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The stability of the MOC as diagnosed from model projections for pre-industrial, present and future climates

The stability of the MOC as diagnosed from model projections for pre-industrial, present and future climates
The stability of the MOC as diagnosed from model projections for pre-industrial, present and future climates
The stability of the Atlantic meridional overturning circulation (MOC) is investigated for various climate scenario runs, using data from the CMIP3 archive of coupled atmosphere-ocean models. Apart from atmospheric feedbacks, the sign of the salt flux into the Atlantic basin that is carried by the MOC determines whether the MOC is in the single or multiple equilibria regime. This salt advection feedback is analyzed by diagnosing the freshwater and salt budgets for the combined Atlantic and Arctic basins. Consistent with the finding that almost all coupled climate models recover from hosing experiments, it is found that most models feature a negative salt advection feedback in their pre-industrial climate: freshwater perturbations are damped by this feedback, excluding the existence of a stable off-state for the MOC. All models feature enhanced evaporation over the Atlantic basin in future climates, but for a moderate increase in radiative forcing (B1 and 2 CO2 scenarios), there is a decrease of the fresh water flux carried by the MOC into the Atlantic (the deficit is made up by increased fresh water transport by the gyre circulation). In this forcing regime the salt advection feedback becomes less negative: for three models from an ensemble of eight it is positive in a 2 CO2 climate, while two models feature a positive feedback in the pre-industrial climate. For even warmer climates (A1B-equilibrium and 4 CO2) the salt feedback becomes more negative (damping) again. It is shown that the decrease in northward fresh water transport at 34°S by the MOC (in B1-equilibrium and 2 CO2) is due to a reduction of the inflow of intermediate waters relative to thermocline waters, associated with a robust shoaling of the MOC in future, warmer climates. In A1B and 4 CO2 climates northward freshwater transport increases again. The MOC keeps shoaling, but both intermediate and thermocline water masses freshen.
0930-7575
1575-1586
Drijfhout, Sybren S.
a5c76079-179b-490c-93fe-fc0391aacf13
Weber, Susanne L.
8fddd340-e824-4412-b353-e020e2eb42c2
Swaluw, Eric
a25649f8-4dd7-4482-9904-c5296484db8f
Drijfhout, Sybren S.
a5c76079-179b-490c-93fe-fc0391aacf13
Weber, Susanne L.
8fddd340-e824-4412-b353-e020e2eb42c2
Swaluw, Eric
a25649f8-4dd7-4482-9904-c5296484db8f

Drijfhout, Sybren S., Weber, Susanne L. and Swaluw, Eric (2011) The stability of the MOC as diagnosed from model projections for pre-industrial, present and future climates. Climate Dynamics, 37 (7-8), 1575-1586. (doi:10.1007/s00382-010-0930-z).

Record type: Article

Abstract

The stability of the Atlantic meridional overturning circulation (MOC) is investigated for various climate scenario runs, using data from the CMIP3 archive of coupled atmosphere-ocean models. Apart from atmospheric feedbacks, the sign of the salt flux into the Atlantic basin that is carried by the MOC determines whether the MOC is in the single or multiple equilibria regime. This salt advection feedback is analyzed by diagnosing the freshwater and salt budgets for the combined Atlantic and Arctic basins. Consistent with the finding that almost all coupled climate models recover from hosing experiments, it is found that most models feature a negative salt advection feedback in their pre-industrial climate: freshwater perturbations are damped by this feedback, excluding the existence of a stable off-state for the MOC. All models feature enhanced evaporation over the Atlantic basin in future climates, but for a moderate increase in radiative forcing (B1 and 2 CO2 scenarios), there is a decrease of the fresh water flux carried by the MOC into the Atlantic (the deficit is made up by increased fresh water transport by the gyre circulation). In this forcing regime the salt advection feedback becomes less negative: for three models from an ensemble of eight it is positive in a 2 CO2 climate, while two models feature a positive feedback in the pre-industrial climate. For even warmer climates (A1B-equilibrium and 4 CO2) the salt feedback becomes more negative (damping) again. It is shown that the decrease in northward fresh water transport at 34°S by the MOC (in B1-equilibrium and 2 CO2) is due to a reduction of the inflow of intermediate waters relative to thermocline waters, associated with a robust shoaling of the MOC in future, warmer climates. In A1B and 4 CO2 climates northward freshwater transport increases again. The MOC keeps shoaling, but both intermediate and thermocline water masses freshen.

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Published date: 2011
Organisations: Ocean and Earth Science

Identifiers

Local EPrints ID: 340326
URI: https://eprints.soton.ac.uk/id/eprint/340326
ISSN: 0930-7575
PURE UUID: 097f790c-7f90-4b9b-87f2-c024b698c721

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Date deposited: 18 Jun 2012 15:37
Last modified: 18 Nov 2019 20:43

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Author: Susanne L. Weber
Author: Eric Swaluw

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