Model Biases in the AMOC Stability Indicator

Abstract

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 Fov index. For example when AMOC transports net freshwater out of the Atlantic sector at 34°S (Fov < 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 Fov 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 Fov index, implying that the AMOC is too stable in these climate model simulations. Here we analyse Fov-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 Fov index drifts from negative values to positive values over a 300-year simulation period. The Fov 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 Fov are responsible the difficulty in simulating realistic AMOC behaviour in climate model simulations. The implication is that climate models with an inconsistent Fov index are not fit for purpose in making AMOC projections.