A Climatology of New Zealand Atmospheric Rivers

Abstract

The occurrence of extreme precipitation events in New Zealand regularly results in devastating impacts to the local society and environment. In order to examine the occurrence of extreme precipitation in New Zealand, extreme horizontal atmospheric moisture fluxes are considered through an application of the emerging concept of atmospheric rivers (ARs). An automated AR detection algorithm is applied to construct the first climatology of ARs in the New Zealand region over the period 1979-2019. A distinct seasonality exists in AR occurrence in New Zealand, which aligns with seasonal variations in the mid-latitude jet streams and consequently, mid-latitude cyclone tracks. In particular, the formation of the unique Southern Hemisphere winter split jet appears to enable AR occurrence to persist in northern regions of New Zealand, while the southern regions of the country exhibit a substantial (30%) reduction in AR occurrence as the polar jet shifts southward during winter. ARs making landfall on the western coast of New Zealand (90% of all events) are characterised by a dominant north-westerly moisture flux associated with a distinct dipole pressure anomaly; low pressure to the south-west and high pressure to the north-east of New Zealand. The hydrological impact of ARs is considered through the adoption of a five-point categorical scale that was developed for the West Coast of the USA. The West Coast of the South Island of New Zealand is identified as the region that experiences the strongest and most dramatic relationship between the occurrence of ARs and precipitation, with up to 74% of total precipitation and 93% of extreme 6-hourly precipitation occurring within 12 hours of detected ARs. ARs account for a maximum of 27% of both total and extreme precipitation at other locations studied around New Zealand. Extreme 6-hourly precipitation on the West Coast is up to 50 times more likely to occur within 12 hours of a detected AR than during non-AR conditions. All other locations exhibit small (a risk factor less than 2) or no increased risk of precipitation during ARs. Precipitation totals from the most intense ARs (category 5) on the West Coast exceed 1000 mm (within 60 hours), which is one of the largest AR precipitation amounts documented globally. In order to further comprehend the influence of ARs on local hydrological extremes and hazards, further examination of the meso-scale atmospheric controls of AR-related extreme precipitation associated with the Southern Alps of New Zealand should remain a priority.