The role of salinity in the decadal variability of the North Atlantic meridional overturning circulation |
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| Authors: | Claude Frankignoul Julie Deshayes Ruth Curry |
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| Institution: | 1.LOCEAN/IPSL,Université Pierre et Marie Curie, Paris 6,Paris Cedex 05,France;2.Woods Hole Oceanographic Institution,Woods Hole,USA |
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| Abstract: | An OGCM hindcast is used to investigate the linkages between North Atlantic Ocean salinity and circulation changes during
1963–2003. The focus is on the eastern subpolar region consisting of the Irminger Sea and the eastern North Atlantic where
a careful assessment shows that the simulated interannual to decadal salinity changes in the upper 1,500 m reproduce well
those derived from the available record of hydrographic measurements. In the model, the variability of the Atlantic meridional
overturning circulation (MOC) is primarily driven by changes in deep water formation taking place in the Irminger Sea and,
to a lesser extent, the Labrador Sea. Both are strongly influenced by the North Atlantic Oscillation (NAO). The modeled interannual
to decadal salinity changes in the subpolar basins are mostly controlled by circulation-driven anomalies of freshwater flux
convergence, although surface salinity restoring to climatology and other boundary fluxes each account for approximately 25%
of the variance. The NAO plays an important role: a positive NAO phase is associated with increased precipitation, reduced
northward salt transport by the wind-driven intergyre gyre, and increased southward flows of freshwater across the Greenland–Scotland
ridge. Since the NAO largely controlled deep convection in the subpolar gyre, fresher waters are found near the sinking region
during convective events. This markedly differs from the active influence on the MOC that salinity exerts at decadal and longer
timescales in most coupled models. The intensification of the MOC that follows a positive NAO phase by about 2 years does
not lead to an increase in the northward salt transport into the subpolar domain at low frequencies because it is cancelled
by the concomitant intensification of the subpolar gyre which shifts the subpolar front eastward and reduces the northward
salt transport by the North Atlantic Current waters. This differs again from most coupled models, where the gyre intensification
precedes that of the MOC by several years. |
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