Extreme european weather regimes

An idealised global atmospheric model is used to investigate mechanisms for the persistent northern hemispheric flow anomalies that led to extreme rainfall and flooding in the UK during summer 2007. The global response is obtained to forcing by idealised tropical heating anomalies, based on the observed OLR and vertical motion anomalies during June-July 2007. Heating anomalies over the East Pacific (associated with a developing La Niņa) and over the Indian Ocean (associated with a more vigorous Indian/Asian Summer Monsoon) give the largest response, reproducing the main features of the observed tropical and sub-tropical anomalous flow. The model response to imposed low level heating associated with warm sea surface temperatures over the north Atlantic is locally baroclinic, consistent with theoretical arguments but in contrast to the observed equivalent barotropic anomalies. The model results suggest that transient eddy feedbacks were important for the observed extra-tropical flow during summer 2007.

The period of record UK rainfall and flooding during autumn 2000 is revisited using the same modelling technique, following previous work which suggested a tropical catalyst for the persistent seasonal flow.