Precise calculations of the existence of multiple AMOC equilibria in coupled climate models. Part I: Equilibrium states

• Published in: Journal of Climate
• Main Authors: Sijp, WP, England, MH, Gregory, JM
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Meteorological Soc 2012
• Subjects: 13 Climate Action; anzsrc-for: 040503 Physical Oceanography; anzsrc-for: 0401 Atmospheric Sciences; anzsrc-for: 0405 Oceanography; anzsrc-for: 0909 Geomatic Engineering; Antarctic; Southern Ocean; The Antarctic; Antarc*; NADW; North Atlantic Deep Water; North Atlantic
• Online Access: http://hdl.handle.net/1959.4/unsworks_41954; https://doi.org/10.1175/2011JCLI4245.1

This study examines criteria for the existence of two stable states of the Atlantic Meridional Overturning Circulation (AMOC) using a combination of theory and simulations from a numerical coupled atmosphere–ocean climate model. By formulating a simple collection of state parameters and their relationships, the authors reconstruct the North Atlantic Deep Water (NADW) OFF state behavior under a varying external salt-flux forcing. This part (Part I) of the paper examines the steady-state solution, which gives insight into the mechanisms that sustain the NADW OFF state in this coupled model; Part II deals with the transient behavior predicted by the evolution equation. The nonlinear behavior of the Antarctic Intermediate Water (AAIW) reverse cell is critical to the OFF state. Higher Atlantic salinity leads both to a reduced AAIW reverse cell and to a greater vertical salinity gradient in the South Atlantic. The former tends to reduce Atlantic salt export to the Southern Ocean, while the latter tends to increases it. These competing effects produce a nonlinear response of Atlantic salinity and salt export to salt forcing, and the existence of maxima in these quantities. Thus the authors obtain a natural and accurate analytical saddle-node condition for the maximal surface salt flux for which a NADW OFF state exists. By contrast, the bistability indicator proposed by De Vries and Weber does not generally work in this model. It is applicable only when the effect of the AAIW reverse cell on the Atlantic salt budget is weak.