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February 1986 was notable for a dramatic blocking pattern in the eastern N Atlantic with part of NW Europe having its coldest February in 300 years. However, the previous December and January had been mild in the same region and the hemispheric anomaly field was characterized by a pattern which could be viewed as a wave-train emanating from the tropical western Pacific. Such an extreme change within a single winter season is unusual and must be understood if reliable monthly and seasonal forecasts are ever to be produced. The tropospheric flow field, isentropic distributions of potential vorticity, and diabatic heating calculated as a residual in the thermodynamic equation are used to illustrate the similarities and differences in the two parts of the winter and to suggest possible reasons for these differences. the mean vorticity equation is also used to investigate the importance of the change in the zonal flow, the transients and the mean divergence. It is concluded that the change in the fluxes due to the synoptic weather systems was crucial, but that a catalyst for the February block could have been provided by an unusual diabatic forcing in the S American-Caribbean region. Implications are drawn for the ingredients which must be included in dynamical models for predicting on the monthly to seasonal time-scales.
Hoskins et al. (Wed,) studied this question.