The Labrador Sea sets a new reference point in the subpolar North Atlantic climate record

Abstract

Abstract The Labrador Sea is the deepest, coldest and freshest subpolar North Atlantic basin. Here, open-ocean convection, driven by surface winter cooling, produces Labrador Sea Water (LSW), a dense voluminous water mass spreading across the ocean, filling and ventilating its intermediate-depth reservoir, and contributing to the Atlantic Meridional Overturning Circulation. The recent multiyear development of recurrently-deepening convection was initiated by increased winter cooling in 2012, continued past 2015, having 1994-2023-high surface cooling, and reached 2000 m, 1996–2023 record deep, in 2018. The convective deepening during 2016–2018 owes to water-column preconditioning by previous winter convections. Convection shallowed in the following winters completing the formation of the densest and largest LSW class since the mid-1990s. The most abrupt shifts in the Labrador Sea convection intensities and depths since the mid-1990s occurred in 2021 and 2023, when winter mixing could only reach 800 m and 700 m, respectfully, shallowing by more than 800 m from 2020 and 2022, and becoming the shallowest since 2011. Consequently, the entire intermediate layer has recently warmed and become less dense. The mentioned cases of abruptly shallowed convection and rapidly warmed ocean can be attributed to the 2021 and 2023 winter collapses of the Polar Vortex, which weakened and reversed the westerly winds, bringing anomalously warm air to the Labrador Sea, reducing the surface cooling to the lowest since 2010, and consequently inhibiting convection.