Influence of Orography on the Extratropical Response to El Niño Events

Abstract

The contribution of the interaction between tropically forced circulation anomalies and the extratropicalmountains in the generation of extratropical circulation anomalies during the 1987/88 and 1988/89 winter seasons is diagnosed using a divergent barotropic model that solves for both the zonal-mean and eddy components of the 200-mb rotational anomalies. Barotropic modeling shows that the orographic modulation of the rotational response to the 200-mb tropical divergence anomaly can be substantial over the Pacific?North American region. The modulation consists of a large-scale wave pattern with a ridge in the central subtropical Pacific, a trough over the Gulf of Alaska, and a weak ridge extending across North America from Baja California to Greenland. These features have an amplitude of ?40 gpm, and the orographic modulation is thus about one-third as strong as the primary wave pattern. The associated 200-mb zonal wind is strongest (?5 m s?1) in the vicinity of the eastern end of the East Asian jet, thus contributing to the southeastward jet extension during El Niño winters. The Himalayan?Tibetan complex is the major locus of orographic interaction in the model, for it alone accounts for all the features and over two-thirds of the amplitude modulation. The eddy and zonal-mean parts of the tropically forced flow anomalies make comparable contributions to orographic modulation. However, the midlatitude eddy anomalies themselves result, in part, from the interaction of the zonal-mean zonal wind anomaly and the climatological vorticity gradients, that is, from ?zonal?eddy?interaction. The strength of this interaction depends on the arbitrarily specified distribution of the compensating zonal-mean subsidence in the model. These findings indicate the potential importance of secondary orographic interaction in the generation of extratropical circulation anomalies in response to tropical heating anomalies. Experiments with more complete dynamical models that predict both the rotational and divergent components of the flow in response to tropical heating anomalies are clearly warranted.