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:

  •  North Atlantic Oscillation, positive phase (NAO+):
    • Negative height anomaly over Greenland and positive height anomaly over the Azores causing a generally enhanced westerly flow across the Atlantic.
    • Typically associated with storminess, mild temperatures and heavy rainfall  over Europe (but with enhanced rain shadow that can deliver relatively dry weather downstream of high topography).
  •  North Atlantic Oscillation, negative phase (NAO­–):
    • Positive height anomaly over Greenland and negative height anomaly over the Azores causing a substantial reduction of the westerly flow across the Atlantic and a strengthening of northerly winds from the Arctic flowing over much of Europe.
    • Typically associated with persistent temperature anomalies over Europe, that are negative in the north.
    • NAO– is also known as Greenland blocking.
  • Blocking (BLO+):
    • Upper high or blocking upper pattern over Europe causing settled unchanging weather over Europe with the potential for continued surface warming or cooling.
    • Low-pressure systems are steered around the periphery of the blocking pattern or may become slow-moving just to the west.
    • Typically associated with severe and persistent temperature anomalies over Europe.  Possible persistent precipitation associated with slow-moving low-pressure system to the west.
    • BLO+ is also known as Scandinavian blocking.

  • Anti-blocking (BLO–):
    • Upper trough over Scandinavia and positive height anomaly over north Atlantic.
    • Typically associated with cold northerly winds from the Arctic over Western Europe and southerly winds further east.
    • The Atlantic Ridge (AR) circulation pattern is a particular case of anti-blocking with a strong positive height anomaly over the north Atlantic around 20W.

Whilst the above is a very helpful conceptual framework, that is backed up by statistical categorisation techniques, it should also be stressed that:

  1. The amplitude of a given pattern will vary from case to case, and from day to day (e.g. coining the terms "strong NAO-" or "weak NAO-")
  2. Sometimes the atmospheric state lies between these regimes

This is discussed further below (see e.g. Fig8.2.2-3).

Useful forecasts of the onset or cessation of anomalous, and even extreme surface weather over Europe may be made if the change from one circulation pattern to another, and the timing of that change, can be predicted well in advance of the event.  Additionally there needs to be an associated assessment of the confidence in any such transitions and their timing.  Errors in forecasting transitions from one type to another can lead to very large forecast errors in absolute (e.g. RMS error) terms. This is especially true in respect of the onset of blocking. 

 

Fig8.2.2-1: Geographical patterns of Euro-Atlantic climatological regimes (both anomalies and full fields) as used at ECMWF. North Atlantic Oscillation, positive phase (NAO+), North Atlantic Oscillation, negative phase (NAO­–), Blocking (BLO+), The Atlantic Ridge (AR) circulation pattern is a particular case of Anti-blocking (BLO–), in which the important feature is the trough over Scandinavia.  Geopotential anomalies (colour shading) and geopotential (contours) at 500 hPa are shown.


Fig:8.2.2-2: 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.

Presentation of circulation patterns

NAO-BLO diagrams

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.

 

Fig8.2.2-3: NAO–BLO phase space.  Well-defined circulation patterns lie towards the periphery of the diagram.  The central circle encloses a region where the circulation system is weak or cannot be confidently identified.

Examples of NAO–BLO phase space diagrams.

Fig8.2.2-4:  Regime Projection Diagrams: 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.  The regime Projection Diagrams are derived using Mirror 2-regime scheme.


NAO–BLO phase space can be used to illustrate the relationship between severe cold European spells. 

 

Fig8.2.2-5: Severe cold European spells, detected using the 2m temperature reanalyses, represented in the NAO–BLO space.  Colours have no special meaning.  The arrangement of the NAO-BLO diagrams corresponds to the arrangement of areas of Europe as shown on the map.

The analysis of severe cold spells in Europe, plotted on NAO-BLO diagrams for different areas is shown in Fig8.2.2-5.  The diagrams show severe cold spells in:

  • Central Europe (Area5) are associated strongly with BLO+ and NAO–,
  • Northwest Europe (Areas 1&2) are associated strongly with NAO– and BLO–,
  • Eastern Europe (Areas 3&6) are associated less strongly, and with a rather scattered distribution, to NAO+, BLO+ and NAO–.

Predictability of the circulation patterns

Fig8.2.2-6: Predictability distribution on NAO-BLO diagram.  Ensemble variance colour coded as the scale.  Ensemble variance (spread) is indicative of predictability.  Thus NAO– has relatively high predictability (probably because it tends to be more persistent than other regimes), BLO+ has relatively lower predictability.  The NAO–BLO space explains about 30% of the daily winter variability over Europe.

Transitions between circulation type or regimes

A 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 Fig8.2.2-7.

Fig8.2.2-7: Frequency (in percentages) of transitions to a given regime; stacked bar colour denotes the previous regime.  Colours show transitions from BLO+ (pale red), from BLO− (purple), from NAO+ (blue), from NAO− (green), no clear initial circulation pattern (grey).  Reanalysis values are shown in the column on the far left of each section.  The other bars indicate the forecast values at Day11, Day16, Day21 and Day31, respectively.  Where the frequency is larger than 5% its value is indicated on the bar.

The results show:

  • Transition to BLO+:
    • NAO+ and persistence are the most probable precursors for BLO+.
  • Transition to NAO­–:
    • BLO+ and, to a rather less extent, persistence are the most probable precursors for NAO–.  This characteristic is clear even in the longest range forecasts. Usually a strong breaking cyclonic wave south of Greenland favours destruction of BLO+ and subsequent more minor eddies tend to establish the NAO–.
    • NAO+ and BLO– are very unusual precursors for NAO–.
  • Transition to BLO–:
    • NAO+ and persistence are the most probable precursors for BLO–.
    • Transitions to BLO– are much less common than other transitions. Hence there is rather less confidence in the associated statistics.
  • Transition to NAO+:
    • Persistence is the most probable precursor for NAO+.  However, BLO– and BLO+ are also significant precursors for transition to NAO+. Transitions into NAO+ do not appear to have a preferred path.


In general:

  • Transitions to BLO+ and NAO– are slightly more frequent than transitions to NAO+.
  • NAO+ somewhat favours transitions into BLO+; BLO+ favours transitions into NAO–.
  • Transitions from blocking conditions are more confidently predicted than transitions into blocking. 
  • The probability of persistence (for more than 12 days) of NAO- is about twice what it is for other circulation types.

The model statistics and relative frequencies, at all forecast ranges, compare well with those from the analysis, indicating that the IFS is well able to simulate transitions, and suggesting that model bias in this context is not a major problem.


Example of circulation pattern and anomaly charts.

Fig8.2.2-8: Forecast mean sea level pressure mean anomaly verifying Day7-14 (23-30 Oct 2023).  The anomaly of surface pressure from ER-M-climate is:

  • higher over northern Scandinavia and Russia.
  • lower across Europe, particularly to the west of Britain.
  • moderately higher between Azores and Canary Isles.

This implies anomalously strong westerlies at around 40N and strong southeasterlies Denmark to Iceland.


Fig8.2.2-9: Forecast precipitation mean anomaly verifying Day7-14 (23-30 Oct 2023).  The anomaly of precipitation from ER-M-climate suggests:

  • a drier spell over Russia and northern Scandinavia, particularly over and to the lee of western Norway.
  • a wetter spell over Europe, particularly over western Europe. Also the easterly winds brought anomalously heavy rain (dark green) to eastern Scotland.
  • a drier than normal spell between Azores and Canary Isles.


Fig8.2.2-10: Forecast precipitation mean anomaly verifying Day7-14 (23-30 Oct 2023).  The anomaly of 2 m temperature from ER-M-climate suggests:

  • a cold spell over Russia and northeast Scandinavia.
  • a warm spell over Europe, particularly over the Balkan states.
  • a warm spell between Azores and Canary Isles.


Fig8.2.2-11: Forecast 500hPa height mean anomaly verifying Day7-14 (23-30 Oct 2023).  The anomaly of 500hPa height from ER-M-climate is:

  • higher over northern Scandinavia and Russia.
  • lower across Europe, particularly to the west of Britain.
  • moderately higher between Azores and Canary Isles.

The 500hPa height pattern is similar to the BLO+ pattern (See Fig8.2.2-1).

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.

Fig8.2.2-12: Regime-based skill measures for ensemble mean fields from various global forecast systems. There is skill where correlation is above 0.5.

Forecasts may be considered to have skill where the anomaly correlation is above 0.5.  Extended range forecasts show predictive skill for:

  • BLO out to between 9 and 13 days
  • NAO out to between 11 and 17 days

The longer period of skill for NAO modes may be associated with the NAO modes being more persistent (notably NAO-), and the fact that models are correctly capturing that persistence.


Fig8.2.2-13: Continuous Ranked Probability Skill Score (CRPSS) for the four Euro-Atlantic Regimes for several forecast models. 

In Fig8.2.2-13, ECMWF (black) shows some skill for NAO-/NAO+ up to 20-23 days while for BLO+ (blocking) and AR (Atlantic Ridge) skill drops to zero at about 16-17 days.  In other words, the ECMWF extended range forecasts have more difficulty predicting episodes of "Blocking" and "Atlantic Ridge" than they do predicting episodes of NAO+ and NAO-.  Note that the plot is based on about 10 years of re-forecast data from all the models shown.  Every ENS forecast is represented on every panel - i.e. the plot does not just relate to questions such as "when NAO- was forecast did it happen?".

In general, the skill of extended range forecasts:

  • for Days12-18 is generally better than both climatology and persistence of Day5-11 forecasts,
  • beyond Day20 is marginal, but for some applications and for some regions may have some utility.

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.  Fig8.2.2-14 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. 

Fig8.2.2-14: The mean ensemble variance as a function of lead time for all forecasts with the ensemble mean entering the BLO+ (red), BLO− (purple), NAO+ (blue), NAO− (green) sectors of an NAO-BLO diagram.  NAO- shows better predictability (less ensemble spread) than other circulation patterns.

In general:

  • NAO– has somewhat higher predictability associated (lowest growth rate of ensemble spread).
  • BLO+, NAO+, BLO– have somewhat lower predictability associated (larger growth rate of ensemble spread).

The reliability of forecasts of cold conditions over certain parts of Europe (notably the north) is:

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

Additional Sources of Information

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