Séminaire
In the last decade, western Europe has seen a number of extreme seasons. For example, anomalously high precipitation totals for autumn 2000 and the last 3 summers, as well as the exceptionally high temperature of summer 2003 and the coldness of the last winter. One common feature in all these examples is the existence of persistent or recurrent, near stationary Rossby wave patterns on the tropopause. For example, in summer 2007 a Rossby wave persisted for 6 weeks with only one week intermission. The three cyclonic weather systems bringing 38% of the season's rainfall stalled under the trough during their development. It appears that in order to predict the anomalous season, it was necessary to predict the Rossby wave pattern.
This talk examines the Rossby waves for these seasons. Discussion focusses on the "phase speed" of large amplitude waves and its meaning, growth in meridional amplitude and their interaction with the background flow. The isolation of a suitable background state is crucial to analysis of wave activity at large amplitude. An approach is explained where the background state relies on integral conservation properties (mass and circulation) and the wave activity is defined by material transport of tracers (potential temperature and potential vorticity). For the first time this has been applied without approximation to analyses revealing some fascinating results linking baroclinic growth, poleward migration of the jetstream, blocking and stratospheric warmings. Some speculation will follow on the nature of climate and its relation to wave activity. Does it make sense to intersect a chain of events to identify a "trigger" for an extreme season or identify a model error resulting in poor forecasts?
Riwal Plougonven, 01 44 32 27 31