Model Biases in the AMOC Stability Indicator

The Atlantic Meridional Overturning Circulation (AMOC) is considered to be&#160;a multi-stable system with a northward overturning and a southward overturning circulation state.&#160;It has been proposed that the stability of the AMOC system can be represented through the net freshwater transport at 34&#176;S (the Atlantic's southern boundary), the so-called&#160;Fov index. For example when AMOC transports net freshwater out of the Atlantic sector at 34&#176;S (Fov < 0),&#160;freshwater (i.e., salinity) perturbations may grow over time through the salt-advection feedback which eventually can induce a state transition.&#160;Present-day observations indicate that Fov is negative and &#160;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&#160;AMOC stability under climate change.&#160;However, most climate models have a tendency of simulating a positive Fov&#160;index, implying that the AMOC is too stable in these climate model simulations.&#160;Here we analyse Fov-related biases using a high-resolution and a low-resolution model version of the Community Earth System Model (CESM).&#160;Under constant pre-industrial conditions, the Fov index drifts from negative values to positive values over a 300-year simulation period.&#160;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.&#160;These numerous processes contributing to Fov are responsible the&#160;difficulty in simulating realistic AMOC behaviour in climate model simulations.&#160;The implication is that climate models with an inconsistent Fov index&#160;are not fit for purpose in making AMOC projections.