Abstract/Summary

European summer weather extremes can have wide-reaching and severe societal impacts, therefore the ability to forecast seasonal anomalies several weeks or months in advance would be extremely beneficial. This thesis investigates the relationship between the circumglobal teleconnection (CGT), the Indian summer monsoon (ISM) and European summer weather. It is shown that the representation of the CGT in the ECMWF seasonal forecast model is too weak. The model has errors in the basic state, including a northward displacement of the jet stream, and these may be important in the representation of the CGT. Results from relaxation experiments, in which the model is corrected to reanalysis in specific regions, suggest that northwest Europe is more important in forcing the CGT and in the downstream development of errors in the CGT pattern than west-central Asia and the ISM, although the link between ISM precipitation and the extratropical circulation is weak in all experiments. Thermal forcing experiments in the ECMWF model suggest that the ISM does force an extratropical, CGT-like response over east Asia, the North Pacific and North America, with upper tropospheric anticyclonic anomalies in these regions associated with increased monsoon heating. However, the response over Europe occurs largely through westward propagating Rossby waves, as opposed to the eastward-propagating waves associated with the CGT. The frequency of blocking over Europe is also shown to be related to variations in ISM heating, with a larger number of blocked days when ISM heating is reduced, which may have implications for European circulation predictability. However, it remains unclear whether this occurs as a result of perturbations to the jet via the North Pacific, or through the westward-propagating response. This westward-propagating response, which is also seen in barotropic model experiments, was found to be crucial in the downstream reinforcement of the wave train between Asia and North America.