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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

Abstract:

The topics discussed include 1) The evolution of extended-range systematic error and skill in forecasting large-scale weather regime transitions; 2) The dependence of extended-range systematic error and skill on model horizontal resolution; 3) Monthly mean forecasts of tropical rainfall; 4) Tropical/extratropical interaction, and the influence of tropical low-frequency variability on extratropical forecast skill; 5) Ensemble forecasting, including the impact of ensemble averaging on forecast skill, and ensemble dispersion as a measure of forecast reliability; and 6) Probabilistic forecasting using phase-space cluster analysis. We believe that operational extended-range forecasting using the ECMWF model may be viable to day 20 - and possibly beyond - following further research on techniques for Monte Carlo forecasting, and when model systematic error in the tropics has been reduced significantly. -from Authors

Abstract:

First, an empirical orthogonal function (EOF) analysis is made of outgoing longwave radiation (OLR) in the tropics over seven winters. Having removed the seasonal cycle and interannual variability, the two leading EOFs describe the 30-60 day oscillation. A composite of extratopical 500 mb geopotential height correlated simultaneously with this mode of tropical variability is constructed. The 500 mb height composite is compared succesfully, with the Simmons, Wallace and Branstator (SWB) mode of barotropic instability, which has similar periodicity and similar spatial structure in both its phase-quadrature components. In the final phase of this study, the ECMWF model has been integrated over four wintertime 20-day periods. -from Authors