Under-ice convective regimes driven by sunlight and sediment temperature control water-ice heat flux.

Seasonal and perennial lake ice covers are declining, yet the mechanisms driving these changes remain insufficiently constrained. While atmospheric forcing is comparatively well understood, under-ice thermal dynamics-driven by sediment heat and solar radiation-are less certain. These energy inputs vary widely across lakes and seasons, and their contributions to ice melt are poorly quantified. We develop a minimal conceptual model that captures three essential elements of under-ice dynamics: the nonmonotonic equation of state, sediment-water heat exchange, and solar heating. Combining theory with direct numerical simulations informed by field observations, we map the under-ice dynamics, revealing five distinct regimes-one conductive and four convective-including three previously unrecognized. For each regime, we derive scaling laws linking governing parameters to the upward heat flux transferred to the ice. This framework provides a predictive foundation for quantifying under-ice heat transfer and parameterizing climate models, with implications for climate projections, biogeochemical cycling, and ecosystem dynamics in cryospheric waters.