Drivers of Deep-Water Winter Cooling and Model Bias in Lake Superior

The transition from fall overturn to winter stratification governs ice formation and spring warming timing in deep dimictic lakes. Here, we investigate the drivers of deep-water warm biases reported in prior lake modeling studies during this transition in Lake Superior using subsurface mooring observations and high-quality surface flux measurements. Large-eddy simulations, a k-ε column model, and a one-dimensional analytical model were evaluated over 60-day periods. At the 212-m Eastern Mooring, incorporating thermobaricity and realistic surface flux forcing largely resolves the warm bias, though deep-water mixing differences emerge between models and with wind direction. In contrast, at the 384-m Southern Mooring, warm biases persist across all models; applied surface cooling rates prove insufficient to match observed water-column cooling. We hypothesize that horizontal processes along steep bathymetric troughs, absent in one-dimensional frameworks, drive this discrepancy. Results highlight the sensitivity of winter stratification to cooling rates, wind forcing, thermobaricity and, potentially, three-dimensional dynamics.