Seasonal forecasting for Europe & stratosphere
Predictions
Prediction beyond weather timescale
NWP
Types of seasonal prediction systems
Skill of tropospheric weather forecasts has increased at rate of 1 day every 10 years
Difference in skill between SH and NH has decreased over time- due to improved global observation systems
Deterministic skill of weather forecasts is low beyond 7-10 days
Small errors in forecast starting conditions grow over time- long range prediction is impossible
Need to run ensemble forecasts to gain insight on long timescales
For seasonal prediction: need to identify 'slowly evolving' components that influence average behaviour of weather systems
Split Earth up into horizontal grid (latitude-longitude)
Split Earth up into vertical grid (height or pressure)
Solve fundamental equations of physics through time and space
Why interested in seasonal predictions?
Supermarkets
Energy traders
Utility suppliers
Transport
Planning ahead
weather can influence determination of stock
looking at energy provision before cold months
Planning ahead
North Atlantic oscillation (NAO)
Positive phase: Winds coming off Atlantic send warm and damp air into britain
Negative phase: High pressure forces cold Arctic winds south towards Britain, and pushes the wetter weather into Europe
Exerts a strong control on European weather and climate on timescales of weeks to many years
Weaker than usual pressure gradient
Strong low pressure
Winter North Atlantic Oscillation Index
NAO index fluctuates from one year to the next
Variations from one decade to the next
Winter North Atlantic Oscillation Index= difference in sea level pressure between Iceland and Azores islands across Atlantic
Suggestion: Postive trend during 20th century was forced by climate change
Case study: Winter 2009/10
Negative NAO conditions- amplitude too weak and extension into eastern Europe not quite right
3 month anomaly in sea level pressure
Total cost: £13 bn
Fourth Road bridge closed for 10 hours due to snow
Eurostar cancelled, Gatwick airport closed
Factors affecting seasonal prediction of NAO
Stratospheric phenomena
Eurasian snow cover in early winter
Autumn sea ice extent
Soil moisture levels
Atlantic & Pacific sea surface temps
Large volcanic eruption
Stratosphere
Temp decreases with height in troposphere (0-15km)
Sudden stratospheric warming in Jan 2009- around 30km, 10hPa
Temp increases with height in stratosphere (15-50km)
Calmer conditions in the summer
Climate in winter: cold over pole- no sunlight
Strong jet stream from West to East in winter- up to 40m/s
West to East jet stream weakens and breaks down
Anomalies in West- East winds propagate downwards in stratosphere over time
Caused by Rossby waves formed when air flows over mountains or from land to sea- contrast in temp
More Rossby waves in NH than SH
Rossby waves propagate in stratosphere and break likes waves on beach- weakens polar vortex -> sudden warming
Occur in around 2/3 winters- occur in NH during winter (Jan-Feb)
Predictability of NAO
Ensemble members with sudden warnings show skilful prediction of NAO from one winter to the next
Ensemble members without sudden warnings show low skill in predicting NAO
Stratosphere plays important role in increased predictive skill
New seasonal forecasting system from Met Office has skill predicting the NAO