Using a physical reference frame to study global circulation variability

The 3D structures of the free oscillations of an adiabatic and hydrostatic atmosphere around a basic state at rest were used as a physical filtering for atmospheric data. This filtering procedure allows for the consideration of the three primitive variables (u, v, [phi]) over the whole atmosphere simultaneously. Accordingly, the computed statistics do not simply rely on the information provided by a single variable of circulation, such as the 500-hPa geopotential field. Using this method, two classical patterns were isolated in the barotropic component of the circulation, one resembling the Pacific-North America (PNA) pattern, the other similar to the North Atlantic Oscillation (NAO) pattern in summer. Associating the barotropic and the second baroclinic components, a coupling in variability was retrieved between the strength of the winter stratospheric polar vortex and the tropospheric circulation over the North Atlantic. Until now these modes had only been recovered by means of statistical analysis. This study shows their existence in physically filtered fields. The obtained results make clear that the observed winter pattern of NAO is not a simple variability mode of the atmosphere, but results instead from mean flow wave interaction that modulates tropospheric planetary Rossby waves. The association between the NAO circulation variability patterns and the anomalies of the 850-hPa temperature field was also investigated. (Manuscript received 16 January 2001, in final form 1 October 2001)