North Atlantic air pressure and temperature conditions associated with heavy rainfall in Great Britain

Severe flooding in the United Kingdom is often linked to the occurrence of heavy rainfall events, which can be characterized by the synoptic scale meteorological conditions over the North Atlantic region. Seasonal heavy rainfall events (summer and winter 1-day maxima) were extracted from 125 locations across Great Britain over the period 1950-2017. For each event, anomaly sea-level pressure and 2 m air temperature conditions across the North Atlantic sector were extracted. In contrast to earlier studies, these two datasets were combined and clustered to identify how the pressure and temperature conditions co-vary within each half-year to produce heavy rainfall events. Distinctly different spatial patterns were found for four classes in summer and for three classes in winter. For all classes there is a negative sea-level pressure anomaly centred over or near the British Isles. However, whereas in summer the low pressures are associated with either predominantly cold or warm anomalies over most of the North Atlantic, in winter two phases of a smaller-scale four-pole temperature pattern emerges. Nevertheless, for one of the winter classes the cold anomaly over the northwest Atlantic is so deep, persistent and widespread that, unusually for the winter season, a significant relationship between the class's frequency of occurrence and the Atlantic Multidecadal Oscillation (AMO) index is found (r = -.39). Further, for both seasons heavy rainfall occurs when the AMO and the North Atlantic Oscillation (NAO) are in opposing phases. Particularly, positive NAO and negative AMO result in heavy rainfall in western Britain. Two classes in each season are consistent with positive and negative phases of the NAO, and the two non-NAO summer classes are associated with a northward extension of the subtropical high pressure and heavy rainfall in the southeast. The association between heavy rainfall and large-scale circulation and temperature drivers can find application in, for example, weather generators.