scholarly journals Ocean Gyres and Abrupt Change in the Thermohaline Circulation: A Conceptual Analysis

2005 ◽  
Vol 18 (13) ◽  
pp. 2403-2416 ◽  
Author(s):  
Hannah Longworth ◽  
Jochem Marotzke ◽  
Thomas F. Stocker
Keyword(s):  
High Latitude ◽  
General Circulation ◽  
Multiple Equilibria ◽  
Thermohaline Circulation ◽  
Abrupt Change ◽  
General Circulation Models ◽  
Water Vapor Transport ◽  
Bifurcation Structure ◽  
Conceptual Foundation ◽  
Freshwater Forcing

Abstract The implications are investigated of representing ocean gyre circulations by a diffusion term in the Stommel and Rooth box models of the thermohaline circulation (THC) in one and two hemispheres, respectively. The approach includes mostly analytical solution and study of the bifurcation structure, but also numerical integration and feedback analysis. Sufficient diffusion (gyre strength) eliminates multiple equilibria from either model, highlighting the need for accurate gyre circulation strength in general circulation models (GCMs) when considering the potential for abrupt climate change associated with THC shutdown. With diffusion, steady-state flow strength in the Rooth model depends on freshwater forcing (i.e., implied atmospheric water vapor transport) in both hemispheres, not only on that in the upwelling hemisphere, as in the nondiffusive case. With asymmetric freshwater forcing, two solutions (strong stable and weak unstable) are found with sinking in the hemisphere with stronger forcing and one solution with sinking in the other hemisphere. Under increased freshwater forcing the two solutions in the hemisphere with stronger forcing meet in a saddle-node bifurcation (if diffusion is sufficiently strong to prevent a subcritical Hopf bifurcation first), followed by flow reversal. Thus, the bifurcation structure with respect to freshwater forcing of the diffusive Rooth model of two-hemisphere THC is similar to that of the Stommel model of single-hemisphere THC, albeit with a very different dynamical interpretation. Gyre circulations stabilize high-latitude sinking in the Stommel model. In the Rooth model, gyre circulations only stabilize high-latitude sinking if the freshwater forcing is weaker in the sinking hemisphere than in the upwelling hemisphere, by an amount that increases with diffusion. The values of diffusion and freshwater forcing at which qualitative change in behavior occurs correspond to the range of the values used in and obtained with GCMs, suggesting that this analysis can provide a conceptual foundation for analyzing the stability of the interhemispheric THC, and also for the potential of the Atlantic THC to undergo abrupt change.

Can Isopycnal Mixing Control the Stability of the Thermohaline Circulation in Ocean Climate Models?

2006 ◽  
Vol 19 (21) ◽  
pp. 5637-5651 ◽  
Author(s):  
Willem P. Sijp ◽  
Michael Bates ◽  
Matthew H. England
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Climate Models ◽  
Thermohaline Circulation ◽  
Vertical Component ◽  
Coupled Model ◽  
General Circulation Models ◽  
Tracer Transport ◽  
Horizontal Diffusion ◽  
The Stability

Abstract Convective overturning arising from static instability during winter is thought to play a crucial role in the formation of North Atlantic Deep Water (NADW). In ocean general circulation models (OGCMs), a strong reduction in convective penetration depth arises when horizontal diffusion (HD) is replaced by Gent and McWilliams (GM) mixing to model the effect of mesoscale eddies on tracer advection. In areas of sinking, the role of vertical tracer transport due to convection is largely replaced by the vertical component of isopycnal diffusion along sloping isopycnals. Here, the effect of this change in tracer transport physics on the stability of NADW formation under freshwater (FW) perturbations of the North Atlantic (NA) in a coupled model is examined. It is found that there is a significantly increased stability of NADW to FW input when GM is used in spite of GM experiments exhibiting consistently weaker NADW formation rates in unperturbed steady states. It is also found that there is a significant increase in NADW stability upon the introduction of isopycnal diffusion in the absence of GM. This indicates that isopycnal diffusion of tracer rather than isopycnal thickness diffusion is responsible for the increased NADW stability observed in the GM run. This result is robust with respect to the choice of isopycnal diffusion coefficient. Also, the NADW behavior in the isopycnal run, which includes a fixed background horizontal diffusivity, demonstrates that HD is not responsible in itself for reducing NADW stability when simple horizontal diffusion is used. Our results suggest that care should be taken when interpreting the results of coarse grid models with regard to NADW sensitivity to FW anomalies, regardless of the choice of mixing scheme.

A Wind-Induced Thermohaline Circulation Hysteresis and Millennial Variability Regimes

2007 ◽  
Vol 37 (10) ◽  
pp. 2446-2457 ◽  
Author(s):  
Yosef Ashkenazy ◽  
Eli Tziperman
Keyword(s):  
Wind Stress ◽  
General Circulation ◽  
Multiple Equilibria ◽  
Thermohaline Circulation ◽  
Stress Amplitude ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Box Model ◽  
Freshwater Flux ◽  
Relaxation Oscillations

Abstract The multiple equilibria of the thermohaline circulation (THC: used here in the sense of the meridional overturning circulation) as function of the surface freshwater flux has been studied intensively following a Stommel paper from 1961. It is shown here that multistability and hysteresis of the THC also exist when the wind stress amplitude is varied as a control parameter. Both the Massachusetts Institute of Technology ocean general circulation model (MITgcm) and a simple three-box model are used to study and explain different dynamical regimes of the THC and THC variability as a function of the wind stress amplitude. Starting with active winds and a thermally dominant thermohaline circulation state, the wind stress amplitude is slowly reduced to zero over a time period of ∼40 000 yr (40 kyr) and then increased again to its initial value over another ∼40 kyr. It is found that during the decreasing wind stress phase, the THC remains thermally dominant until very low wind stress amplitude at which pronounced Dansgaard–Oeschger-like THC relaxation oscillations are initiated. However, while the wind stress amplitude is increased, these relaxation oscillations are present up to significantly larger wind stress amplitude. The results of this study thus suggest that under the same wind stress amplitude, the THC can be either in a stable thermally dominant state or in a pronounced relaxation oscillations state. The simple box model analysis suggests that the observed hysteresis is due to the combination of the Stommel hysteresis and the Winton and Sarachik “deep decoupling” oscillations.

scholarly journals Precession modulation of the South Pacific westerly wind belt over the past million years

2019 ◽  
Vol 116 (47) ◽  
pp. 23455-23460 ◽  
Author(s):  
Frank Lamy ◽  
John C. H. Chiang ◽  
Gema Martínez-Méndez ◽  
Mieke Thierens ◽  
Helge W. Arz ◽  
...  
Keyword(s):  
General Circulation ◽  
Mixed Layer Depth ◽  
Atacama Desert ◽  
South Pacific ◽  
General Circulation Models ◽  
Water Vapor Transport ◽  
Intermediate Water ◽  
Westerly Wind ◽  
The South ◽  
The Past

The southern westerly wind belt (SWW) interacts with the Antarctic Circumpolar Current and strongly impacts the Southern Ocean carbon budget, and Antarctic ice-sheet dynamics across glacial–interglacial cycles. We investigated precipitation-driven sediment input changes to the Southeast Pacific off the southern margin of the Atacama Desert over the past one million years, revealing strong precession (19/23-ka) cycles. Our simulations with 2 ocean–atmosphere general circulation models suggest that observed cyclic rainfall changes are linked to meridional shifts in water vapor transport from the tropical Pacific toward the southern Atacama Desert. These changes reflect a precessional modulation of the split in the austral winter South Pacific jet stream. For precession maxima, we infer significantly enhanced rainfall in the southern Atacama Desert due to a stronger South Pacific split jet with enhanced subtropical/subpolar jets, and a weaker midlatitude jet. Conversely, we derive dry conditions in northern Chile related to reduced subtropical/subpolar jets and an enhanced midlatitude jet for precession minima. The presence of precessional cycles in the Pacific SWW, and lack thereof in other basins, indicate that orbital-scale changes of the SWW were not zonally homogeneous across the Southern Hemisphere, in contrast to the hemispherewide shifts of the SWW suggested for glacial terminations. The strengthening of the jet is unique to the South Pacific realm and might have affected winter-controlled changes in the mixed layer depth, the formation of intermediate water, and the buildup of sea-ice around Antarctica, with implications for the global overturning circulation and the oceanic storage of atmospheric CO2.

Comparison of the Stability of the Atlantic Thermohaline Circulation in Two Coupled Atmosphere–Ocean General Circulation Models

2007 ◽  
Vol 20 (17) ◽  
pp. 4293-4315 ◽  
Author(s):  
Jianjun Yin ◽  
Ronald J. Stouffer
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Large Scale ◽  
Thermohaline Circulation ◽  
General Circulation Models ◽  
Ocean General Circulation Models ◽  
Intercomparison Project ◽  
Ocean General Circulation ◽  
Atlantic Thermohaline Circulation ◽  
Atmospheric Feedback

Abstract Two coupled atmosphere–ocean general circulation models developed at GFDL show differing stability properties of the Atlantic thermohaline circulation (THC) in the Coupled Model Intercomparison Project/Paleoclimate Modeling Intercomparison Project (CMIP/PMIP) coordinated “water-hosing” experiment. In contrast to the R30 model in which the “off” state of the THC is stable, it is unstable in the CM2.1. This discrepancy has also been found among other climate models. Here a comprehensive analysis is performed to investigate the causes for the differing behaviors of the THC. In agreement with previous work, it is found that the different stability of the THC is closely related to the simulation of a reversed thermohaline circulation (RTHC) and the atmospheric feedback. After the shutdown of the THC, the RTHC is well developed and stable in R30. It transports freshwater into the subtropical North Atlantic, preventing the recovery of the salinity and stabilizing the off mode of the THC. The flux adjustment is a large term in the water budget of the Atlantic Ocean. In contrast, the RTHC is weak and unstable in CM2.1. The atmospheric feedback associated with the southward shift of the Atlantic ITCZ is much more significant. The oceanic freshwater convergence into the subtropical North Atlantic cannot completely compensate for the evaporation, leading to the recovery of the THC in CM2.1. The rapid salinity recovery in the subtropical North Atlantic excites large-scale baroclinic eddies, which propagate northward into the Nordic seas and Irminger Sea. As the large-scale eddies reach the high latitudes of the North Atlantic, the oceanic deep convection restarts. The differences in the southward propagation of the salinity and temperature anomalies from the hosing perturbation region in R30 and CM2.1, and associated different development of a reversed meridional density gradient in the upper South Atlantic, are the cause of the differences in the behavior of the RTHC. The present study sheds light on important physical and dynamical processes in simulating the dynamical behavior of the THC.

scholarly journals Validation of River Flows in HadGEM1 and HadCM3 with the TRIP River Flow Model

2011 ◽  
Vol 12 (6) ◽  
pp. 1157-1180 ◽  
Author(s):  
Pete Falloon ◽  
Richard Betts ◽  
Andrew Wiltshire ◽  
Rutger Dankers ◽  
Camilla Mathison ◽  
...  
Keyword(s):  
Annual Cycle ◽  
High Latitude ◽  
General Circulation ◽  
River Flow ◽  
General Circulation Models ◽  
River Basins ◽  
River Flows ◽  
Discharge Data ◽  
Basin Scale

Abstract The Total Runoff Integrating Pathways (TRIP) global river-routing scheme in the third climate configuration of the Met Office Unified Model (HadCM3) and the newer Hadley Centre Global Environmental Model version 1 (HadGEM1) general circulation models (GCMs) have been validated against long-term average measured river discharge data from 40 stations on 24 major river basins from the Global Runoff Data Centre (GRDC). TRIP was driven by runoff produced directly by the two GCMs in order to assess both the skill of river flows produced within GCMs in general and to test this as a method for validating large-scale hydrology in GCMs. TRIP predictions of long-term-averaged annual discharge were improved at 28 out of 40 gauging stations on 24 of the world’s major rivers in HadGEM1 compared to HadCM3, particularly for low- and high-latitude basins, with predictions ranging from “good” (within 20% of observed values) to “poor” (biases exceeding 50%). For most regions, the modeled annual average river flows tended to be exaggerated in both models, largely reflecting inflated estimates of precipitation, although lack of human interventions in this modeling setup may have been an additional source of error. Within individual river basins, there were no clear trends in the accuracy of HadGEM1 versus HadCM3 predictions at up- or downstream gauging stations. Relative root-mean-square error (RRMSE) scores for the annual cycle of river flow ranged from poor (>50%) to “fair” (20%–50%) with an overall range of 20.7%–1023.5%, comparable to that found in similar global-scale studies. In both models, simulations of the annual cycle of river flow were generally better for high-latitude basins than in low or midlatitudes. There was a relatively small improvement in the annual cycle of river flow in HadGEM1 compared to HadCM3, mostly in the low-latitude rivers. The findings suggest that there is still substantial work to be done to enable GCMs to simulate monthly discharge consistently well over the majority of basins, including improvements to both (i) GCM simulation of basin-scale precipitation and evaporation and (ii) hydrological processes (e.g., representation of dry land hydrology, floodplain inundation, lakes, snowmelt, and human intervention).

Sign in / Sign up

Export Citation Format

Share Document