LAUSR

The effect of large-scale modes of climate variability on extreme UK daily rainfall, together with potential trends is investigated with non-stationary extreme value analysis. Extreme rainfall is identified from 25 km gridded observations spanning 1958–2012 and to which generalized extreme value (GEV) distributions are fitted. The GEV location and scale parameters are assessed for their dependence on indices of the El Nino-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO) and Atlantic Multi-decadal Oscillation (AMO), together with any evidence of trend. The influence of indices and trend are assessed individually and in combination, for individual months and 3-month rolling seasons. To improve signal-to-noise ratio all data below 200 m elevation is pooled. The NAO is found to have the greatest impact with positive NAO reducing the likelihood of extreme rainfall from spring to autumn, but increasing it in winter. London's 50 year return level for JJA (DJF) ranges from 34(26) mm day−1 at maximum NAO to 51(24) mm day−1 for minimum NAO over the observed period. A weak ENSO influence is only found for early winter (NDJ) and no influence detected for PDO or AMO. Trends towards more extreme rainfall were found for OND and DJF; however, the inclusion of NAO resulted in reduced magnitude and significance for DJF trends. Robust trends were found for the winter half year irrespective of NAO influence, with London's 50-year return level increasing by 6%. Extreme rainfall changes associated with NAO are consistent with NAO driven changes in extra-tropical cyclones. Positive NAO non-winter months have fewer less intense storms crossing the UK in contrast to winter where they are more frequent and intense. The speed of storms is also higher during positive NAO winters which can mitigate increases in the rarest events.