Precise Calculations of the Existence of Multiple AMOC Equilibria in Coupled Climate Models. Part II: Transient Behavior

Abstract

n Part I of this paper an evolution equation for the Atlantic salinity and the reverse cell strength in the North Atlantic Deep Water (NADW) OFF state was formulated. Here, an analytical solution to this equation is used to test its validity in the context of transient solutions. In this study several transient scenarios in the general circulation model are examined to determine the accuracy of the predictions made with the material in Part I. The authors also determine how well the basic premises of Part I hold throughout these transient behaviors. The unstable equilibria that mark the upper boundary of the OFF state attraction basin are elucidated by the time-dependent behavior shown here. Transient equilibration from one stable NADW OFF state to another in response to changes in the anomalous salt flux H is accurately described by the evolution equation. The theory also explains the distribution of decay times for the NADW OFF state around the maximum critical Atlantic surface flux. Exceedingly long collapse times in excess of 50 000 years are found for surface flux values slightly in excess of the critical value.