Seasonal forecasting for europe and the stratosphere Flashcards
what is a deterministic weather forecast
predict the precise evolution of the weather at a specific place for a time in the future
what is a probabilistic forecast
information about what the weather is more or less likely to do
what does NAO stand for
North Atlantic Oscillation
what is the NAO
is a dominant mode of variability in large-scale atmospheric circulation in the Northern hemisphere winter.
what happens in the NAO
fluctuations in strength of pressure gradient between iceland and the azores islands
what happens in neg and pos phase of NAO
pos - stronger pressure strong westerly
neg - weak pressure weaker westerlies
how does the NAO relate to extreme weather events
pos NAO - stronger maritime influence more warm moist air to UK and Europe. alos more severe storms and flooding
Neg NAO - cold dry air from europe and siberia on uk and west europe
Give four factors that are thought to affect the phase of the winter North Atlantic Oscillation index.
- Autumn sea ice extent
- Tropical sea surface temperature anomalies (e.g. El Nino)
- Eurasian snow cover
- A large volcanic eruption
- Other phenomena that affect the stratosphere such as the 11 year solar cycle
what causes a stratosperic sudden warming to occur
Variations in the stratospheric winds are driven by atmospheric “planetary” waves. The waves are generated in the troposphere (the lowest part of the atmosphere) and propagate up into the stratosphere where they break, like waves breaking on a beach. The wave breaking causes the strong westerly winds in the high latitude winter stratosphere to slow and temperatures in the polar stratosphere to rise (hence the name sudden stratospheric warming). If enough wave breaking occurs during a particular time period, then eventually the stratospheric winds will reverse in direction altogether (i.e. become easterly) and temperatures at the pole may rise by several tens of degrees C. We then define that a stratospheric sudden warming has occurred.
How do sudden stratospheric warmings affect weather in Europe?
Sudden stratospheric warmings occur in winter and typically cause a more negative NAO index. They are therefore typically associated with cold and dry winters in the UK and western Europe. Because sudden stratospheric warming events can persist for up to a couple of months, they can have a long lasting effect on the weather throughout a winter season. An example is the winter of 2009/10 when a sudden stratospheric warming occurred at the end of January and we experienced cold snowy weather for several weeks.
