Subpolar Gyre Variability in CMIP6 Models: Is there a Mechanism for Bistability?

Abstract

The subpolar gyre is at risk of crossing a tipping point which would result in the collapse of convection in the Labrador Sea. It is important to understand the mechanisms at play and how they are represented in climate models. In this study we use causal inference to verify whether the proposed mechanism for bistability of the subpolar gyre is represented in CMIP6 models. In many models an increase of sea surface salinity leads to a deepening of the mixed layer resulting in a cooling of the water at intermediate depth, in line with theory. The feedback from the subsurface temperature through density to the strength of the gyre circulation is more ambiguous, with fewer models indicating a significant link. Those that do show a significant link do not agree on its sign. One model (CESM2) contains all interactions, with both a negative and delayed positive feedback loop.