Characteristics and large‐scale drivers of atmospheric rivers associated with extreme floods in New Zealand

Abstract

AbstractThe Southern Alps in the South Island of New Zealand are one of wettest places globally, making it critical to understand the mechanisms for delivery of extreme precipitation and river flooding. Atmospheric rivers (ARs) are recognized as key causes of extreme precipitation in New Zealand, but relatively little is known about their large‐scale meteorological drivers. Here, we aim to investigate these hydroclimatological connections for five major South Island catchments located from south–north along the axis of the Southern Alps: Te Anau, Matukituki, Pūkaki, Rakaia and Waiau Toa. For each catchment, the top eight flood events over a 38‐year period are characterized. Specifically, vertically integrated horizontal water vapour transport (IVT), 500 hPa geopotential and 300 hPa winds are examined to quantify the large‐scale atmospheric drivers of flood events. The Kidson synoptic weather classification is also employed to understand better the connection of IVT to New Zealand weather patterns. Intense IVT is associated with flood events for all five catchments and, in most instances, corresponds to AR‐type events. Spatial patterns of IVT display substantial variation between flood events and catchments; however, there are some generalizable patterns. First, clear AR‐type IVT patterns dominate for all catchments (except the Waiau Toa) with AR orientation ranging from westerly to northwesterly. For the most northerly catchment (Waiau Toa), cyclonic zones of high IVT occur instead for some events. Second, AR length varies from those restricted to the Tasman Sea (~2,000 km) to those extending to the west or north of Australia. Third, northwesterly ARs are associated with the passage of a depression in the general westerly circulation, characterized in particular by the Kidson “Trough” weather type. Finally, AR orientation is associated strongly with 300 hPa winds, and in many cases to the general wave characteristics of Southern Hemisphere circulation, particularly the Zonal Wave 3 pattern.