Use of Barotropic Models in the Study of the Extratropical Response to Tropical Heat Sources

Abstract

The responses of an expanded hierarchy of barotropic vorticity models to the anomalous tropical divergence associated to the El Nino event of February 1983 are analyzed. The discussion is focused on questions related to the interpretation of the results based on similar models by other authors. The main points are : (1) the importance of the vorticity advection by the divergent component of the wind for the extratropical response in an ENSO event, (2) the reason of the apparent success of more simple models which do not include this term, (3) the role of the upper level convergence near Indonesia for the development of the PNA pattern and (4) the relative insensitivity of AGCM's extratropical response to the longitudinal position of the equatorial convection along the Pacific Ocean, reported by Geisler et al. This paper emphasizes the major drawback of the barotropic model : the need for a correct specification of the divergence/ convergence associated with an anomalous tropical heat source, in view of the fact that subtropical anomalies in the divergence field, induced by tropical heating, make significant contributions to the midlatitude response.