562 / 2024-09-18 13:05:10

Lagrangian Perspective on the Decadal Variability of the Subpolar AMOC

Atlantic Meridional Overturning Circulation, TRACMASS, Water Mass Transformation, Lagrangian trajectory

Session 41 - The Atlantic Meridional Overturning Circulation Variability and its Climatic Impacts

The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in redistributing mass, heat and freshwater across the globe, with significant impacts on both regional and global climate. Thus, determining the AMOC’s variability and its governing mechanisms is of high priority in understanding current climate and its change. On decadal time scales, many studies underline a critical role of the Labrador Sea convection as the determinant of the AMOC variability. Specifically, stronger convection enhances deep water production and subsequent southward export, leading to a strengthened AMOC. What is less clear in this process is the three-dimensional evolution of water mass pathways and property changes. In this study, we aim to provide a three-dimensional view of the subpolar AMOC and its decadal changes by simulating Lagrangian trajectories in an ocean circulation model and a coupled climate model. Our findings indicate a strong coupling between gyre circulation and the AMOC in the subpolar basin such that water masses initialized at 40°N take multiple cyclonic circuits before they become dense enough and join the AMOC lower limb. The along-track thermohaline evolution is quantified by constructing the Lagrangian thermohaline streamfunction, which reveals substantial cooling and freshening of the water masses mainly along the subpolar topographic boundary. The decadal variability is further investigated by comparing trajectories simulated during a decade of strong AMOC and a decade of weak AMOC. We find that during the decade of strong AMOC, the cyclonic circulation pathways intensify with a significant westward shift, implying a strengthened and contracted subpolar gyre. Interestingly, while the cooling of water masses enhances, the freshening weakens, both of which lead to the densification of water masses and thus a stronger AMOC. Future work will aim at disentangling the governing mechanisms of the along-track thermohaline changes by quantifying contributions from temperature and salinity divergence under the water mass transformation framework. Moreover, the gyre circulation, which is the main pathway for the AMOC upper limb, can be focused on to explore the relevance of its size or strength to the AMOC's variability.