Weather Logistics UK

Atlantic High Blocking Index

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Description of an Atlantic High Blocking Index

Our blocking high index (classification system) demonstrates how seasonal predictions can be produced by ranking high pressure patterns by their relative impact upon weather. By the use of climatology data, forecasts can relate each long term weather patterns to the 1961 - 1990 climate averages. Our seasonal forecast model is run to produce outputs for four different fixed atmospheric (climatic) states, with high pressures placed strategically over the Atlantic Ocean judged by the forecaster based on: current sea-surface temperature patterns, North Atlantic Oscillation (NAO), Arctic Oscillation (AO), El Niño Southern Oscillation (ENSO) indices and previous blocking character over the past month.

A monthly mean and standard deviation in the jet stream positions and strength feed the simulation, in combination with a field of average or current sea surface temperature data from the US National Oceanic and Atmospheric Administration (NOAA) reported on a monthly basis. As only the current SSTs are known, it is often best to examine the SST anomaly and then make the relevant adjustment to the SST climatology for the future months. The entire model is based on a perturbation of the climate average, to make only adjustments based on observations whilst constraining the results to the current climate signal. Air flow is diverted around high pressures using planetary wave theory in accordance with the negative NAO phase at an indexed value of 1.0 (one standard deviation above a 30 year reference period). The model is therefore most applicable in an atmospheric state where high pressure dominates the north-west Atlantic Ocean. In this case we assume a north-westerly deflection is more common than south-westerly component. In an observed blocking scenario the beta-plane approximation infers a poleward shift of upper air flow that steers low pressures over the Arctic by a planetary wave "path obstruction". In this case the upper air flow is represented by the Coriolis deflection - a function of northerly flow diversion - which is corrected for as the jet stream returns back the stable zonal mean latitude.

The jet-stream flow diversion model generates flow simulations for four different blocking high inputs. The results are reported on a seasonal basis at lead times of t-100 days. The climate states are labelled as ensemble numbers of 1 to 4 in ascending order of blocking impact - producing a seasonal mean for each variable. The outputs from our deflection simulation are then ranked to produce a set of 24 outcomes of temperature, storminess and other derivatives. The blocking high pressure patterns are classified in terms of their relative positions to the UK (15 or 25 degrees Westward of Greenwich) and their diameters (geometric planetary wave amplitude) of low-level deflection of curvature diameter 503km or 750km (double flow path).

The effect of these blocking patterns are given an indexed value from 1 to 10, with 10 representing the highest resistance to Atlantic low pressure passage, where winter seasonal temperatures are up to 5 degrees Celsius below average and storminess and precipitation events up to half of the 1961 - 1990 climate average.

High Pressure Blocking Index Showing Scenarios for Autumn and Winter with Increasing Storminess and Temperature.

The system functions by logistics regression, with the central four columns of 0.4 (40%), 0.3 (20%), 0.2 (20%) and 0.1 (10%) indicating the relative likelihoods of observing a particular pattern during the course of a 3-month period. Since these values add to unity, they are multiplied by the blocking number, in the top-left square of the blocking index sheet. More accurate weighting of each blocking scenario over the past month could be obtained from detailed analysis of archived synoptic chart data, or by computationally expensive numerical modelling using input of monthly jet stream climatology data and sea surface temperatures. With the latter method initial conditions could then implicitly input the blocking high pressure states artificially. Two pieces of information are required; the high's intensity, or position, and there remains significant uncertainty in both domains that limits the accuracy of our results. The most extreme case possible is where blocking patterns are ranked from most to least influential, i.e. 4,3,2,1 or 1,2,3,4. A blocking index of 10.0 on our scale represents a theoretical state where for almost half of the seasonal period a strong jet stream deflection, continental winds and atypical weather dominates the British Isles weather patterns.

A low blocking score (index) of 1.0 indicates that either a distant and weak blocking pattern dominates for only a small fraction of the season, or there is little significant tendency to any high or low pressure regime. Since high pressures are common in parts of the North Atlantic, but their positions are variable it is likely that for around half of the season a small high pressure influence will impact upon the temperatures and storminess of the British Isles. Large-scale and long-term high pressure blocking scenarios such as this are rare, whilst an extreme Eurasian blocking with Easterly winds can become stable due to he outflow of cold-dry air into Western Europe that prohibits the passage of cyclones. In addition snowfall and clear-skies over the vast Northern continent helps deepen a Eurasian high pressure blocking pattern through sustained radiative surface cooling.

Easterly Winds - Extreme Blocking Patterns and Western European Cold

The strongest possible blocking pattern in this model is when a large high pressure sits just to the west of the UK persistently, that consistently resists the prevailing westerly flow, and which prevents almost all Atlantic frontal systems from passing. The model cannot currently encompass easterly continental blocking patterns, since planetary layer radiation budget and continental snow cover uncertainties make this a complex task. It is however acknowledged from meteorological observations that frequent high pressure blocking patterns over the Atlantic Ocean usually precede a full continental blocking pattern, which is thought to be attributed to a reduced influx of water vapour that reduces cloud cover and surface temperatures over Europe and leads to intense radiative cooling. During the winter months this is marked by long-term extreme cold in the form of a "dirty high" pressure system, with associated north sea fog and on occasions persistent snowfall that extends from Denmark to the East coasts of Britain.

Blocking patterns of 8 to 10 on the index are almost always followed by extreme Eurasian cold. Yet Easterly winds can also occur due to Sudden Stratospheric Warmings (SSWs), but rarely without previous significant Atlantic flow blocking close to the British Isles.

Planetary Wave Breaking Events: Extreme High Pressure Blocking

High pressure systems are often elongated and larger than low pressure systems, which tend to be much more compact and rounded. This is a consequence of the geostrophic (pressure gradient and Coriolis) and cyclostrophic (pressure gradient and centripetal) force balances. However these forces are not regular, particularly in the mid-latitudes where the Coriolis force changes readily with latitude and where North Atlantic high pressure systems are frequently observed to have a extensive northward length-scale. The northerly end of these high pressure systems experience much more of the Coriolis "apparent turning" force due to the closer proximity of the polar axis. In addition, the upper level temperature gradient is much stronger when the planetary wave amplitude is higher, which based on thermal wind approximations give rise to a fierce jet-stream across the North flank of the high pressure system. North Atlantic wave breaking in high amplitude planetary waves is perhaps more common than other parts of the world due to Greenland's high altitude peninsula. This flow obstruction introduces significant low-level friction that prohibits the rapid flow required to sustain a northerly intruding anticyclone. It is therefore more likely that the anticyclone "opens up", joining the Greenland high briefly and opening up the system to cold Arctic "doors". The high pressure will then shift readily north establishing its more stable position to the North of the British Isles.

"Planetary wave breaking is the rapid, irreversible overturning of potential vorticity (PV)" [1]Dr C Strong, 2008, JGR. Atmospheric vorticity is a measure of the local spin of a fluid element from local and planetary rotation forces and [2]PV is a conserved atmospheric quantity that also incorporates that effects of atmospheric layering (stratification). During wave breaking events the PV imposed on a latitudinally stretched (high amplitude planetary wave) is susceptible to top-end collapse to produce a stable reconfiguration. The process initiates when the North-South extent of a planetary wave extends a critical amplitude and the wave crest at the poleward side becomes unsustainably steep. This process is known as a planetary wave breaking process.

Abrupt transitions to cold weather are highly likely during planetary wave breaking events. In the British Isles they are regularly characterised by a polar air front sweeping south on a strong northerly jet stream. This comes in the form of an Arctic maritime flow, associated with heavy snow showers during the Winter season. Relatively high horizontal sheering at the top (Poleward end) can whip low pressure systems over to the North of an anticyclone and act to twist the N-S asymmetric high pressure clockwise and eastward. The end result is a more stable atmospheric state where the blocking anticyclone extends W-E along more-or-less constant upper air temperature and Coriolis geophysical field. The result is the introduction of extreme cold from Scandinavia and West Siberia, referred to frequently as the "Beast from the East". Planetary wave breaking events frequently occur when the amplitude of the Atlantic high pressure blocking pattern reaches the high end of the "blocking high index" i.e. characterised by a persistent 8 to 10 on our scale.

Storminess (Precipitation Events) and Atlantic High Blocking Patterns

High pressure blocking index chart for Winter storminess anomaly scenarios with increasing "localised" blocking
UK storminess during Winter when blocking index is increased for positive NAO index values

Please view our Atlantic blocking high index for further details on the blocking high index.

See “Designer Weather” – Our Seasonal Predictions are Based on Hard Science for more details.

The average temperatures in the northern hemisphere winter season are strongly dependent upon the sign and magnitude of the North Atlantic Oscillation (NAO), which in turn acts as the "gate-holder" of the upper air flow. During the negative NAO phase, the westerly flow component is weakened, as the N-S gradient in pressure is reduced. In terms of jet stream blocking patterns, a firm Atlantic high in the north-east Atlantic centred close to or just to the West British Isles weakens the NAO and increases the incidence of negative NAO events.

There is a negative correlation between storminess (frequency of precipitation events) and intense Atlantic blocking high patterns in the close proximity to the British Isles. In contrast, strong and distant high pressure blocking patterns over the mid-Atlantic Ocean just south of Greenland increase the prevalence of fierce winter storms. This is particularly so if the polar jet is deflected over the tip of Greenland, where intensification of the upper level winds and a strong polar maritime air-stream is plunged southward. When the cool and moist air meets tropical maritime air further south, rapid storm formation known as cyclogenesis leads to the development of strong winds and heavy rains over Northern England, Northern Ireland and Scotland.

The upper air flow modelling at Weather Logistics Ltd simulates a 50% variability in the seasonal storminess associated with blocking patterns. The windy and wettest weather typically occurs when the blocking index is low or distant from the UK, whilst calmer weather is more likely when it is high. There is a negative correlation between NAO values and Atlantic blocking patterns.

Temperature Anomalies and Atlantic High Blocking Patterns

Blocking index chart for Winter temperature anomaly scenarios with respect to increasing blocking index
UK temperatures during Winter as blocking highs incidence increases for positive NAO index values

Please view our Atlantic blocking high index for further details on the blocking high index.

During negative NAO periods the prevailing trajectories of low pressure systems at the surface are directed northwest over cooler polar waters. The converse is true when a positive NAO dominates, with a strong west to south-westerly air-stream blowing from the warm waters of the subtropical Pacific. As a direct result of the blocking impact upon the lower level path of air, there is strong negative correlation to temperatures. The correlation is strongest during December to February, as the monthly average jet stream latitude is situated over the centre of the UK.

There is typically a +/-1.5C difference in observed temperatures between low and high blocking indices, with strong and proximal high pressure patterns leading to severe winter cold spells and a tendency toward a continental weather pattern.

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