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Multimodel analysis of the response of the AMOC under an increase of radiative forcing and its symmetrical reversal

Multimodel analysis of the response of the AMOC under an increase of radiative forcing and its symmetrical reversal
Multimodel analysis of the response of the AMOC under an increase of radiative forcing and its symmetrical reversal
The response of the Atlantic meridional overturning circulation (AMOC) to an increase of radiative forcing (ramp-up) and a subsequent reversal of radiative forcing (ramp-down) is analyzed for four different global climate models. Due to changes in ocean temperature and hydrological cycle, all models show a weakening of the AMOC during the ramp-up phase. Once the external forcing is reversed, the results become model dependent. For IPSL-CM5A-LR, the AMOC continues its weakening trend for most of the ramp-down experiment. For HadGEM2-ES, the AMOC trend reverses once the external forcing also reverses, without recovering its initial value. For EC-EARTH and MPI-ESM-LR the recovery is anomalously strong yielding an AMOC overshoot. A robust linear dependency can be established between AMOC and density difference between North Atlantic (NA) deep water formation region and South Atlantic (SA). In particular, AMOC evolution is primarily controlled by a meridional salinity contrast between these regions. During the warming scenario, the subtropical Atlantic becomes saltier while the NA experiences a net freshening which favours an AMOC weakening. The different behaviour in the models during the ramp-down is dependent on the response of the ocean at the boundaries of NA and SA. The way in which the positive salinity anomaly stored in the subtropical Atlantic during the ramp-up is subsequently released elsewhere, characterizes the recovery. An out-of-phase response of the salinity transport at 48∘N and 34∘S boundaries is able to control the meridional density contrast between NA and SA during the transient experiments. Such a non-synchronized response is mainly controlled by changes in gyre salinity transport rather than by changes in overturning transport, thus suggesting a small role of the salt advection feedback in the evolution of the AMOC.
0930-7575
1429–1450
Sgubin, Giovanni
1209a2a2-fa99-47ce-afae-bcc498ad66ae
Swingedouw, Didier
740cbc3a-9dc6-4457-99bb-870e5b2cd6dc
Drijfhout, Sybren
a5c76079-179b-490c-93fe-fc0391aacf13
Hagemann, S
a5f45c81-3df4-437d-aa7d-2e316ebe1c56
Robertson, E
fc560669-8c72-4927-bebd-c7b5cb0369da
Sgubin, Giovanni
1209a2a2-fa99-47ce-afae-bcc498ad66ae
Swingedouw, Didier
740cbc3a-9dc6-4457-99bb-870e5b2cd6dc
Drijfhout, Sybren
a5c76079-179b-490c-93fe-fc0391aacf13
Hagemann, S
a5f45c81-3df4-437d-aa7d-2e316ebe1c56
Robertson, E
fc560669-8c72-4927-bebd-c7b5cb0369da

Sgubin, Giovanni, Swingedouw, Didier, Drijfhout, Sybren, Hagemann, S and Robertson, E (2015) Multimodel analysis of the response of the AMOC under an increase of radiative forcing and its symmetrical reversal. Climate Dynamics, 45 (5), 1429–1450. (doi:10.1007/s00382-014-2391-2).

Record type: Article

Abstract

The response of the Atlantic meridional overturning circulation (AMOC) to an increase of radiative forcing (ramp-up) and a subsequent reversal of radiative forcing (ramp-down) is analyzed for four different global climate models. Due to changes in ocean temperature and hydrological cycle, all models show a weakening of the AMOC during the ramp-up phase. Once the external forcing is reversed, the results become model dependent. For IPSL-CM5A-LR, the AMOC continues its weakening trend for most of the ramp-down experiment. For HadGEM2-ES, the AMOC trend reverses once the external forcing also reverses, without recovering its initial value. For EC-EARTH and MPI-ESM-LR the recovery is anomalously strong yielding an AMOC overshoot. A robust linear dependency can be established between AMOC and density difference between North Atlantic (NA) deep water formation region and South Atlantic (SA). In particular, AMOC evolution is primarily controlled by a meridional salinity contrast between these regions. During the warming scenario, the subtropical Atlantic becomes saltier while the NA experiences a net freshening which favours an AMOC weakening. The different behaviour in the models during the ramp-down is dependent on the response of the ocean at the boundaries of NA and SA. The way in which the positive salinity anomaly stored in the subtropical Atlantic during the ramp-up is subsequently released elsewhere, characterizes the recovery. An out-of-phase response of the salinity transport at 48∘N and 34∘S boundaries is able to control the meridional density contrast between NA and SA during the transient experiments. Such a non-synchronized response is mainly controlled by changes in gyre salinity transport rather than by changes in overturning transport, thus suggesting a small role of the salt advection feedback in the evolution of the AMOC.

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Published date: September 2015
Organisations: Paleooceanography & Palaeoclimate

Identifiers

Local EPrints ID: 407497
URI: http://eprints.soton.ac.uk/id/eprint/407497
ISSN: 0930-7575
PURE UUID: 0f3294b2-c753-4ce5-9206-0f384508e68f
ORCID for Sybren Drijfhout: ORCID iD orcid.org/0000-0001-5325-7350

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Date deposited: 13 Apr 2017 01:02
Last modified: 16 Mar 2024 04:12

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Contributors

Author: Giovanni Sgubin
Author: Didier Swingedouw
Author: S Hagemann
Author: E Robertson

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