lake. Key Points The key physical mechanism governing deep-water renewal in very deep Great Slave Lake is the conditional thermobaric instability Waters below the thermobaric horizon (similar to 200 m) maintain permanent stratification due to thermobaric effects and are ventilated by downwelling events and intrusions 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 degrees C