Extreme European weather regimes

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 and autumn 2000. From past research the global response was obtained to forcing by idealised tropical heating anomalies based on the observed tropical OLR 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, in the observations. The extra-tropical anomalies were well reproduced in autumn 2000 but not in summer 2007. 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. Two different methodologies were then considered, an inverse modelling technique and relaxation. In the former the model was forced with the anomalous seasonal forcing derived from the model itself while in the latter the model variables in a target region were relaxed towards analyses. Both methods stressed the importance of the tropical and extra-tropical vorticity forcing in reproducing the observed anomalies. The main forcing in the extra-tropics was found to be associated with the mid-latitude transients, while in the tropics it was associated with cumulus momentum transport and the transient nature of tropical convection in addition to heating by convection. These processes were represented in the model and it was concluded that they improve the tropical and extra-tropical responses to idealised tropical heating anomalies. They contribute significantly to the vorticity balance and must be accounted for in idealised models and diagnostic calculations.EThOS - Electronic Theses Online ServiceGBUnited Kingdo