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Introduction

Important sources of predictability for the extended range forecasts are:

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It should be noted that biases have been reduced in 47R1.  In particular the lower stratosphere cold bias evident in 46R1 has a marked improvement in 47R1 (especially in summer).

Circulation patterns in the Euro-Atlantic Region

The large-scale circulation patterns that impact upon the European area can be categorised into four main classes:

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Fig:8.2.5.2: Anomalies in 2m temperature associated with the persistence (periods longer than 5 days) of the (a) positive North Atlantic Oscillation, NAO+, (b) negative North Atlantic Oscillation, NAO-, (c) Scandinavian Blocking, BL+, and (d) Atlantic Ridge (AR) (or Anti Blocking BLO-) regimes.

NAO-BLO diagrams

If NAO and BLO circulation systems are considered as orthogonal, a NAO-BLO phase space 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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The NAO–BLO space explains about 30% of the daily winter variability over Europe.

Transitions between circulation patterns

A study of a large number of reanalyses (36 years of ERA-interim data) gives an indication of the frequency of transitions from one circulation type to another (giving a “climatology” of transitions).  Another study using available extended range re-forecasts (12 years of re-forecasts) gives an indication of the ability of the forecasts to capture similar transitions that occurred during the six-day period preceding various selected forecast lead times (Day11, Day16, Day21, Day 31).  The results are shown in Fig8.2.5.6.

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Fig8.2.5.7: NAO-BLO diagram showing transition of circulation pattern with time.  Colours indicate the elapse of time.  The initial NAO+ circulation pattern becomes a BLO+ circulation pattern by T+72hr, and finally becomes a NAO– circulation pattern by about T+168hr.

Skill

A measure of skill is the anomaly correlation between the observed and the ensemble mean forecasts of the principle circulation patterns - i.e. components associated with westerly/easterly flow across the Atlantic (NAO+/NAO–), blocked/anti-blocked flow over Scandinavia (BLO+/BLO­–), and the bivariate correlation using both of these.

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The skill in predicting heat waves or cold spells in the extended range may be limited by the ability of the forecast model to represent transitions to anticyclonic circulation regimes (BLO+, NAO-) over Europe.  However, once an NAO- circulation pattern has formed there is a tendency for it to persist in reality and in the IFS.

Uncertainty and Predictability

The ensemble variance or spread is an indicator of forecast uncertainty and normally increases with forecast lead time.  The rate at which the spread grows during the forecast can be used as an estimate of predictability.  Fig8.2.5.10 shows the change in spread with elapsed time.  Beyond day 3, forecasts with the ensemble mean first entering the NAO− sector have a lower mean ensemble variance than those with the ensemble mean entering any other sectors.  The differences between the mean ensemble variances could be associated with the fact that, by entering into a circulation pattern (NAO–) associated with higher predictability, the forecast uncertainty increases at a slower rate. 

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• Fairly high with NAO–
• Moderate with transitions to BLO+

Teleconnections

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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Fig8.2.5.12: Variation of the MJO index bivariate correlation with forecast lead-time for two older ECMWF model cycles (40r1 and 40r3).  Beyond Day20 correlation falls below 0.7. By Day 27 it falls below 0.6, implying marginal skill.

Important Caveats for Forecasters

Forecasters must also be aware that high confidence in forecasts of a circulation pattern may not equate to high confidence in forecasts of the associated surface weather, in certain locations. For example, an exceptionally confident winter-time forecast of NAO- can be accompanied, over southern England, by exceptionally large spread in forecasts of 2m temperature. This is due mainly to the locations of strong atmospheric thermal gradients undergoing regime-related shifts away from their climatological positions.And in other scenarios/locations other synoptic factors will come into play.

MJO Teleconnection Example

The forecast based on 00UTC 25 Feb 2019 illustrates the tele-connection effect of tropical deep convection over the Indian Ocean upon subsequent downstream developments of NAO+ type over the North Atlantic/Europe. 

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The pressure pattern with low pressure over the Iceland area and high pressure over the Azores are is representative of NAO+ type.  The forecast temperatures anomalies correspond to the NAO+ type temperature anomalies shown in Fig8.2.5.1 above. 

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

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