Drivers of extreme daily rainfalls in New Zealand
Extreme daily rainfalls (‘Rx1day’) from 22 high-quality station rainfall records across New Zealand were assessed at a monthly, seasonal and annual time scale, and associated with a corresponding daily Kidson synoptic classification (‘weather’) type. Three Kidson weather types were over-represented with regards annual Rx1day occurrence at many of the stations (TSW, TNW, and NE), but clear regional distinctions were observed in the dominant weather type associated with extreme daily rainfalls. This finding is consistent with the differing circulation related to each Kidson type and the large orographic component of New Zealand daily rainfall. Subtle seasonal variations in Kidson weather type dominance were observed. Monthly Rx1day anomalies were averaged for each of the four seasons, and correlated with seasonal average measures of the El Niño Southern Oscillation and the Southern Annular Mode, which are known to influence Kidson type frequency, as well as the Indian Ocean Dipole. Useful relationships were found, which varied by season and by region. However, further work is required to develop a skilful predictive scheme of extreme rainfall risk based on circulation drivers. Recent advances in the prediction of Kidson weather types, using NWP or ensemble techniques such as those currently employed at NIWA, may mean that Kidson weather typing becomes the most likely route to skilful, intra-seasonal prediction of extreme daily rainfalls in New Zealand.