Stratospheric Control of the Linkage between the AMOC and Atlantic Multidecadal Variability

Abstract

The ocean’s role in Atlantic multidecadal variability (AMV) remains intensely debated. The core issue is whether AMV, as an internal climate mode, is driven by variations in the Atlantic meridional overturning circulation (AMOC) or by atmospheric processes. Climate models exhibit a wide range of AMOC–AMV linkages, producing temporal correlations between 0.3 and 0.8, but no robust explanation for these differences exists. Here, using both multimodel intercomparison and perturbation experiments, we propose a dynamical mechanism relating the strength of AMOC–AMV linkage in climate models to stratospheric temperature. This mechanism includes 1) tropospheric midlatitude jet response to stratospheric mean-state temperature anomalies in midlatitudes and 2) resulting ocean surface density changes that alter the spatial structure of deep-water formation in the subpolar North Atlantic and hence AMOC–AMV connection. Specifically, colder stratospheric temperatures produce tighter linkage through a northward jet shift and a stronger AMOC, with enhanced deep-water formation in the Labrador and Irminger Seas relative to the Nordic seas. Models with a warm stratospheric bias tend to produce weaker linkage. Perturbation experiments imposing stratospheric cooling at mid- to high latitudes within two independent climate models support these conclusions. Furthermore, we find that models with stronger AMOC–AMV linkage predict a stronger North Atlantic “warming hole” and weaker twenty-first-century Arctic amplification. We conclude that these results have significant implications for climate prediction and projections.