The Existence of Free Rossby Waves during January 1979

Abstract

The large-scale transient components of atmospheric flow have been studied for many years. Observational studies indicate that large amplitude regularly westward propagating waves appear episodically in the atmosphere. These waves have spatial structures and frequencies enticingly similar to those of external Rossby modes obtained theoretically from the linearized baroclinic equations. In the present study, the episode of 10?28 January 1979 is studied by expanding global objective analyses into the normal modes of a global baroclinic quasi-geostrophic model linearized about the observed nonseparable zonal mean wind field. Several coherent regularly propagating waves are found. One of the most significant of these (the R31 or 16 day mode) is examined in detail. Although the observed structure is similar in some ways to the theoretically derived structure, there are important discrepancies. These discrepancies are examined more closely using upper air soundings from Antarctica. The experimental results suggest that for the episode of 10?28 January 1979 the identification of the observed regularly propagating 16 day wave with the theoretically derived R31 mode is doubtful and more sophisticated explanations may be required.