Ashkenazy and Tziperman (2007) showed, using the MITgcm ocean general circulation model in an idealized configuration plus a three-box model, that the thermohaline circulation (meridional overturning) exhibits hysteresis when the surface wind-stress amplitude is used as the slowly varied control parameter: pronounced Dansgaard–Oeschger-like relaxation oscillations set in at low wind stress on the decreasing branch and persist to substantially larger wind stress on the increasing branch, so that a steady thermally dominant state and an oscillatory state coexist under identical wind forcing. Whether this wind-induced hysteresis and regime coexistence survives in more realistic settings — Atlantic-like basin geometry, higher (eddy-permitting) resolution, alternative convective-adjustment schemes, and coupled atmospheric feedbacks — has never been tested, and the sensitivity of oscillation onset, amplitude, and weakly chaotic character to model and parameter choices remains unquantified. Here 'weakly chaotic character' denotes the irregular, non-periodic form of the oscillations.

Background

The Atlantic meridional overturning circulation (AMOC) — also called the thermohaline circulation (THC) — is the global-scale density- and wind-driven circulation that transports heat and salt meridionally in the ocean. Since Stommel (1961), the THC has been known to admit multiple equilibria (a thermally dominant 'on' state with sinking in the north, and a salinity-dominated collapsed state) connected by hysteresis when the surface freshwater flux is varied as a control parameter; this freshwater-control hysteresis has been studied intensively in models of all complexity levels, from box models to fully coupled climate models. A second classical dynamical phenomenon is the 'deep decoupling' oscillation of Winton and Sarachik (1993): under strong steady salinity forcing, ocean general circulation models (OGCMs) self-sustain millennial-scale relaxation oscillations in which the deep ocean alternately couples to and decouples from the surface convecting layer, producing Dansgaard–Oeschger (DO)-like warm/cold swings analogous to the abrupt millennial climate events of the last glacial period. Ashkenazy and Tziperman (2007) united these two threads under a new control parameter: in the MITgcm (the Massachusetts Institute of Technology ocean general circulation model) in an idealized double-hemisphere sector configuration, coupled to a simple three-box model for interpretation, they slowly reduced the wind-stress amplitude from its full value to zero over about 40 000 years and then restored it over a similar period. On the decreasing branch the circulation remained thermally dominant until very low wind stress, at which pronounced DO-like relaxation oscillations began; on the increasing branch these oscillations persisted up to significantly larger wind stress. Hence, at one and the same wind-stress amplitude the system can be either in a steady thermally dominant state or in a pronounced oscillatory state — a wind-induced hysteresis that the box model attributes to the combination of Stommel-type hysteresis and the deep-decoupling oscillation. The wind-driven mixing term MwM_w in the box model and the relatively large salinity fluxes used there are acknowledged simplifications. The authors themselves framed the unresolved issue: the result is established only in the idealized model, and its bearing on real millennial climate variability depends on whether it generalizes to realistic model settings.

Problem Statement

Determine whether the wind-stress-induced hysteresis of the thermohaline circulation found by Ashkenazy and Tziperman (2007) in an idealized double-hemisphere MITgcm configuration and a three-box model — in particular the coexistence, at one and the same wind-stress amplitude, of a steady thermally dominant overturning state and a pronounced Dansgaard–Oeschger-like relaxation-oscillation state — persists when the experiment is moved toward realistic settings, tested with a wind-stress-amplitude ramp demonstrably slow compared with the circulation's adjustment timescale, in at least one realistic OGCM setting. The overarching objective is to establish whether wind stress is a genuine bifurcation and hysteresis control parameter of the overturning beyond the idealized model. The realistic settings at issue are higher (eddy-permitting) spatial resolution, alternative convective-adjustment schemes, Atlantic-like basin geometry in place of the idealized sector configuration, and coupled atmospheric feedbacks — interactive wind stress, sea surface temperature, and moisture/freshwater transport — in place of prescribed surface forcing. The question also concerns the sensitivity of oscillation onset, amplitude, and weakly chaotic character to model choices and parameters such as freshwater-flux magnitudes and the wind-driven mixing parameterization, and whether the box-model wind-driven mixing term MwM_w — whose specific form the source acknowledges to be clearly arbitrary — and its unrealistically large salinity-flux values can be reconciled with full general-circulation results.

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