The Impact of Wind Stress Feedback on the Stability of the Atlantic Meridional Overturning Circulation

2011 ◽  
Vol 24 (7) ◽  
pp. 1965-1984 ◽  
Author(s):  
Olivier Arzel ◽  
Matthew H. England ◽  
Oleg A. Saenko
Keyword(s):  
Wind Stress ◽  
Multiple Equilibria ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Freshwater Flux ◽  
Overturning Circulation ◽  
Atlantic Basin ◽  
Meridional Overturning ◽  
The Impact ◽  
Glacial Climate

Abstract Recent results based on models using prescribed surface wind stress forcing have suggested that the net freshwater transport Σ by the Atlantic meridional overturning circulation (MOC) into the Atlantic basin is a good indicator of the multiple-equilibria regime. By means of a coupled climate model of intermediate complexity, this study shows that this scalar Σ cannot capture the connection between the properties of the steady state and the impact of the wind stress feedback on the evolution of perturbations. This implies that, when interpreting the observed value of Σ, the position of the present-day climate is systematically biased toward the multiple-equilibria regime. The results show, however, that the stabilizing influence of the wind stress feedback on the MOC is restricted to a narrow window of freshwater fluxes, located in the vicinity of the state characterized by a zero freshwater flux divergence over the Atlantic basin. If the position of the present-day climate is farther away from this state, then wind stress feedbacks are unable to exert a persistent effect on the modern MOC. This is because the stabilizing influence of the shallow reverse cell situated south of the equator during the off state rapidly dominates over the destabilizing influence of the wind stress feedback when the freshwater forcing gets stronger. Under glacial climate conditions by contrast, a weaker sensitivity with an opposite effect is found. This is ultimately due to the relatively large sea ice extent of the glacial climate, which implies that, during the off state, the horizontal redistribution of fresh waters by the subpolar gyre does not favor the development of a thermally direct MOC as opposed to the modern case.

scholarly journals Noise-induced transitions of the Atlantic Meridional Overturning Circulation in CMIP5 models

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Daniele Castellana ◽  
Henk A. Dijkstra
Keyword(s):  
Transition Probabilities ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Multiple Equilibrium ◽  
Cmip5 Models ◽  
Freshwater Flux ◽  
Overturning Circulation ◽  
Southern Boundary ◽  
Meridional Overturning ◽  
The Impact

AbstractBy studying transition probabilities of the Atlantic Meridional Overturning Circulation (AMOC) in an ensemble of CMIP5 climate models, we revisit one of the stability indicators of the AMOC, i.e. the freshwater transport carried by the AMOC at the southern boundary of the Atlantic basin. A correction to this indicator, based on the transition probabilities, is suggested to measure whether an AMOC state is in a multiple equilibrium regime or not. As a consequence, the AMOC of all CMIP5 models considered is in a multiple equilibrium regime and hence, in principle, a collapsed AMOC state should exist in each of these models. The results further demonstrate the dependence of the Atlantic surface freshwater flux on the AMOC and the impact of extreme events in the AMOC on temperatures in the North Atlantic region.

scholarly journals Effect of Atmospheric Feedbacks on the Stability of the Atlantic Meridional Overturning Circulation

2012 ◽  
Vol 25 (12) ◽  
pp. 4081-4096 ◽  
Author(s):  
Matthijs den Toom ◽  
Henk A. Dijkstra ◽  
Andrea A. Cimatoribus ◽  
Sybren S. Drijfhout
Keyword(s):  
Atlantic Meridional Overturning Circulation ◽  
Surface Fluxes ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Freshwater Flux ◽  
Overturning Circulation ◽  
Freshwater Fluxes ◽  
Atmospheric Feedbacks ◽  
Meridional Overturning ◽  
The Impact

Abstract The impact of atmospheric feedbacks on the multiple equilibria (ME) regime of the Atlantic meridional overturning circulation (MOC) is investigated using a fully implicit hybrid coupled model (HCM). The HCM consists of a global ocean model coupled to an empirical atmosphere model that is based on linear regressions of the heat, net evaporative, and momentum fluxes generated by a fully coupled climate model onto local as well as Northern Hemisphere averaged sea surface temperatures. Using numerical continuation techniques, bifurcation diagrams are constructed for the HCM with the strength of an anomalous freshwater flux as the bifurcation parameter, which allows for an efficient first-order estimation of the effect of interactive surface fluxes on the MOC stability. The different components of the atmospheric fluxes are first considered individually and then combined. Heat feedbacks act to destabilize the present-day state of the MOC and to stabilize the collapsed state, thus leaving the size of the ME regime almost unaffected. In contrast, interactive freshwater fluxes cause a destabilization of both the present-day and collapsed states of the MOC. Wind feedbacks are found to have a minor impact. The joint effect of the three interactive fluxes is to narrow the range of ME. The shift of the saddle-node bifurcation that terminates the present-day state of the ocean is further investigated by adjoint sensitivity analysis of the overturning rate to surface fluxes. It is found that heat feedbacks primarily affect the MOC stability when they change the heat fluxes over the North Atlantic subpolar gyre, whereas interactive freshwater fluxes have an effect everywhere in the Atlantic basin.

Reduced Stability of the Atlantic Meridional Overturning Circulation due to Wind Stress Feedback during Glacial Times

2008 ◽  
Vol 21 (23) ◽  
pp. 6260-6282 ◽  
Author(s):  
Olivier Arzel ◽  
Matthew H. England ◽  
Willem P. Sijp
Keyword(s):  
Sea Ice ◽  
Wind Stress ◽  
Surface Wind ◽  
Meridional Overturning Circulation ◽  
Nonlinear Dependence ◽  
Upper Ocean ◽  
Overturning Circulation ◽  
Climate Conditions ◽  
Meridional Overturning ◽  
The Impact

Abstract A previous study by Mikolajewicz suggested that the wind stress feedback stabilizes the Atlantic thermohaline circulation. This result was obtained under modern climate conditions, for which the presence of the massive continental ice sheets characteristic of glacial times is missing. Here a coupled ocean–atmosphere–sea ice model of intermediate complexity, set up in an idealized spherical sector geometry of the Atlantic basin, is used to show that, under glacial climate conditions, wind stress feedback actually reduces the stability of the meridional overturning circulation (MOC). The analysis reveals that the influence of the wind stress feedback on the glacial MOC response to an external source of freshwater applied at high northern latitudes is controlled by the following two distinct processes: 1) the interactions between the wind field and the sea ice export in the Northern Hemisphere (NH), and 2) the northward Ekman transport in the tropics and upward Ekman pumping in the core of the NH subpolar gyre. The former dominates the response of the coupled system; it delays the recovery of the MOC, and in some cases even stabilizes collapsed MOC states achieved during the hosing period. The latter plays a minor role and mitigates the impact of the former process by reducing the upper-ocean freshening in deep-water formation regions. Hence, the wind stress feedback delays the recovery of the glacial MOC, which is the opposite of what occurs under modern climate conditions. Close to the critical transition threshold beyond which the circulation collapses, the glacial MOC appears to be very sensitive to changes in surface wind stress forcing and exhibits, in the aftermath of the freshwater pulse, a nonlinear dependence upon the wind stress feedback magnitude: a complete and irreversible MOC shutdown occurs only for intermediate wind stress feedback magnitudes. This behavior results from the competitive effects of processes 1 and 2 on the midlatitude upper-ocean salinity during the shutdown phase of the MOC. The mechanisms presented here may be relevant to the large meltwater pulses that punctuated the last glacial period.

A Diagnostic Indicator of the Stability of the Atlantic Meridional Overturning Circulation in CCSM3

2013 ◽  
Vol 26 (6) ◽  
pp. 1926-1938 ◽  
Author(s):  
Wei Liu ◽  
Zhengyu Liu
Keyword(s):  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Overturning Circulation ◽  
Atlantic Basin ◽  
Climate System Model ◽  
Freshwater Transport ◽  
Meridional Overturning ◽  
The Stability ◽  
Diagnostic Indicator

Abstract A diagnostic indicator ΔMov is proposed in this paper to monitor the stability of the Atlantic meridional overturning circulation (AMOC). The ΔMov is a diagnostic for a basinwide salt-advection feedback and defined as the difference between the freshwater transport induced by the AMOC across the southern border of the Atlantic Ocean and the overturning liquid freshwater transport from the Arctic Ocean to the North Atlantic. As validated in the Community Climate System Model, version 3 (CCSM3), for an AMOC in the conveyor state, a positive ΔMov (freshwater convergence) in the Atlantic basin indicates a monostable AMOC and a negative ΔMov (freshwater divergence) indicates a bistable AMOC. Based on ΔMov, the authors investigate the AMOC stability in the Last Glacial Maximum (LGM) and analyze the modulation of the AMOC stability by an open/closed Bering Strait. Moreover, the authors estimate that the real AMOC is likely to be bistable in the present day, since some observations suggest a negative ΔMov (freshwater divergence) is currently in the Atlantic basin. However, this estimation is very sensitive to the choice of the observational data.

Effect of the Atlantic Meridional Overturning Circulation on atmospheric pCO2

2021 ◽  
Author(s):  
Daan Boot ◽  
Henk Dijkstra
Keyword(s):  
Carbon Cycle ◽  
Multiple Equilibria ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Stable States ◽  
Overturning Circulation ◽  
Dimensional Parameter ◽  
Meridional Overturning ◽  
Project Model ◽  
Atmospheric Pco2

<p>The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in regulating the climate of the Northern Hemisphere. Several studies have shown that the AMOC can be in two stable states under equal forcing. This bistability, and associated tipping behavior, has been suggested as a mechanism for climate transitions in the past such as the Dansgaard-Oescher events. The relationship between AMOC variability and that in atmospheric pCO2 concentration is still unclear since different studies provide  contradictory results. Here, we investigate this  relationship using the Simple Carbon Project Model v1.0 (SCP-M), which we extended to represent a suite of nonlinear  carbon cycle feedbacks. By implementing SCP-M in the continuation and bifurcation software AUTO-07p, we can efficiently explore the multi-dimensional parameter space to address the AMOC - pCO2 relationship while varying the strengths of the carbon cycle feedbacks. We do not find multiple equilibria in the carbon-cycle dynamics, with fixed AMOC, but there are  intrinsic oscillations due to Hopf bifurcations with multi-millennial periods. The mechanisms of  this variability,  related to biological production and to calcium carbonate compensation, will be presented and their relevance  is addressed. </p>

Impact of the Equatorial Wind Stress on the Indian Ocean Shallow Meridional Overturning Circulation During the IOD Mature Phase

2020 ◽  
Author(s):  
Linfang Zhang ◽  
Yaokun Li ◽  
Jianping Li
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Overturning Circulation ◽  
Mature Phase ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Upper Level ◽  
The Impact

<p>            This paper investigates the impact of the equatorial wind stress on the Indian Ocean Shallow Meridional Overturning Circulation (SMOC) during the India Ocean Dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper level. In normal years, the wind stress in the Indian Ocean is easterly between 30°S-0°and the westerly wind is between 0°and 30°N, which contributes to a southward Ekman transport at the upper level to form the climatological SMOC. During the years of positive IOD events, abnormal easterly wind near the equator, accompanying with the cold sea surface temperature anomaly (SSTA) along the coast of Sumatra and Java and the warm SSTA along the coast of East Africa, brings southward Ekman transport south of the equator while northward Ekman transport north of the equator. This leads the seawaters moving away from the equator and hence upwelling near the equator as a consequence, to form a pair of small circulation cell symmetric about the equator.</p>

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