Tim Palmer

Abstract:

During the winter of 1988/89, a real-time experimental scheme to predict skill of the ECMWF operational forecast was devised. The scheme was based on statistical relations between skill scores (the predictands) and a number of predictors including consistency between consecutive forecasts, amplitude of very short-range forecast errors, and indices of large-scale regime transitions. The results of the experiment are assessed with particular attention to a period with large variations in the skill of the operational forecast. -Authors

Abstract:

Recent GCM integrations with El Nino sea surface temperature anomalies are reviewed, and the question of whether the extratropical response can be explained in terms of simple barotropic model dynamics is examined. -Author

Abstract:

Regimes of the northern extratropical circulation in winter are identified in this paper as clusters of atmospheric states in a low‐dimensional phase space generated by the leading EOFs of eddy geopotential fields. In order to define the clusters, our algorithm seeks points corresponding to local maxima for the density of atmospheric states; subsequently, a cluster is defined around each density maximum as that portion of the phase space in which the observed density can be locally approximated by a unimodal function. Two analyses were performed, using a 5‐dimensional and a 3‐dimensional space respectively, and they provided consistent results. Six clusters were found. the largest cluster includes 40% of the fields in our sample; its centroid is close to the climatological winter state, but it possesses a positive projection on the Pacific‐North American (PNA) pattern. the other five clusters represent anomalous flow regimes and include 52% of the fields. One of them shows a low amplitude of the planetary waves; the remaining four represent states with large wave amplitude but different phases. the variability between clusters accounts for the bimodality in the amplitude of planetary waves detected in previous observational studies. Our analysis reveals that this bimodality is much enhanced in the region of the phase space where the PNA index is negative, and the separation among the clusters is stronger. Finally, frequencies of transitions between clusters are presented, which show an asymmetric behaviour in the transitions between regimes with low and high amplitude of planetary waves. Copyright © 1990 Royal Meteorological Society