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SUMMARY:Optimal excitation of Atlantic ocean variability and implications 
 for predictability - Laure Zanna\, University of Oxford
DTSTART:20091103T163000Z
DTEND:20091103T173000Z
UID:TALK19837@talks.cam.ac.uk
CONTACT:Alex Piotrowski
DESCRIPTION:The Atlantic meridional overturning circulation (MOC) plays an
 \nimportant role in regulating high-latitude climate. Evidence from\nnumer
 ical models indicates that the strength of the MOC varies on a\nwide range
  of timescales with a relatively high potential\npredictability on decadal
  timescales. In order to explore the\nvariability and predictability of th
 e Atlantic MOC\, we study the\nfastest growing perturbations arising from 
 the ocean dynamics in a\ngeneral circulation model.\n\nThe analysis reveal
 s that the MOC is most sensitive to deep density\nperturbations located in
  the northern part of the basin. The\nexcitation of such perturbations can
  generate a large amplification of\nMOC anomalies after approximately 7 ye
 ars. This information may help\nin recognizing for example where additiona
 l observations could better\nconstrain interannual and multi-decadal varia
 bility associated with\nthe growth of perturbations. The mechanism associa
 ted with the growth\nof MOC anomalies can be understood by examining the t
 ime evolution of\ndeep zonal density gradients (related to the MOC via the
  thermal wind\nrelation). The propagation of density anomalies\, which dep
 ends on the\nmean flow and the mean density gradient\, determines the grow
 th time\nscale of the MOC anomalies.\n\nMoreover\, the growth timescale of
  the perturbations gives an estimate\nof error growth in initial condition
 s\, and therefore an estimate on\nhow such growth can potentially limit th
 e predictability of the MOC.\nIn the present model\, MOC anomalies are fou
 nd to grow faster when deep\nocean perturbations are allowed rather than w
 hen only the surface of\nthe ocean is perturbed.  Therefore\, uncertaintie
 s in the deep ocean\nmay limit the predictability of the ocean circulation
  and\npredictability experiments in which only the atmospheric state\n(equ
 ivalent to perturbing only the ocean surface) is perturbed may\noverestima
 te of the ocean predictability time.\n\nReferences:\nZanna L\, P. Heimbach
 \, A.M. Moore and E. Tziperman\, 2009:  On the\npredictability and variabi
 lity of the North Atlantic ocean.  Submitted\nto J. of Climate\nZanna L\, 
 P. Heimbach\, A.M. Moore  and E. Tziperman\, 2009:  Excitation\nof interan
 nual variability of the meridional overturning circulation.\nSubmitted to 
 J. of Phys. Oceanogr.\n
LOCATION:Tilley Lecture Theater\, Department of Earth Sciences
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