Weather regimes

The large-scale air-flow over a region of the earth, which is governed by the height contour patterns, can be considered as inducing, controlling or representing typical weather conditions in an area.   The patterns are spatially fairly large, they affect the weather over regions as large as continents and can have a persistence of ten days or more.  These patterns form the basis of a number of ECMWF graphical "weather regime" products, for the Euro-Atlantic region, and can be described as follows:  

North Atlantic oscillation (NAO) regimes:

  • +ve NAO (NAO+) has generally strong pressure differences:  Westerly winds predominate bringing mild, wet and stormy winter conditions to northern Europe and eastern USA while southern Europe is more likely to see cold, dry winter conditions.
  • -ve NAO (NAO-) has generally weaker pressure differences:  Spells of easterly winds bring cold, dry and calm winter weather with fewer and weaker storms to northern Europe (and eastern USA). 

Blocking (BLO) regimes:

  • +ve Blocking (BLO+) with ridging over Northwestern Europe and Scandinavia:  Spells of wintertime easterly winds bring increased occurrences of cold or very cold and snowy weather to much of Europe while Scandinavia sees very cold, dry and calm conditions. 
  • -ve Blocking (BLO-) low pressure is dominant over Northern/Central Europe, with some ridging to the west, over the Atlantic:  Spells of north to northwesterly flow bring an increased risk of wintertime snow to northwest Europe while Scandinavia and central Europe can be unsettled and eastern Europe is typically milder
  • Atlantic Ridge (ATR) could be said to be similar to BLO-, albeit with the major components of the broadscale pattern (and associated weather features) shifted somewhat to the east. 

The use of regimes allows one to home in on some of the more predictable larger-scale features in the sub-seasonal range, offsetting the decreasing predictability of smaller-scale synoptic features. Furthermore, some probabilistic assessment may be made of the timing of transitions from one regime to another.  There is some connection between weather regimes and surface weather and weather extremes.  Specific regimes particularly provide the environmental conditions conducive to large-scale cold spells in winter, heat waves in summer, and widespread heavy precipitation or thunderstorm activity.

One downside of regime classification is that there can be a lot of local variations within a given regime. For example within NAO- in winter southern England can be cold and dry, or there may be snow, or it can become very wet with higher temperatures.

Note also that the average temperature anomaly patterns for a given regime in summer can be somewhat opposite to what they are for the equivalent regime in winter.

Schemes for identifying regimes

ECMWF currently displays sub-seasonal range forecast data in regime format, based on Euro-Atlantic regimes, using two related schemes:

  • 4-regime scheme.  Shown as cumulative histograms using a four regime scheme (NAO+, NAO-, Blocking, ATR) where NAO denotes North Atlantic Oscillation, and ATR denotes Atlantic ridge.  The signs show positive or negative characteristics.  All regimes are independent of one another.  There is also a fifth category of no (clear-cut) regime.  This scheme is used to construct regime frequency histograms.
  • Mirrored 2-regime scheme.  Shown as phase-space diagrams using a two regime scheme ( NAO and BL) where NAO denotes North Atlantic Oscillation and BL denotes Blocking (both represented somewhat differently to the 4-regime scheme).   Each has a negative and a positive component that are in both cases exactly opposite to one another; these are NAO+/- and BLO+/-. There is also a fifth category of no (clear-cut) regime which would be denoted by the area close to the centre of the phase-space diagram.  This scheme is used to construct to construct Phase Space or Regime Projection diagrams.

ATR (Atlantic ridge) has some similarities with BLO- (negative Blocking) but is not the same.

Both schemes have their advantages:

  • 4-regime scheme reflects more accurately the true atmospheric patterns.  Typically the 4 regimes explain 67% of 500hPa height anomaly variance.  The Mirrored 2-regime scheme only explains about 50% of the 500hPa height anomaly variance.
  • Mirrored 2-regime scheme structure (orthogonality) lends itself to plotting on a phase space diagram, which in turn can indicate the strength of the regime fit.  This is not indicated with the 4-regime scheme.

Use of the schemes:

Regime assignment of forecast data is in each case computed in Empirical Orthogonal Function (EOF) space using "calibrated" anomalies of 500mb height/geopotential.

In order to achieve regime assignment:

  • first the anomalies are computed with respect to the re-forecast 20-year climate (SUBS-M-Climate).
  • then those anomalies are compared with reference points for the regime definitions. The regime definitions are themselves based on re-analysis data.

Fig7.4-1 and Fig7.4-2 show what these regime reference points look like in 500mb anomaly space for the two schemes.  These are for illustrative purposes only – the user should not expect a model forecast assigned to a particular regime to look exactly like these; ordinarily there will just be some vague resemblance.  

For the 4-regime scheme, whilst two sets of anomalies are shown, in reality the reference points against which forecast anomalies are compared actually vary smoothly through the year.  This is done by giving different weights to these patterns depending upon the month.  The reference points for forecasts for the main winter and summer months are almost identical to that shown in Fig7.4-1.  For intermediate month forecasts they will be a weighted combination.


Fig7.4-1: Diagrams depict the Euro-Atlantic sector around which the regime definitions used in some ECMWF products are based. 4-regime scheme (used for the Sub-Seasonal histogram products), shows the mean pattern of 500hPa geopotential (contoured, m) and anomaly (shaded, m) for each of the four regimes used. They were derived using the ERA-Interim reanalysis data for 29 years.  The panel shows:

  • left: the patterns derived for the winter period (defined here as October through to April)
  • right: the patterns derived for the summer period (defined here as May through to September).

Note: The NAO+ and NAO- are not the exact opposite of one another, in winter or summer.  The relative frequencies of each regime are normalised to sum to 100%, but the user should also note that the prevalence of different regimes does also vary over time (e.g. over decades).


Fig7.4-2: Diagrams depict the Euro-Atlantic sector around which the regime definitions used in some ECMWF products are based. The mirrored 2-regime scheme, (used for the Phase Space products), shows only anomalies for the positive modes of the two regimes (NAO+ and BLO+).  These patterns are used throughout the year.  The negative modes (NAO- and BLO-) are the symmetrical opposites of these - imagine multiplying all the anomalies shown by –1.

The regimes in the two schemes can, on the face of it, appear nominally rather similar.  However, NAO+ and NAO- regimes in the two schemes, although synonymous, have slightly different meanings.  Blocking is similar to BLO+ but they are not quite the same and, as stated above, ATR is evidently somewhat different to BLO-.


The use of regimes can be explored at medium range regime charts and sub-seasonal range regime charts.

Read more about the use of "Weather regimes" at ECMWF (2020 article)



(FUG associated with Cy50r1)