Extreme Seasons in Europe: Summer 2007

An idealised global atmospheric model has been used to investigate mechanisms for the persistent northern hemispheric flow anomalies that led to extreme rainfall and flooding in the UK during summer 2007. From past research the global response was obtained to forcing by idealised tropical heating anomalies based on the observed tropical OLR and vertical motion anomalies. The largest features of the observed tropical and sub-tropical anomalous flow were reproduced but the idealised forcing also created major features not seen, or much weaker than, in the observations. The extra-tropical anomalies were not reproduced. The lack of response to tropical forcing in the extra-tropical regions of the summer hemisphere is consistent with the fact that the tropical easterlies do not allow influence through Rossby wave propagation into the extra-tropics and may also suggest other mechanisms were important.

Relaxation experiments, where the model variables in a target region are relaxed towards analyses, were also performed. It was found that successfully reproducing the tropical flow is crucial for the model to reproduce the observed extra-tropical anomalies. The response to tropical relaxation obtained with the idealised model was compared to ensembles of seasonal forecasts obtained with the ECMWF’s model, a state-of-the-art GCM. The responses were found to be similar suggesting the possible use of a much simpler model as a tool for understanding the impact of aspects of tropical convection on higher latitudes. The arctic sea ice extent also reached an unprecedented minimum in this season. Relaxation experiments showed that the flow over the Arctic was important for the observed extra-tropical anomalies.