Changes in New Zealand daily rainfall extremes 1930 - 2004 | The Meteorological Society of New Zealand

Changes in New Zealand daily rainfall extremes 1930 - 2004

G.M. Griffiths

Atmospheric circulation has a major impact on mean rainfall in New Zealand due to the large orographic component of rainfall in this country. Since 1930, both New Zealand-average air temperatures and westerly circulation have increased, with the largest increases occurring after 1950. A detailed analysis of New Zealand extreme rainfall for two periods (1930-2004 and 1950-2004) was undertaken, using 11 indices of extremity based on high-quality daily station data. Of interest was whether extremes differed in the later (warmer) period. Regional variation in trends was strongly evident, with a west-east pattern across major mountain ranges showing increased rainfall extremity in the west, but decreased extremity and increased dry spell duration in the east, consistent with increased westerly circulation over New Zealand.  This zonal pattern persisted over both periods, even against a background of warming. Index trends showed temporal consistency between the periods, except for a simple intensity measure (rainfall per rain day). Changes in extreme daily rainfalls were strongly related to changes in mean rainfall, although the relationship was weaker in the later (warmer) period. Station 1-day rainfall extremity was highly correlated to westerly circulation (zonal flow) across the country, and to a much lesser extent, meridional flow and New Zealand-averaged temperature anomaly.

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