http://69.16.188.128/t6v3x7a6/cds/winter/winterforecast2009-10.pdf?dopvhost=media.netweather.tv&doppl=0e6478c945684efd&dopsig=4a5e6225f3370adf2909486b07b71aa3
‘Back to the 1970s’
First we get a tv programme set in the 1970s, industrial action increases echoing the ‘winter of discontent’, and guess what, the weather looks like playing its part with a seasonal outlook for December through February which has many parallels with a particular weather-climate episode during the mid to late 1970s.
Our winter bulletin provides a detailed background to this winter forecast. It describes in detail the factors which have influenced this forecast including reasoned statements. A copy of the bulletin may be found here.
2009/10 Forecast key messages:
* El Nino conditions are now well established in the Pacific although the weather patterns will not be typical to this climate phenomenon due to other and often conflicting global signals.
* The three winter months are likely to see considerable variability in temperature and rainfall with January featuring the coldest weather relative to normal.
* There is low confidence in the February forecast.
* Due to the monthly variability, there is low confidence in overall temperature and rainfall departures.
* Overall we forecast temperatures close to the long term average and rainfall above average.
How this forecast was prepared:
This forecast has been prepared using the following methods:
* An understanding of coupled ocean-atmosphere process which governs the interaction of climatic events such as El Nino with the overall global circulation.
* The use of historical analogues where important atmospheric variables were similar.
* Netweather.tv’s customised predictive model output based upon the Climate Forecast System (CFS) data provided by the National Centre for Environmental Prediction.
The ocean – atmosphere process
(1) El Nino
Latest buoy data identifies a large body of warmer than normal extending across the Equatorial Pacific Ocean. This pool of warm water extends to well over 150m down with surface temperature departures of +1.2C quite widely with the core of the warmer than normal water located in the central and western parts of the Pacific where surface values are approaching 2C greater than normal. An eastward developing wave is present in the subsurface which is helping to force waters which are up to 6C warmer than normal closer to the surface.
These warm waters are characteristic of an El Nino event. The current magnitude of the event is officially measured by the temperature departure over a period of three months although on current evidence the event would be classed as a moderate strength.
Bursts of westerly winds have helped to draw these warm subsurface waters to the surface. These events have taken place on roughly 6 to 8 week intervals. The most recent in mid October helped to trigger a significant increase in the strength of El Nino and there is evidence of the next burst beginning in the Indian Ocean.
This will continue to aid the development of warmer than normal waters across much of the Equatorial Pacific. Given that there will likely be at least one further burst of westerly winds this winter (probably mid to late January 2010 as the mean frequency for these tropical waves is 6-8 weeks), and the impressive relative warmth of the subsurface waters, it is very likely that this El Nino event will persist and strengthen from a moderate event during the first part of the winter to a high end moderate or strong event by the early part of February 2010.
Global weather model forecasts are in agreement about the continuation of this El Nino event although there is some uncertainty over the strength and likely duration.
The typical atmospheric response to El Nino is described within our winter bulletin. Broadly, this involves a strong sub-tropical jet stream across Florida extending into the Atlantic with low pressure and unsettled and mild conditions for much of Europe. Some El Nino winters have produced colder weather conditions for Europe, most notably those of weak strength and those where the core of the warmer than normal waters was centred further west in the central Pacific Ocean. Much of what we know about the weather-related impacts El Nino events is however subject to the influence of inter-decadal cycles.
The normal expectation would be for a high end moderate / strong El Nino event to result in an above average temperature for Europe during the winter. However, this has been anything but a typical El Nino event. From its rather unusual evolution where the warmest of the waters have developed further west (most Ninos develop off the coast of Peru and work the warmest waters westwards over time) to weather patterns more indicative of La Nina at times, the El Nino of 2009 / 10 is very atypical.
By far the most unusual behaviour of this El Nino event has been the stubborn resistance of the global weather pattern to ‘go along’ with the normal ‘forcing’. Although the pressure anomaly chart for the Northern Hemisphere over the last 30 days indicates that the general circulation is now being influenced by El Nino, there remains a sizeable disconnect between El Nino and the overall net balance of winds or Atmospheric Angular Momentum (AAM).
The correlation between El Nino and AAM during the winter months is about +0.9. This means the stronger the El Nino, the more westerly (positive) the angular momentum budget. Current data suggests that AAM is bumping around just below normal levels (zero anomaly) which is in contrast to where it should be (somewhere around +1.2) given El Nino.
There have been instances in the past where angular momentum was below average during an El Nino phase. Analysis of these years suggests a notable variation with the standard El Nino pattern with a particular plunge of Arctic air through Scandinavia into central Europe. This is in line with the theory that low angular momentum moderates the propensity for an over-amplified flow normally associated with El Nino suppressing the downstream ridge which is normally associated over western Europe which keeps temperatures mild. The forecast of where angular momentum might go in the next few months is therefore fundamental to how El Nino might manifest itself over western Europe. This is discussed on part (4).
(2) Pacific and Atlantic Sea Surface Temperatures
Since the publication of our winter bulletin, the overall spatial arrangement of sea surface temperature anomalies (SSTAs) has remained the same. The colder than normal pool in the north-east Pacific has continued to develop and intensify whilst the Atlantic cold pool off Newfoundland has been variable with a trend for these cooler waters to shift eastwards.
Comparison of years with a similar arrangement of SSTAs to those today and the broad atmospheric response indicates a southward displaced jet stream and low pressure in the Atlantic extending over western Europe with cold Arctic air filtering into Scandinavia and central Europe during January.
Warmer than normal waters persist in the eastern Atlantic which may serve to warm any south-westerly airstream over the UK.
(3) Atmospheric processes
Our winter bulletin identified several key factors which were likely to affect the upper and middle parts of the atmosphere during the course of the winter. Most of these are related to the Quasi Biennial Oscillation (QBO), a stratospheric wind flow which switches between east and west over the tropics.
Monitoring of these variables continues to reinforce our thoughts that this winter will see higher than normal pressure over the Arctic with a negative phase of the Arctic Oscillation (AO) the dominant regime. Negative AO phases are associated with below average temperatures across much of Europe.
Key atmospheric indicators are as follows:
* The temperature of the stratosphere over the tropics remains well below normal whilst the temperature of the stratosphere over both the Arctic and Antarctic has been above normal for much of the last month.
* Concentrations of ozone are at the highest levels over the Arctic for at least a decade for this time of year.
* Upper level winds are consistent with similar years.
* There are signs that an upwelling wave will commence over the Arctic around the start of the winter and will reach its peak intensity favouring high pressure over the Arctic in mid to late January.
All of these indicate that the upper atmosphere is behaving exactly as it should do and that the polar vortex (which is heavily influenced by stratospheric temperatures and ozone concentration) will be considerably weaker than at any winter in the last decade, possibly since the 1980s. The strength of the polar vortex has a major influence on the Arctic Oscillation and a weak vortex is likely to further promote higher pressure over the Arctic region displacing colder air into the mid latitudes.
It is also worth noting that the extent of snow cover across the Northern Hemisphere is currently above the long term average indicating that variables such as the low solar activity, QBO and El Nino are driving a stronger than normal polar cell indicative of colder winter conditions across parts of Europe.
(4) Atmospheric angular momentum (AAM)
Atmospheric angular momentum or globally averaged angular momentum is below average. This may be attributed to a persistent oceanic signal in the Pacific and Atlantic which favours easterly winds over westerlies. The ongoing solar minima (see our winter bulletin for more details) allied to the east QBO are all contributing negative tendency (east winds, low angular momentum) to the atmosphere. The QBO should remain strongly negative for the duration of the winter whilst the Pacific and Atlantic (in both hemispheres) have large pools of anomalously cold water outside of the tropics (colder water than normal favours east winds at the surface) which should ensure that there is a significant easterly component to angular momentum throughout the winter.
Crucially, this will restrain the full impacts of El Nino (which is a westerly wind phenomenon) on the global circulation preventing the fully amplified flow pattern seen in most strong or moderate El Nino years. It is not until February (following the 2nd westerly wind burst of the season) that we should see angular momentum lifted high enough to cooperate with the El Nino signal and even this is uncertain.
Historical analogues
We have selected historical analogues on the basis of:
* El Nino winters with disconnect with atmospheric angular momentum (1958/59, 1963/64, 1965/66, 1979/80, 2002/03)
* Comparable Sea Surface Temperatures across the Northern Hemisphere (1963/64, 1965/66, 1977/78, 1979/80, 2002/03)
* Years with low solar activity and east QBO (1965/66, 1974/75, 1976/77)
The complete list is: 1958/59, 1963/64, 1965/66, 1974/75, 1976/77, 1977/78, 1979/80, 2002/03.
Several other analogues were considered on the basis of the upper atmosphere but not included due to lack of support from El Nino. These also resulted in a considerable skew towards colder than normal temperatures.
The historical analogues point towards:
* An unsettled December with wind and rain and temperatures close to average over the UK, above average for large pats of Europe but below average over Scandinavia.
* A cold January with average or below average temperatures and drier than normal over much of northern Europe.
* Milder than normal temperatures over central and western Europe during February with above average rainfall.
It’s also worth noting that the potential range for cold in January is between -0.5C and -2C.
Climate Forecast System (CFS) model
The CFS has varied in its updates of late. The broad theme has been of an unsettled and average or slightly below December, a cold January and February with below average rainfall. However, the most recent update projects an average or milder than average January and colder February.
The broad theme of the pressure charts does however validate the historical analogues which support the idea of higher pressure over the Arctic displacing colder weather south into Europe.
Forecast
Temperatures for the UK are forecast to be close to average during the period December 2009 through February 2010. Rainfall (including snowfall) is forecast to be above average. These departures are likely to hide significant monthly variations.
December is forecast to have average or slightly above average temperatures. For continental Europe, temperatures are forecast above average but closer to average or even below average over Scandinavia. Rainfall is expected to be above average throughout.
January is expected to feature below average temperatures for central Europe, Scandinavia and the southern part of the UK in particular. Rainfall is expected to be below average for Northern Europe and above average for southern Europe. There is low confidence on the degree of departure for cold temperatures which are likely to range between 0.5 and 2C below average for much of central Europe and Scandinavia. This impacts the confidence level for the winter as a whole as a cold January would result in a colder start to February and increase the probability of colder than average winter as a whole.
The forecast confidence for February is very low. Our current forecast is for average or slightly above average temperatures for much of western and central Europe including the UK based on historical analogues. Rainfall is expected to be above average.
Our forecast is for the negative phases of the Arctic Oscillation and North Atlantic Oscillation which would normally favour below average temperatures across Europe. A cold forecast has not been issued overall due to warm sea surface temperatures in the eastern Atlantic and potential draw of south-winds during February resultant from a surge in angular momentum during this month and guidance from historical analogues.
Forecaster: Stewart Rampling
Long range forecaster, Netweather.tv
http://www.netweather.tv/index.cgi?action=other;type=winter-forecast;sess=
Quote selectie