Modeling changes in ice dynamics and subsurface thermal structure in Lake Michigan-Huron between 1979 and 2021

The world's largest lakes, including the Laurentian Great Lakes, have experienced significant surface warming and loss of ice cover over the last several decades. Although changing surface conditions have received substantial research interest, changes below the surface remain largely unexplored, despite their importance for turbulent mixing, nutrient cycling, and primary production. In this study, we investigate changes in subsurface thermal structure and timing in Lake Michigan-Huron related to ongoing climate warming. This work utilizes atmospheric reanalysis data to drive the Great Lakes Finite Volume Community Ocean Model (GL-FVCOM), providing three-dimensional hydrodynamic and ice simulations between 1979 and 2021. Results are used to analyze trends in ice and temperature dynamics, revealing significant changes in annually averaged ice cover (- 2.1- - 5.2%/decade), ice thickness (- 0.68 --2.0 cm/decade), surface temperature (+0.47-+ 0.51 ?C/decade), and bottom temperature (+ 0.26- + 0.29 ?C/decade) over the last 40 years, especially in ecologically important bays (e.g., Green Bay, Saginaw Bay). Significant warming was observed at all depth layers (0-270 m), with warming trends in the epilimnion and hypolimnion that agreed well with recent analysis of observational data in Lake Michigan. Shifting stratification dynamics led to dramatic changes in modelled overturning behavior, and earlier spring turnover dates (- 2.2- - 7.5 days/decade) and later fall turnover dates (+ 2.5- + 6.3 days/decade) led to a net lengthening of the stratified period. This study presents one of the most comprehensive analyses of changes in Great Lakes subsurface temperatures to date, providing important context for future climate modelling and coastal management efforts in the region.