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Atmospheric Rivers and Weather Types in Aotearoa New Zealand: a two-way story
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  • Benjamin Pohl,
  • Hamish D Prince,
  • Jonathan D Wille,
  • Daniel G Kingston,
  • Nicolas J. Cullen,
  • Nicolas C. Fauchereau
Benjamin Pohl
CRC / Biogéosciences

Corresponding Author:[email protected]

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Hamish D Prince
Center for Western Weather and Water Extremes, Scripps Institution of Oceanography, University of California, San Diego
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Jonathan D Wille
Université Grenoble Alpes
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Daniel G Kingston
University of Otago
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Nicolas J. Cullen
School of Geography
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Nicolas C. Fauchereau
National Institute of Water and Atmospheric Research
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Here, we analyze the inter-relationships between weather types (WTs) and atmospheric rivers (ARs) around Aotearoa New Zealand (ANZ), their respective properties, as well as their combined and separate influence on daily precipitation amounts and extremes. Results show that ARs are often associated with 3-4 WTs, but these WTs change depending on the regions where ARs landfall. The WTs most frequently associated with ARs generally correspond to those favoring anomalously strong westerly wind in the mid-latitudes, especially for southern regions of ANZ, or northwesterly anomalies favoring moisture export from the lower latitudes, especially for the northern regions.
WTs and ARs show strong within-type and inter-event diversity. The synoptic patterns of the WTs significantly differ when they are associated with AR occurrences, with atmospheric centers of actions being shifted so that moisture fluxes towards ANZ are enhanced. Symmetrically, the location, angle, and persistence of ARs appear strongly driven by the synoptic configurations of the WTs. Although total moisture transport shows weaker WT-dependency, it appears strongly related to zonal wind speed to the south of ANZ, or the moisture content of the air mass to the north. Finally, WT influence on daily precipitation may completely change depending on their association, or lack thereof, with AR events. WTs traditionally considered as favorable to wet conditions may conceal daily precipitation extremes occurring during AR days, and anomalously dry days or near-climatological conditions during non-AR days.