Model Biases in the AMOC Stability Indicator
crossref(2023)
摘要
<p>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<sub>ov</sub> index. For example when AMOC transports net freshwater out of the Atlantic sector at 34°S (F<sub>ov</sub> < 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<sub>ov</sub> is negative and  hence the present-day AMOC is in its multi-stable regime.</p> <p>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<sub>ov </sub>index, implying that the AMOC is too stable in these climate model simulations. Here we analyse F<sub>ov</sub>-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<sub>ov</sub> index drifts from negative values to positive values over a 300-year simulation period. The F<sub>ov</sub> 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<sub>ov</sub> are responsible the difficulty in simulating realistic AMOC behaviour in climate model simulations. The implication is that climate models with an inconsistent F<sub>ov</sub> index are not fit for purpose in making AMOC projections.</p>
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