A Thermodynamic Audit of the Surface Energy Budget of the Hudson Bay Complex (HBC) from 1980 – 2024 Using the ECMWF ERA5 Climate Reanalysis Model

Abstract

This study conducts a comprehensive thermodynamic audit of the Hudson Bay Complex (HBC) from 1980 to 2024 using the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis model to quantify long-term changes in the regional surface energy budget. Results reveal significant basin-wide warming characterized by near-equal increases in net shortwave and incoming longwave radiation, despite the absence of statistically significant trends in incoming solar radiation. These changes are primarily attributed to declining surface albedo and enhanced greenhouse forcing, amplifying regional energy retention over the study period. Subsurface heat fluxes demonstrate a marked spatial redistribution, with net warming progressing northward from James Bay into central Hudson Bay and Hudson Strait. Concurrently, theice-covered season has shortened, and the open-water period has lengthened by several weeks, with mean annual sea-ice cover declining between 2–4% per decade. The observed energy imbalance reflects a shift from an ice-dominated to a thermodynamically active, open-water regime. This transition has critical implications for ocean-atmosphere coupling, primary productivity, and ecological stability within the HBC. A possible relationship between surface temperature warming and the increase in incoming longwave radiation is postulated as a main factor influencing the shifting sea-ice seasons and thermodynamic changes. Collectively, these findings demonstrate that the HBC constitutes one of the most climatically sensitive marine systems within the circumpolar Arctic.