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The Different Nature of the Interdecadal Variability of the Thermohaline Circulation under Mixed and Flux Boundary Conditions
 

Summary: The Different Nature of the Interdecadal Variability of the Thermohaline Circulation
under Mixed and Flux Boundary Conditions
OLIVIER ARZEL, THIERRY HUCK, AND ALAIN COLIN DE VERDIÈRE
Laboratoire de Physique des Océans (UMR 6523 CNRS IFREMER UBO), Brest, France
(Manuscript received 7 July 2004, in final form 30 January 2006)
ABSTRACT
The differences between the interdecadal variability under mixed and constant flux boundary conditions
are investigated using a coarse-resolution ocean model in an idealized flat-bottom single-hemisphere basin.
Objective features are determined that allow one type of oscillation to be distinguished versus the other.
First, by performing a linear stability analysis of the steady state obtained under restoring boundary
conditions, it is shown that the interdecadal variability under constant flux and mixed boundary conditions
arises, respectively, from the instability of a linear mode around the mean stratification and circulation and
from departure from the initial state. Based on the budgets of density variance, it is shown next that the two
types of oscillations have different energy sources: Under the constant-flux boundary condition (the thermal
mode), the downgradient meridional eddy heat flux in the western boundary current regions sustains
interdecadal variability, whereas under mixed boundary conditions (the salinity mode), a positive feedback
between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal
oscillation. Furthermore, the positive correlations between temperature and salinity anomalies in the forc-
ing layer are shown to dominate the forcing of density variance. In addition, the vertical structure of
perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux, while

  

Source: Arzel, Olivier - Climate Change Research Centre, University of New South Wales
Huck, Thierry - Laboratoire de Physique des Océans, Université de Bretagne Occidentale

 

Collections: Geosciences