Decadal climate variability of extreme rainfalls in New Zealand | The Meteorological Society of New Zealand

Decadal climate variability of extreme rainfalls in New Zealand

C.S. Thompson

Design rainfalls rely on the assumption that the extremes in storms are independent of each other, and that the expected frequency of high intensity rainfall does not change from year to year. Recent climate studies in New Zealand on rainfall and river flow have shown that persistent climate states, such as the Interdecadal Pacific Oscillation (IPO), can have significant impacts on these variables in the north and northeast of the North Island and the southwest and south of the South Island (McKerchar, 2002). As a consequence, the underlying assumption of stationarity in the high intensity rainfall frequency may be invalid. For a number of locations in New Zealand, this study describes the influence of the IPO on annual maximum rainfall series for 1-, 12- and 24-hour durations over the period 1946-1999, which coincides with the two identified phases of the IPO. At 1-hour the IPO influence appears to be quite small, if present at all, but at 24-hours the influence is stronger with some significant results in the same regions as indicated above. Current rainfall and flood frequency analysis does not conditionally stratify data according to climate state, and this has implications for assessing the risk in the more frequently occurring severe storms.

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