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TALK: "Toward a Theory of the North Atlantic Oscillation"
Professor Michael Ghil, Ecole Normale Supérieure (ENS) and UCLA, Department of Atmospheric & Oceanic Sciences
ABSTRACT:
For the last 15 years, the author of this talk and several associates have developed a theory of interannual variability in the wind-driven, mid-latitude ocean circulation A key feature of this theory is the existence of a "gyre mode" with a periodicity of 7-8 years. The period of this mode, as well as several of its surface features agree with the spatio-temporal structure of sea-surface temperatures (SSTs) and sea-level pressures of the North Atlantic Oscillation. In a barotropic and a baroclinic model, respectively, a strong and narrow SST front, like that of the Gulf Stream, can induce a vigorous jet in the atmosphere above. The atmospheric jet undergoes intraseasonal oscillations whose period and spatio-temporal characteristics depend on the SST front's strength. These results depend on the resolution of the atmospheric model and disappear for a grid size of 100 km or larger, which is still typical of GCMs used in simulations of past, present and future climate. Simulations that use an atmospheric GCM with a zooming capability over the Gulf Stream region, and high-resolution Mercator SST data for the lower boundary, support the idealized-model result. This hypothesis is further examined in (i) coupled ocean-atmosphere model studies with realistic North Atlantic geometry; and (ii) data over the western North Atlantic from the Simple Ocean Data Assimilation (SODA) data reanalysis. The recent results represent joint work with S. Brachet, Y. Feliks and E. Simonnet.
Presentation (PDF file).
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| Department of Atmospheric Sciences |
Institute of Geophysics and Planetary Physics |
University of California Los Angeles |
