Full Summer Weather Forecast 2012
Last winter, the global weather-climate system went into a La Nina state where colder than normal waters persisted across the eastern and central equatorial Pacific. Aided and abetted by the pattern in the upper atmosphere, this gave us our unseasonably mild weather pattern in western Europe, punctuated by a notable cold mid-winter spell across Europe.
Contextually, summers following La Nina events tend not be great for sun-seekers, typically cooler and wetter than normal related to the cooling effect of below average sea temperatures in the Pacific during the winter months, akin to having the freezer open in a room all winter long.
Figure 1: air temperatures (850hPa) for summers following La Ninas across the Northern Hemisphere suggests notably below average temperatures (blues), particularly across the tropical Atlantic Ocean and Europe.
However, before you start booking cheap flights abroad, consider that the atmosphere is throwing us a lifeline and there are a number of pointers towards a much more clement, dare one mention it traditional British summer with a no one weather type dominating throughout and passages of fine weather interspersed with more unsettled conditions giving rise to a forecast which suggests temperatures to be most probably at or slightly above average and rainfall below average.
There are growing indications of a trend to more settled conditions in July and August.
A key aspect of the forecast is the possible development of low pressure in the Atlantic and a weaker than normal Azores ridge (the ridge of semi-permanent high pressure to the south-west of Europe which is usually associated with settled summery weather). Whenever you get this situation, there is a chance for much warmer, humid conditions to dominate as high pressure systems block the weather pattern over Europe sending warm air towards the UK.
This was the situation as we saw it last spring and our summer 2011 forecast incorrectly called this. Although this pattern didn.t manifest until the end of the summer, it did arrive and delivered a much warmer and drier than average autumn so we wouldn.t discount the chance of a much more drier and warmer summer this time round although on balance of probability and global forecast modelling, our percentage call is for rather more mixed and average conditions, the warmer and drier just shading the cooler and wetter phases with the best of the weather during July and August. There is a reasonable chance that the start of the summer holidays at the end of July will see the best of the summery conditions.
The upper air pattern this spring
The profile of winds across the globe high up in the upper atmosphere has, since last October, been running along very distinctive and potentially useful lines for a forecast starting point. Roughly every eighteen months, the direction of winds in the stratosphere (some 13km above the earth.s surface) reverses and this is referred to as the Quasi Biennial Oscillation or QBO. Currently we are in an easterly phase of the QBO and there is a notable similarity between the wind flow patterns experienced in the last eight months and those in a number of years since 1949. The QBO tends to operate in reasonably predictable patterns and we can use to this our advantage when putting a seasonal forecast together.
Figure 2: This plot represents the mean wind flow high up in the atmosphere across a section of the atmosphere from the South Pole to the North Pole over a period of time from October 2011 through to early May 2012. Blue indicates mean easterly wind flow, red and yellow mean westerly flow.
The easterly flow across the equator is clearly evident, indicative of an easterly phase of the QBO.
The QBO is an important seasonal forecast tool as it can give us an insight into where the centres of tropical thunderstorm activity will be located and how intense these areas might be, particularly where solar activity is broadly increasing. This has knock in effects for the entire global weather pattern through the formation of high and low pressure centres in the sub-tropics and mid-latitudes.
Figure 3: comparison of two similar years to the current profile (1968 and 2010) shows just how the QBO can be used as a predictive tool.
The profile is extremely good in terms of the pattern observed since October and suggests that the mean easterly winds over the equator will persist well into the autumn with a weak mean easterly wind persisting across the northern hemisphere from 30N poleward. Note also how the southern hemisphere will experience strong westerly winds during their winter according to this analysis.
Analysis of similar years suggests some very important considerations for this summer:
Figure 4: For our list of similar QBO years, we can analyse the distribution of tropical convection (thunderstorm activity). This suggests that there will be anomalous convection (negative outgoing longwave radiation) or cloudiness in two main areas, to the east of the International Dateline and over the tropical African continent. Note also that there is a signal for increased convection (westerly winds) in the North Pacific, potentially lowering pressure in this area which in turn could set up a pattern of ridges and troughs (high and low pressure centres) with low pressure signal over the Atlantic.
Transitions to El Nino
- The centre for tropical thunderstorm activity is likely to be focused in several areas, notably over Africa, the western equatorial Pacific and east of the International Dateline in the Pacific. This tends to favour lower than normal air pressure over the Pacific and Atlantic.
- Activity is likely to be moderate or strong and at least one strong episode of tropical waves (the Madden Julian Oscillation) is expected.
- This pattern of upper atmosphere winds is unlikely to alter significantly during the summer and there is no strong signal for a dramatic shift in our weather pattern because the entire system will be effectively locked . what variability there is coming from one guiding force and the theme will be of steady evolution and there is no indication whatsoever of a 2007 type summer which was characterised by a sharp increase in easterly winds over the Arctic.
The surface temperature of the equatorial Pacific Ocean exerts a significant influence on global weather patterns through the positioning of the Walker Cell, one of the major circulatory patterns in the global weather . climate system. There are four key areas of the Pacific which are of most interest to long range forecasts: extending from .Nino Region 1. in the eastern equatorial Pacific off the coast of South America; through to Nino Region 4 in the western central equatorial Pacific.
Warmer than normal events are known as El Nino, colder events are known as La Nina. Since the late winter, the temperature of sub-surface waters in the eastern Pacific have warmed and these waters have risen to the surface. Currently, temperatures in the eastern equatorial Pacific are between 1.5C and 2C above normal.
El Nino events tend to start with a warming of this nature but at present, the official status of this event is neutral due to more normal surface waters further west in the central Pacific. However, this is missing the point as the current strength of the warmer than normal waters in the eastern Pacific rates of one of the largest in the last sixty years. Current long range models indicate that this situation will not alter dramatically in the coming months meaning that we will likely see a continuation of anomalously warm sea temperatures and a significant influence of the pressure patterns across North America , the Atlantic and Europe.
Figure 5: Graphical representation of the four key regions of the equatorial Pacific where La Nina and El Nino are recorded (courtesy of Climate Prediction Centre).
Figure 6: The typical profile of sea surface temperatures associated with a strong positive anomaly (warmer waters) in El Nino Regions 1 and 2 during the early stages of an El Nino event.
Figure 7: This data plot shows the current sea surface temperatures off the coast of South and Central America. Temperatures are around 1.5C above normal, the warmest they have been for several years and higher than the EL Nino event in 2008.
Figure 8: Model forecasts for this part of the Pacific continue to suggest a sustained warming of sea temperatures, falling gradually back during the second half of the summer as the warmer waters transfer westwards.
Reanalysis of warmer than normal waters over El Nino Regions 1 and 2 (eastern equatorial Pacific) shows the type of weather pattern typically experienced. Breakdown of the monthly weather patterns reveals June to be more unsettled and showery with July and August faring better for NW Europe with more in the way of high pressure.
Figure 9: This shows how the weather .climate system adapts to warming waters in the eastern Pacific associated with El Nino Regions 1 and 2. Notice how there is lower than normal pressure (blues) over the North Pacific and Atlantic with higher than normal pressure over NW Europe.
Transitions form La Nina events to El Nino events generally are cooler and unsettled. The historical parallels with warmer than normal waters in the eastern Pacific (stronger warming in the eastern Pacific) tend to suggest slightly above average temperatures due to a greater influence of high pressure.
The Madden Julian Oscillation (MJO)
The MJO is a tropical wave which traverses the tropics. It is associated with strong thunderstorm activity and typically takes between 45 and 60 days to complete a circuit. It is an important inter-seasonal influence on global weather patterns, setting up circulation patterns in the atmosphere which have downstream impacts throughout the mid and high latitudes over time.
During the spring, the MJO was .active. with a vigorous wave recorded. It has subsequently decayed although given the numerous centres of tropical thunderstorm activity forecast (related to the upper air pattern), and increasing influence of warmer than normal sea temperatures over the eastern equatorial Pacific, there is every prospect of the MJO being re-invigorated in July which could significantly influence weather patterns into August.
Figure 10: Plot showing the MJO in the last 90 days. Notice the amount of time the MJO has spent in phases 5, 6, 7 and 8 which indicate a shift in the global weather patterns trending towards an El Nino event.
A noticeable feature of the MJO during April and May has been its reluctance to move from its position over Africa and the central / eastern equatorial Pacific (so called phase 8). If the MJO becomes active in the summer around these areas, the net influence is likely to be high pressure forming over Europe as a result of lower than normal pressure over the Atlantic.
Sea surface temperatures
Analysis of the current pattern of sea surface temperatures across the Northern Hemisphere can provide a useful benchmark for long range forecasts.
The current profile of sea surface temperature anomalies looks like this.
Figure 11: current profile of sea surface temperature anomalies across the Northern Hemisphere. Note: the horseshoe of colder than normal waters in the Pacific: the warming waters off the South American coast associated with El Nino; colder than normal waters over the tropical Atlantic and warmer than normal waters over the North Atlantic.
For years with a similar pattern, we get the following pressure anomaly pattern.
Figure 12: pressure pattern associated with years with a similar sea surface profile. Note the lower than normal pressure over Europe indicating unsettled conditions.
On the face of it, this would suggest the entire summer to be unsettled. However, during July and August, the pattern observed for similar years is a lot different and the overall picture is being skewed somewhat by June. The plot for July is a lot different.
Figure 13: July pressure patterns for similar years for sea surface temperatures. Notice how the yellow colours (higher than normal pressure) are located over the British Isles.
The Climate Forecast System (National Centre for Environmental Prediction) is one of the best performing long range predictive models. It correctly forecast last summer and the general pattern last winter (possibly missing the mid-winter cold snap). Broadly, the model has a strong emphasis on the dynamical ocean-atmosphere process and therefore is very much respected.
The model has, for some considerable time, consistently forecast a generally average temperature summer, marginally drier than average. The pressure forecast is very similar to that based upon the overall sea surface temperature profile with marginally higher than normal pressure centred over NW Europe during the second half of the summer and slightly warmer than average temperatures and marginally lower than normal rainfall overall, but with monthly variations for temperature, rainfall and pressure. Of late, the model has begun to sniff the possibility of a warmer than normal July and has maintained a theme of generally average temperatures and rainfall.
Figure 14: CFS forecast for air pressure during June with higher than normal pressure giving more settled and warmer weather.
Figure 15: CFS forecast for air pressure during July. The model suggests high pressure to be located further west in the Atlantic giving largely average temperatures and rainfall, warmer in the west.
Figure 16: CFS forecast for August suggesting high pressure to be located over and to the north-east of the UK with low pressure centred to our south-west feeding up warm and humid weather.
Figure 17: CFS temperature forecast for August indicating warmer than normal weather, especially for the north of the UK.
By looking at historical examples under a similar set of atmospheric variables, we can get an idea of what the broad weather pattern might look like. Key emphasis here has been placed on the profile of upper level winds across the planet, the overall state of solar activity, warmer than normal sea temperatures over the eastern equatorial Pacific and the likely distribution of tropical thunderstorm activity.
Three similar years are examined: 1959, 1968 and 1989.
Figure 18: Pressure forecast based on three similar years. This suggests higher than normal pressure (oranges) centred over the UK and Scandinavia with lower pressure (blues) over North Africa, Europe and the Atlantic.
Figure 19: Temperature forecast based on the three similar years, suggesting above average temperatures over the UK, especially western parts.
Reanalysis of these years underlines the key points of the forecast:
- Weaker than normal pressure over Africa associated with stronger than normal thunderstorm activity
- Weaker than normal pressure in the Atlantic due to the upstream influence from the Pacific and developing high pressure centre over central America tied into warmer than normal waters in the eastern equatorial Pacific
- Higher than normal pressure over NW Europe
Putting these key elements together, allied to a reasonably consistent CFS model forecast (albeit some disagreement with June), we are able to make the following prediction:
Forecaster: Stewart Rampling, Long Range Weather, Netweather.tv
- Temperatures close to or slightly above average most favoured
- Air pressure likely to be higher than normal, particularly during July and August with rainfall below average overall
- Notably more unsettled during June where rainfall may be around or just above average and temperatures around average
- Mean trough or area of low pressure to be located over the UK for the first few weeks of June, but tending to become more centred further west in the Atlantic over time, feeding warmer and more humid air up towards the UK
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