SEMINAR: AOML Seminar - September 7, 2010 - 10:00 a.m. - Dr. Sybren Drijfhout - "What is the best location to monitor the stability of the Atlantic MOC?”

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AOML Seminar

DATE :           Tuesday, September 7, 2010

TIME :            10:00 a.m. – refreshments at 9:45 a.m.

LOCATION:    AOML First-Floor Conference Room

SPEAKER :     Dr. Sybren Drijfhout

  KNMI, Royal Netherlands Meteorological Institute, Netherlands

TITLE :           "What is the best location to monitor the stability of the Atlantic MOC?”

Abstract:The convergence of the salt transport by the Meridional Overturning Circulation (MOC) in the Atlantic basin, or Mov, determines the sign of the salt advection feedback that operates on the Thermohaline Circulation (THC). Convergence amplifies a salinity anomaly when excess freshwater acts to reduce convection and the MOC/THC. Divergence damps such anomalies. In many parts of the Atlantic Mov is divergent, but south of the deep convection sites, between 40N and 60N, it is strongly convergent. At first sight it seems natural the monitor Mov in this region. But despite the strongly amplifying feedback on salinity anomalies by this, the stability of the THC is not affected by Mov at these latitudes, as the resulting salt advection feedback is coupled to a lagging damping feedback, leading to oscillations. The damping feedback is provided by either the Zonal Overturning Circulation, or the subpolar gyre, or transport from the Arctic. A second region of convergent Mov occurs near the southern boundary of the Atlantic, but the feedback associated with the local convergence is again counteracted by a lagged response of the gyre. The much weaker, basin-scale divergence/convergence of Mov, however, is not associated with a lagged response of opposite sign. Therefore, a basin-scale convergence of Mov may lead to amplification of subpolar freshwater anomalies. Its convergence controls the strength of the more local feedbacks associated with oscillations. Because Mov in the very north is weak, the basin-scale convergence of Mov is approximately equal to its sign at the southern boundary of the Atlantic, say 35S. If salt transport is northward there, the THC has multiple equilibria, if it is southward only the thermally driven branch (on-state) exists.

A second argument to monitor the THC at 35S stems from the existence of a monitoring array at 26N, and the fact that shorter timescale MOC-variability is uncorrelated at 35S and 26N, while the Atlantic Multidecadal Oscillation and anthropogenic trends do correlate over the whole basin, leading to a significantly increased signal-to-noise ratio when having two monitoring arrays situated in two different hemispheres, being beneficial for decadal prediction initialization and detecting abrupt chang