Explaining asymmetry between weakening and recovery of the AMOC in a coupled climate model

Haskins, Rosalind; Oliver, Kevin I. C.; Jackson, Laura; Drijfhout, Sybren; Wood, Richard

doi:10.1007/s00382-018-4570-z

Cited by 20 publications

(20 citation statements)

References 50 publications

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“…In the experiments where the overturning reached zero, the overturning strength increased back fairly quickly but then reached a new equilibrium in the throughflow circulation mode (i.e., negative overturning and positive Atlantic inflow). Interestingly, we do not see an overshoot in the overturning right after the recovery as in a recent experiment with a coupled model (Haskins et al, 2018). This supports the hypothesis that the overshoot is created by density change in the South Atlantic, which is not accounted for in this box model.…”

Section: Weakening and Recovery Transition Speed Under A 250-year Fresupporting

confidence: 88%

A Stella® version of the Arctic Mediterranean Double Estuarine Circulation model: SAMDEC v1.0

Thibodeau¹,

Lambert²

2019

Preprint

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The Arctic Mediterranean can be described as a double estuarine circulation regime. This observed circulation feature, which connects the North Atlantic to the Arctic Ocean, is composed of two interconnected branches of circulation: an overturning circulation, where dense water formed in the Nordic Seas returns toward the Atlantic and an estuarine circulation, where the East Greenland Current exits the Arctic Mediterranean. A conceptual box model has previously built upon Henry Stommel’s original version, concluding that a double estuarine circulation is less sensitive to perturbations in northern freshwater input than an overturning circulation in isolation. This extended model exhibits a similar freshwater sensitivity to several coupled atmosphere-ocean general circulation models (AOGCMs), which require about 1 Sv of freshwater input to induce a transition to a qualitatively weakened overturning in the Atlantic Ocean. Besides the amount of freshwater that would be required to abruptly weaken the Atlantic overturning circulation, it is essential to determine over what time scale such a transition could occur. To address this temporal aspect of potential abrupt transitions in a double estuarine circulation, we built a numerical version of the box-model in Stella®. The Stella® version of the Arctic Mediterranean Double Estuarine Circulation model (SAMDEC) is thus a new and widely-available numerical box-model representing the Arctic Mediterranean Double Estuarine Circulation and is intended to provide a numerical tool to easily solve this double estuarine theoretical framework. In addition to its simplicity of use, one of the most important added values of SAMDEC is the ability to easily determine and visualise transition times between two circulation regimes at very low computing cost, making it a valuable tool for research and education. This allows for a quantitative assessment of the response of the Arctic Mediterranean circulation to variable freshwater fluxes and temperature changes over short and long time scales. To highlight the features of SAMDEC, we showcase here two freshwater flux scenarios; 1) increased freshwater input in the Nordic Seas, which is most comparable to common ‘hosing’ experiments; and 2) increased freshwater input in the Nordic Seas and the Arctic. Similar to previous experiments performed with AOGCMs, the weakening caused by realistic freshwater addition is relatively slow during the first 100 years of simulation and increases thereafter. Finally, under a realistic freshwater increase, SAMDEC indicates that 88 to 176 years are needed to achieve a 15% weakening of the overturning. Overall, SAMDEC can provide insights into the dynamic of transition between circulation regimes under changes in freshwater input for both near-future and geological timescale investigations. A light version of the model can be accessed online with any internet browser and the full model can be downloaded, modified and used on a personal computer.

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Section: Weakening and Recovery Transition Speed Under A 250-year Fresupporting

confidence: 88%

A Stella® version of the Arctic Mediterranean Double Estuarine Circulation model: SAMDEC v1.0

Thibodeau¹,

Lambert²

2019

Preprint

View full text Add to dashboard Cite

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“…The density changes in the south modulated the overall weakening. These roles in the north and south were identified in FAMOUS by Haskins et al (2019), and the present study shows similar behaviour in an eddy-permitting model. The density in the north and south is then split into the AMOC perturbation driven by changes in temperature and salinity, Fig.…”

Section: Role Of Temperature and Salinity In Amoc Changessupporting

confidence: 87%

“…The same experimental design was applied to each simulation, with varying forcing strengths. This experimental design has been previously applied in both of the models used in this study (Jackson and Wood 2018;Haskins et al 2019), and is used to explore the sensitivity of the AMOC to freshening, and not the fate of the freshwater itself. The northern and southern densities are calculated using full sections of temperature and salinity taken at 50°N and 33°S in the Atlantic.…”

Section: Model Simulationsmentioning

confidence: 99%

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Temperature domination of AMOC weakening due to freshwater hosing in two GCMs

et al. 2019

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Anthropogenic climate change is projected to lead to a weakening of the Atlantic meridional overturning circulation (AMOC). One of the mechanisms contributing to this is ice melt leading to a freshening of the North Atlantic Ocean. We use two global climate models to investigate the role of temperature and salinity in the weakening of the AMOC resulting from freshwater forcing. This study finds that freshwater hosing reduces the strength of the AMOC, but in some situations it is not through reduced density from freshening, but a reduction in density from subsurface warming. When the freshwater is mixed down it directly reduces the density of the North Atlantic, weakening the strength of the AMOC. As the AMOC weakens, the mixed layer depth reduces and surface properties are less effectively mixed down. A buoyant surface cap forms, blocking atmospheric fluxes. This leads to the development of a warm anomaly beneath the surface cap, which becomes the primary driver of AMOC weakening. We found that the mean North Atlantic salinity anomaly can be used as a proxy for AMOC weakening because it describes the extent of this surface cap.

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“…Overall the box model dynamics appears to provide insights into the behaviour and design of AOGCM hosing experiments. However as a caveat we note that recently Haskins et al [44] have analysed a "press" scenario in the FAMOUS AOGCM, in a case which shows an oscillatory AMOC recovery after the forcing is removed. While this oscillatory recovery looks qualitatively similar to our box model solutions, it appears to be driven by variations in the South Atlantic density, a processes that is not captured in the three box, and possibly not the five box model.…”

Section: Relationship To Amoc Tipping In Complex Climate Modelsmentioning

confidence: 94%

Basin bifurcations, oscillatory instability and rate-induced thresholds for Atlantic meridional overturning circulation in a global oceanic box model

et al. 2019

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The Atlantic Meridional Overturning Circulation (AMOC) transports substantial amounts of heat into the North Atlantic sector, and hence is of very high importance in regional climate projections. The AMOC has been observed to show multi-stability across a range of models of different complexity. The simplest models find a bifurcation associated with the AMOC "on" state losing stability that is a saddle node. Here we study a physically derived global oceanic model of Wood et al with five boxes, that is calibrated to runs of the FAMOUS coupled atmosphere-ocean general circulation model. We find the loss of stability of the "on" state is due to a subcritical Hopf for parameters from both pre-industrial and doubled CO 2 atmospheres. This loss of stability via subcritical Hopf bifurcation has important consequences for the behaviour of the basin of attraction close to bifurcation. We consider various time-dependent profiles of freshwater forcing to the system, and find that rate-induced thresholds for tipping can appear, even for perturbations that do not cross the bifurcation. Understanding how such state transitions occur is important in determining allowable safe climate change mitigation pathways to avoid collapse of the AMOC.

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Explaining asymmetry between weakening and recovery of the AMOC in a coupled climate model

Cited by 20 publications

References 50 publications

A Stella® version of the Arctic Mediterranean Double Estuarine Circulation model: SAMDEC v1.0

A Stella® version of the Arctic Mediterranean Double Estuarine Circulation model: SAMDEC v1.0

Temperature domination of AMOC weakening due to freshwater hosing in two GCMs

Basin bifurcations, oscillatory instability and rate-induced thresholds for Atlantic meridional overturning circulation in a global oceanic box model

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