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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 or controlling typical weather conditions in an area.   The patterns are spatially fairly large, they affect the weather over regions as large as continents and have a persistence of typically ten days or more.   These patterns over the Euro-Atlantic region may be described by a few weather regime definitions:  

North Atlantic oscillation (NAO) regimes:

• +ve NAO (NAO+) have generally stronger 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-) have generally weaker pressure differences:  Spells of easterly winds bring cold dry and calm winters with fewer and weaker storms to northern Europe and eastern USA. 

Blocking (BLO) regimes:

• +ve Blocking (BLO+) with ridging over Western Europe and Scandinavia:  Spells of wintertime easterly winds bring increased occurrences of very cold and snowy weather to much of Europe while Scandinavia sees very cold, dry and calm conditions. 
• -ve Blocking (BLO-) with ridging over the Atlantic:  Spells of northerly winds bring increased occurrences of wintertime snow to western Europe while Central Europe is more likely to see very cold, dry and calm winters 

The use of regimes allows increased predictability in the sub-seasonal range, offsetting the decreasing predictability of synoptic features.   Further, 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. 

Schemes for identifying regimes

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

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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.

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Fig7.4-1: Diagrams depict the Euro-Atlantic sector around which the regime definitions used in ECMWF products are based.

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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).

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Fig7.4-2: Diagrams depict the Euro-Atlantic sector around which the regime definitions used in ECMWF products are based.

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