Bistability of the thermohaline circulation identified through comprehensive 2-parameter sweeps of an efficient climate model

Abstract

The effect of changes in zonal and meridional atmospheric moisture transports on Atlantic overturning is investigated. Zonal transports are considered in terms of net moisture export from the Atlantic sector. Meridional transports are related to the vigour of the global hydrological cycle. The equilibrium thermohaline circulation (THC) simulated with an efficient climate model is strongly dependent on two key parameters that control these transports: an anomaly in the specified Atlantic-Pacific moisture flux (DF a ) and atmospheric moisture diffusivity (K q ). In a large ensemble of spinup experiments, the values of DF a and K q are varied by small increments across wide ranges, to identify sharp transitions of equilibrium THC strength in a 2-parameter space (between Conveyor ''On'' and ''Off'' states). Final states from this ensemble of simulations are then used as the initial states for further such ensembles. Large differences in THC strength between ensembles, for identical combinations of DF a and K q , reveal the coexistence of two stable THC states (Conveyor ''On'' and ''Off'')-i.e. a bistable regime. In further sensitivity experiments, the model is forced with small, temporary freshwater perturbations to the mid-latitude North Atlantic, to establish the minimum perturbation necessary for irreversible THC collapse in this bistable regime. A threshold is identified in terms of the forcing duration required. The model THC, in a ''Conveyor On'' state, irreversibly collapses to a ''Conveyor Off'' state under additional freshwater forcing of just 0.1 Sv applied for around 100 years. The irreversible collapse is primarily due to a positive feedback associated with suppressed convection and reduced surface heat loss in the sinking region. Increased atmosphere-to-ocean freshwater flux, under a collapsed Conveyor, plays a secondary role.

First description of the Minnesota Earth System Model for Ocean biogeochemistry (MESMO 1.0)

“…First, we implemented FWflux in order to restore the Atlantic MOC that was reduced to 9 Sv in K v prof. With FWflux, the Atlantic-to-Pacific freshwater flux is increased by 25% from the value given by NGSA-II (Table 1). As shown earlier for GENIE-1 (Marsh et al, 2004, this has the effect of increasing the Atlantic MOC by increasing salinity and thus density of the Atlantic surface waters. The interbasin water transport is an needed in GENIE-1 to compensate for the insufficient transport in the energy moisture balance model of the atmosphere (Edwards and Marsh, 2005).…”

“…Also, the computational efficiency of GENIE-1 has allowed it to sweep entire parameter spaces with respect to the Atlantic meridional overturning circulation (MOC) so that the model's MOC behavior is now fairly well characterized (Marsh et al, 2004).…”

Comparing transient, accelerated, and equilibrium simulations of the last 30 000 years with the GENIE-1 model

“…In order to maintain significant North Atlantic deep water formation in a standard simulation (Edwards and Marsh, 2005), there is a constant surface moisture flux correction which transports fresh water from the Atlantic to the Pacific in three zonal bands, totaling 0.32 Sv (variable F a in Table 1; see Fig. 1 of Marsh et al, 2004). The model is essentially the same as is used in the work of except that we run the land model with prescribed vegetation and realistic orography, and ocean biogeochemistry is not included in our simulations.…”

Interhemispheric coupling, the West Antarctic Ice Sheet and warm Antarctic interglacials

“…The intermediate complexity model GENIE-1 has been extensively applied to investigations of the thermohaline circulation (Marsh et al, 2004) and provides the computational efficiency required to perform large ensembles and quantify model uncertainty. The physical model comprises a 3-D frictional geostrophic ocean with eddy-induced and isopycnal mixing coupled to a 2-D fixed wind-field EnergyMoisture Balance Model (EMBM) atmosphere and a dynamic and thermodynamic sea-ice component (Edwards and Marsh, 2005).…”

“…GCM simulations suggest an uncertainty of ± 0.15 Sv in this flux, especially relevant to climate states that differ from modern (Zaucker and Broecker, 1992). However, although the neglect of atmospheric transport feedbacks quantitatively alters the modelled sensitivity to transient meltwater fluxes (Marsh et al, 2004), the ensemble is designed to cover the range of possible sensitivities, varying APM in the range 0.05 to 0.64 Sv across the LPC ensemble members.…”