Tim Palmer

Abstract:

A number of tropical Pacific SST anomaly experiments, run on the UK Meteorological Office 11-level general circulation model in perpetual January mode, are described. It is found that the model's extratropical response can be statistically significant as far downstream as the European continent depending on the anomaly used, but is sensitive to the specification of orography in the model. A realistic response to a composite El Niño SST anomaly is obtained in the extratropics provided envelope orography replaces the standard orographic specification. A negative East Pacific anomaly run revealed that aspects of the tropical response are not readily explained by linear theory and some possible reasons for this are proposed. The extratropical response appears in some experiments to be qualitatively consistent with a downstream Rossby wavetrain and this has been tested using a baro-tropic model. In other experiments, and in the Southern Hemisphere, this is less clear. The extent to which the extratropical response is maintained by cyclogenesis in mid-latitudes remains to be firmly established though preliminary results suggest it is important. The extratropical response is sensitive to relatively weak SST anomalies in the tropical West Pacific. © 1985, Elsevier B.V. All rights reserved.

Abstract:

We present several atmospheric general circulation experiments which model the atmospheric response to SST anomalies in the tropical East and West Pacific. We have found that the extratropical response to a relatively small West Pacific anomaly can be stronger than, and qualitatively different from the response to a much larger East Pacific anomaly. These experiments suggest a possible explanation for the difference in mid-latitude response during the 1976-77 El Nino winter and the El Nino winters of 1972-73 and 1982-83.-from Authors