Model Biases in the AMOC Stability Indicator

The Atlantic Meridional Overturning Circulation (AMOC) is considered to be a multi-stable system with a northward overturning and a southward overturning circulation state. It has been proposed that the stability of the AMOC system can be represented through the net freshwater transport at 34°S (the Atlantic's southern boundary), the so-called F_ov index. For example when AMOC transports net freshwater out of the Atlantic sector at 34°S (F_ov < 0), freshwater (i.e., salinity) perturbations may grow over time through the salt-advection feedback which eventually can induce a state transition. Present-day observations indicate that F_ov is negative and  hence the present-day AMOC is in its multi-stable regime.

AMOC state transitions have regional and global impacts and it is therefore important to study the AMOC stability under climate change. However, most climate models have a tendency of simulating a positive F_ov index, implying that the AMOC is too stable in these climate model simulations. Here we analyse F_ov-related biases using a high-resolution and a low-resolution model version of the Community Earth System Model (CESM). Under constant pre-industrial conditions, the F_ov index drifts from negative values to positive values over a 300-year simulation period. The F_ov biases are related to biases in the E-P fluxes, freshwater runoff from Greenland, Agulhas leakage, Southern Ocean deep convection and the (meridional) location of the Antarctic Circumpolar Current front. These numerous processes contributing to F_ov are responsible the difficulty in simulating realistic AMOC behaviour in climate model simulations. The implication is that climate models with an inconsistent F_ov index are not fit for purpose in making AMOC projections.