Physics
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agusm.a22c..10g&link_type=abstract
American Geophysical Union, Spring Meeting 2002, abstract #A22C-10
Physics
3319 General Circulation, 3362 Stratosphere/Troposphere Interactions, 1620 Climate Dynamics (3309)
Scientific paper
NAO has long been viewed as a variability pattern confined to the North Atlantic, i.e. a zonally non-symmetric pattern. Recently a zonally symmetric concept was introduced that views the NAO as the local expression of a hemispheric mode, the Northern Annular Mode (NAM) reaching from the surface through the whole stratosphere. Here we will introduce a concept that takes both modes as physically independent, but interacting. The NAO is basically a barotropic mode with highest weight in the troposphere, NAM is a baroclinic mode with highest weight in the stratosphere. The life cycle of the NAO has been determined as about 10 days, resulting from the interaction of high frequency synoptic eddies with planetary Rossby waves. The life cycle of NAM is much longer (order of several weeks) due to the regime character of the stratospheric circulation in winter as produced by positive feedbacks between the strength and structure of zonal mean winds and vertically propagating planetary wave energy. Low frequency variability is introduced to the NAO by the effect of NAM on the vertical propagation of planetary waves. The structure of the EOF1 of Northern Hemispheric surface pressure (the pattern of the "Arctic Oscillation") is extremely similar to the simple difference of surface pressure between the two regimes of the (stratospheric) NAM. The interpretation of the NAM as an atmospheric variability mode reaching from the surface all the way to the upper stratosphere is therefore based on an artefact of the statistical analysis of the data without consideration of the underlying physics.
Castanheira J. M.
Graf Hans-F.
Silvestre J. M.
Walter Katrin
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