Climate Dynamics

, Volume 37, Issue 7–8, pp 1575–1586 | Cite as

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

  • Sybren S. Drijfhout
  • Susanne L. Weber
  • Eric van der Swaluw
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.

Keywords

Thermohaline circulation Multiple equilibria Abrupt climate change Salt advection feedback Stability South Atlantic ocean 

Notes

Acknowledgments

Computer resources were funded by the National Computing Facilities Foundation (NCF). We thank Michael Kliphuis for technical support and Vanessa Pedinotti for computational support. The manuscript benefitted from remarks from Geert-Jan van Oldenborgh and Andrea Cimatoribus. We acknowledge the international modeling groups for providing their data for analysis, PCMDI and ESG for making these data available. Part of this research was carried out in the framework of the Dutch Climate Change and Spatial Planning program (BSIK).

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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • Sybren S. Drijfhout
    • 1
  • Susanne L. Weber
    • 1
  • Eric van der Swaluw
    • 1
  1. 1.Royal Netherlands Meteorological Institute (KNMI)De BiltThe Netherlands

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