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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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- 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 anomalyover 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.
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Fig:8.2.7-2: 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.
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Fig8.2.7-3: 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.
Fig8.2.7-3: 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.
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- 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
Fig8.2.7-5: Predictability distribution on NAO-BLO diagram. Ensemble variance colour coded as the scale.
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The NAO–BLO space explains about 30% of the daily winter variability over Europe.
Transitions between circulation patterns
NAO-BLO diagrams may be used to illustrate the sequence of transitions from one circulation type to another (e.g. NAO+ to BLO+ to NAO–).
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 Fig8.2.7-7: NAO-BLO diagram for the medium range 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.
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 Fig8.2.7-6.
Fig8.2.7-7: 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–. See Fig8.2.7-7 for an example.
- 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.
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.
Fig8.2.7-8: 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.
Fig8.2.7-9: Continuous Ranked Probability Skill Score (CRPSS) for the four Euro-Atlantic Regimes for several forecast models.
In Fig8.2.7-9, 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. Fig8.2.7-10 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.
Fig8.2.7-10: 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 (e.g. Fig8.2.7-7). 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+
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.
In particular, the presence of and the phases of significant MJO events can be linked to an increase of skill in forecasting NAO– circulation patterns Fig8.2.7-11 compares the bivariate correlation (NAO & BLO) against analysis for forecasts initiated with and without an MJO event. Forecasts initiated with an MJO event show higher skill (an improvement of the order of one day) between Day8 and Day15. Correlations are significantly increased for Day11 and Day12 at a 90% confidence level, and for Day10 to Day13 at an 80% confidence level.
Some broad remarks can be made:
- The probability of the development of a positive NAO signal is significantly increased about ten days after the MJO is in phase 3 (i.e. enhanced convection over the Indian Ocean), and significantly decreased about ten days after the MJO is in phase 6 (i.e. enhanced convection over the Western Pacific and suppressed convection over Indian Ocean).
- The probability of the development of a negative NAO signal is significantly decreased about ten days after the MJO is in phase 3 (i.e. enhanced convection over the Indian Ocean), and significantly increased about ten days after the MJO is in phase 6 (i.e. enhanced convection over the Western Pacific and suppressed convection over Indian Ocean).
- The impact of the MJO on two other Euro-Atlantic weather regimes (the Atlantic Ridge and Scandinavian blocking) is much weaker.
Research has shown that in the winter half of the year the reliability of 2m temperature forecasts for Europe is influenced by whether or not there is a substantive MJO at analysis time. The IFS can model the evolution of a pre-existing MJO quite well and also can also capture the consequent influence at mid-latitude effects. However, the IFS has trouble generating new, substantive MJOs in the right place, which then impedes the predictive skill when there is no MJO at start time.
Thus the 2m temperature anomaly forecasts for Europe tend to be:
- more reliable if there is a substantive MJO at analysis time, (i.e. lying outside the circle on the Wheeler-Hendon diagram).
- less reliable if there is not a substantive MJO at analysis time.
Forecasters should be able to use these results to good effect when examining the monthly forecasts.
Fig8.2.7-11(left): Bivariate correlation (NAO & BLO) for forecasts with (red) and without (black) an MJO event in the initial conditions. Colours refer to initial conditions for the forecasts:
- black - no initial MJO.
- red - initiated with an MJO (shows higher skill between Day8 and Day15).
Fig8.2.7-11(right): Brier Skill Scores (BSS) for NAO– predictions according to the initial MJO phase. Colours refer to initial conditions for the forecasts:
- black - no initial MJO.
- red - MJO phases 2–3 (enhanced convection over the Indian Ocean).
- blue - MJO phases 4–5 (enhanced convection over the Maritime Continent).
- green - MJO phases 6–7 (enhanced convection over the western Pacific).
- brown - MJO phases 1 and 8 (suppressed convection over the Maritime Continent).
The MJO influences skill in forecasts concerning NAO– circulation pattern:
- During MJO phases 4&5 skill is increased for the Day13-20 period.
- During MJO phases 6&7 skill is increased for the Day8-13 period.
The corresponding increases in skill for NAO+, BLO+ and BLO– are small.
Fig8.2.7-12: 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.
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.
Fig8.2.7-13: Meteosat (IODC) IR (Channel 4) image DT 00UTC 25 Feb 19. A large area of convection lies over an equatorial region of the Indian Ocean typical of Madden-Julian Oscillations (MJO
Fig8.2.7-14: MJO Wheeler-Hendon diagram for the monthly forecast based on DT 00UTC 25 February 2019. The colours represent ENS forecasts at various lead times as given by the key above the diagram. Initially the MJO lies within Sector1 (Western Indian Ocean) and is forecast to progress into Sector2 (Eastern Indian Ocean) by Day5 with a fairly limited spread among ENS members. The mean position of the ENS forecasts during the subsequent ten days show continuing westward progress although with an increasing spread before weakening (moving into the central circle) by Day20.
Fig8.2.7-15: Day15-21 forecast Mean Surface Level Pressure Anomaly verifying Day15-21 (11-17 Mar 19). In the North Atlantic, the forecast suggests the pressure anomaly is likely to be 10-20hPa below ER-climate across areas in the north and 5-10hPa above ER-climate further south implying an anomalously strong westerly flow towards Europe (i.e. a NAO+ regime).
Fig8.2.7-16: Day15-21 forecast 2m Temperature Anomaly verifying Day15-21 (11-17 Mar 19). The forecast suggests an anomalously mild period over central and western Europe where the 2m temperature is forecast to be 0°C-3°C above ER-climate.
Fig8.2.7-17: Day15-21 forecast Precipitation Anomaly verifying Day15-21 (11-17 Mar 19). The forecast suggests an anomalously wet spell over western Europe where precipitation is forecast to be 0-10mm above ER-climate over western Europe, and 10-30mm over Ireland and the United Kingdom.
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6.
Fig8.2.7-7: 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–. See Fig8.2.7-7 for an example.
- 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.
Fig8.2.7-15: 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.
Fig8.2.7-16: 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.
- a drier than normal spell between Azores and Canary Isles.
Fig8.2.7-17: 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.
Fig8.2.7-17: 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 Fig8.2.7-1).
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.
Fig8.2.7-8: 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.
Fig8.2.7-9: Continuous Ranked Probability Skill Score (CRPSS) for the four Euro-Atlantic Regimes for several forecast models.
In Fig8.2.7-9, 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. Fig8.2.7-10 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.
Fig8.2.7-10: 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 (e.g. Fig8.2.7-7). 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+
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