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If NAO and BLO circulation systems are considered as orthogonal, a NAO-BLO phase space diagram (Wheeler-Hendon diagram) may be used to investigate and illustrate the relationship between circulation type and other forecast or observed parameters.  The NAO–BLO phase space can offer the advantage of a simplified framework for assessing model performance in predicting temperature extremes.

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Fig8.2.7-3:  NAO/BL phase space diagrams (Wheeler-Hendon diagrams) for the periods: Days11-17 and Days25-31, from extended range ensembles DT 20 Oct 2023.  Colours represent the proportion of members that have a similar mean solution during each seven day period.  The shaded area gives an indication of the spread of regime types.  Colours represent different proportions (taken as probabilities) of a combination of regime types. Note the colours represent different probabilities on each diagram.  Within the central circle there is only a weak indication of regime type.

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Circulation patterns like NAO are often associated with global teleconnections through propagation of Rossby wave trains.  El Nino-Southern Oscillation (ENSO) events, Sudden Stratospheric Warmings (SSW) and pronounced Madden-Julian Oscillation (MJO) events have been found to enhance the predictability and skill of forecast circulations in the North Atlantic/European area.

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