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An idealized ice–ocean model is used to study the time-dependent Atlantic meridional overturning circulation (AMOC) responses to a sudden uniform surface warming and/or an amplified evaporation minus precipitation (E−P) forcing. At transient time scales, the AMOC initially weakens in response to both types of forcing as a result of buoyancy gain in the North Atlantic, but the amplified E−P response is an order of magnitude smaller when its amplitude is chosen based on the Clausius–Clapeyron scaling, consistent with its weaker initial buoyancy flux anomaly. At equilibrium, the AMOC here weakens under warming, contrasting with previous idealized modeling studies. The difference is attributed to a larger role of North Atlantic warming (acting to weaken the AMOC) and a weaker role of reduced brine rejection around Antarctica (acting to deepen and strengthen the AMOC). When E−P forcing is amplified, the AMOC strengthens, qualitatively consistent with a previously proposed passive response that predicts an enhancement of the existing salinity pattern in equilibrium, although the amplification of the salinity contrast is significantly damped by a negative salt advection feedback. For a small-amplitude change in both temperature and E−P, the AMOC response can be approximated by the linear combination of the individual responses. However, large-amplitude warming and amplified E−P forcing can lead to a positive salt advection feedback that collapses the AMOC in our simulations. To understand why the sign of the salt advection feedback varies across different simulations, its multifaceted roles are further investigated using box model theories, and their relevance to comprehensive models is discussed.
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Surface Constraints on the Depth of the Atlantic Meridional Overturning Circulation: Southern Ocean versus North Atlantic
Paleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean.
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- PAR ID:
- 10139478
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 33
- Issue:
- 8
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- p. 3125-3149
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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