Abstract
The consistency of ensemble forecasts from three global medium‐range prediction systems with the observed transition behaviour of a three‐cluster model of the North Atlantic eddy‐driven jet is examined. The three clusters consist of a mid jet cluster taken to represent an undisturbed jet and south and north jet clusters representing southward and northward shifts of the jet. The ensemble forecasts span a period of three extended winters (October–February) from October 2007–February 2010. The mean probabilities of transitions between the clusters calculated from the ensemble forecasts are compared with those calculated from a 23‐extended‐winter climatology taken from the European Centre for Medium‐Range Weather Forecasts 40‐Year Re‐analysis (ERA40) dataset. No evidence of a drift with increasing lead time of the ensemble forecast transition probabilities towards values inconsistent with the 23‐extended‐winter climatology is found. The ensemble forecasts of transition probabilities are found to have positive Brier Skill at 15 day lead times. It is found that for the three‐extended‐winter forecast set, probabilistic forecasts initialized in the north jet cluster are generally less skilful than those initialized in the other clusters. This is consistent with the shorter persistence time‐scale of the north jet cluster observed in the ERA40 23‐extended‐winter climatology. Copyright © 2011 Royal Meteorological Society