The Potential Mechanisms of AMOC Multidecadal Periods in CMIP6/CMIP5 Preindustrial Simulations

Abstract

Observations, theoretical analyses, and climate models show that the period of multidecadal variability of the Atlantic Meridional Overturning Circulation (AMOC) is related to westward temperature propagations in the subpolar North Atlantic, which is modulated by oceanic baroclinic Rossby waves. Here, we find major periods of AMOC variability of 12-28 years and associated westward temperature propagations in the preindustrial simulations of 9 CMIP6/CMIP5 models. Comparison with observations shows that the models reasonably simulate ocean stratifications in turn oceanic Rossby waves in the subpolar North Atlantic. The timescales of Rossby waves propagating on a static background flow across the subpolar North Atlantic basin overestimate the AMOC periods. The mean flow effects involving westward geostrophic self-advection and eastward mean advection largely shorten and greatly improve the estimate of AMOC periods through increasing Rossby wave speeds. Our results illustrate the importance of considering mean flow effects on Rossby wave propagations in the estimate of AMOC periods.