Dynamic and steric sea-level changes due to a collapsing AMOC in the Community Earth System Model

Abstract

A collapse of the Atlantic Meridional Overturning Circulation (AMOC) leads to a redistribution of dynamic sea level (DSL) across the global ocean surface. Here, we investigate DSL and steric sea-level responses under different AMOC strengths using the Community Earth System Model and two stand-alone ocean configurations (strongly eddying and parameterising eddy effects) with the Parallel Ocean Program. For our analysis, we employ various quasi-equilibrium freshwater hosing experiments in which AMOC collapses were reported. As the AMOC begins to collapse, the DSL substantially rises over the Atlantic Ocean and Arctic Ocean. Regions outside the Atlantic basin display a relatively small DSL drop. The largest DSL trends are found over the North Atlantic Ocean and reach +6 mm yr−1 over a 100-year period, with DSL trends near densely-populated coastal regions of up to +4 mm yr−1. This is a considerable contribution to local sea-level rise compared to the observed global mean sea-level rise of +3.3 mm yr−1. DSL trends obtained from the quasi-equilibrium experiments include a contribution from the freshwater hosing itself (≈ +0.2 mm yr−1), which is typically a factor 10–20 smaller than DSL response during the AMOC collapse. Moreover, a collapsed AMOC increases the net oceanic heat uptake leading to more than 50 cm of global mean thermosteric sea-level rise on millennial timescales (> 2000 years). These results highlight the potential value of accounting for an AMOC collapse scenario when developing or applying sea-level rise projections for the North Atlantic Ocean.