Hysteresis and Resilience of the AMOC in an Eddy‐Permitting GCM

Abstract

We show a quasi‐irreversible shutdown (hysteresis) of the Atlantic meridional overturning circulation (AMOC) in an unfluxadjusted global climate model with an eddy‐permitting ocean. This global climate model is a prototype of one submitted to the sixth Coupled Model Intercomparison Project and is the most comprehensive model to show this behavior. The AMOC has a resilience timescale for a given hosing: If the hosing exceeds the critical hosing for collapse for only a limited time, the AMOC recovers when the hosing is stopped. Beyond the critical time, when the hosing stops the AMOC stays in a weak state with no recovery within 200 years. We show that simple, observable metrics such as AMOC strength and mixed layer depth can indicate whether this critical resilience time has passed and that the mechanism controlling the presence of recovery is a positive advective feedback.

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

“…However, flux adjustments should be applied with caution: Dijkstra and Neelin () showed how the flux correction procedure, as applied by, for instance, Manabe and Stouffer (), could artificially affect the stability of the AMOC in models; the flux‐corrected configuration could have multiple equilibria in a regime where the non‐flux‐corrected configuration would have a single equilibrium. That multiple equilibria can also be found in models that do not have flux adjustments have been shown in more recent studies (Hawkins et al, ; Hu et al, ; Jackson & Wood, , ; Mecking et al, ).…”

“…Mecking et al () showed that freshwater convergence within the North Atlantic is an important feedback for opposing the salinification caused by the ITCZ shift discussed in the previous section. Jackson and Wood (, ) also discuss this feedback in different experiments with the same GCM, showing that both collapsed and recovering AMOC states after hosing feature an increasing salinity in the subtropical gyre and decreasing salinity in the subpolar gyre. In both states the basin‐integrated salinity is quite similar.…”

“…Under present‐day condition with an open Bering Strait, the AMOC hysteresis is weaker or even absent depending on the background climate (Figure ; blue and green curves). However, more recent model studies have shown multiple equilibria of the AMOC with an open Bering Strait (Jackson & Wood, , ). They found that transport changes through the Bering Strait still salinified the North Atlantic, even though they did not lead to an AMOC recovery.…”

“…This contradicts the findings of Sijp et al () who reported that the on‐branch and off‐branch for the AMOC are likewise coupled to two pan‐Atlantic salinity states of high and low salinity. However, in Mecking et al () and Jackson and Wood (, ) the collapsed state is characterized by a shallow overturning circulation in the same direction as the on‐state AMOC, rather than a reversed circulation, and experiments are limited to a few hundred years. Hence, it may not be a true equilibrium and it is possible that F ov at the southern boundary becomes relevant on longer timescales.…”

“…The paradigm of the AMOC has shifted from a circulation pattern where convection‐driven downwelling in the North Atlantic is balanced by diffusive upwelling in the rest of the ocean basins to one where wind‐driven upwelling and eddy‐driven transports in the Southern Ocean play a dominant role (e.g., Haertel & Fedorov, ; Toggweiler & Samuels, ). Currently, a new class of medium‐ and high‐resolution ocean and climate models are coming online, which will increase our ability to model the ocean circulation in the eddying and nondiffusive regime that better supports our evolving views of the AMOC and its controls (e.g., Jackson & Wood, , ; Weijer et al, ). Finally, we hope that this review will provide some context for the current debate about whether the current generation of climate models is overestimating the stability of the AMOC in the context of future global warming (e.g., Gent, ; Liu et al, ).…”