Concurrent warming, freshening and cessation of deep convection in the Labrador Sea raised its sea level to a record high

The Labrador Sea, a major North Atlantic carbon sink and source of ventilated intermediate-depth water masses, is a vital component of the global climate system. Since the 1950s, it has seen significant heat and freshwater content shifts, resulting in arguably the largest full-depth oceanic temperature and salinity changes ever recorded. Here, we quantitatively assess the relative contributions of these changes to sea level variability. Using satellite altimetry in conjunction with profiling Argo float and ship-based hydrographic measurements, we show that between 2017 and 2025 the central Labrador Sea experienced an exceptionally fast sea level rise to record high. Six concurrent factors contributed to this – reduced winter cooling, enhanced summer warming, anomalous freshening, ceased deep convection, reduced deep-water density, and water-column mass gain. The temperature-driven sea level changes are controlled by surface heat fluxes. The salinity effects switched from counterbalancing temperature effect (1948–2015) to reinforcing (2015–2023), making the unprecedented Labrador Sea freshening and feeding it extreme Arctic sea ice losses (with a two-year lag) essential contributors to the 2017–2025 sea level rise.