The occurrence of extreme precipitation events in New Zealand regularly results in devastating impacts to the local society and environment. An automated atmospheric river (AR) detection technique (ARDT) is applied to construct a climatology (1979-2019) of extreme mid-latitude moisture fluxes conducive to extreme precipitation. A distinct seasonality exists in AR occurrence aligning with seasonal variations in the mid-latitude jet streams. The formation of the Southern Hemisphere winter split jet enablesARoccurrence to persist through all seasons in northern regions of New Zealand, while southern regions of the country exhibit a substantial (50%) reduction in AR occurrence as the polar jet shifts southward during the cold season. 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. Precipitation totals during AR events increases with AR rank (five-point scale) throughout the country, with the most substantial increase on the windward side of the Southern Alps (South Island), with the largest events (rank 5 ARs) producing 3-day precipitation totals exceeding 1000 mm. ARs account for up to 78% of total precipitation and up to 94% of extreme precipitation on the West Coast of the South Island. Assessment of the multi-scale atmospheric processes associated with AR events governing extreme precipitation in the Southern Alps of New Zealand should remain a priority given their hydrological significance and impact on people and infrastructure.

Atmospheric rivers (ARs) are filamentary corridors of enhanced atmospheric water vapor transport that can produce extreme precipitation in mid-latitude and polar regions, particularly when an AR undergoes topographic ascent (Nash et al., 2018;Neiman et al., 2008;Zhu & Newell, 1998). The magnitude and duration of the moisture flux directly relates to the intensity of precipitation with the highest precipitation rates being associated with strong, prolonged ARs (Eiras- Barca et al., 2021;Konrad & Dettinger, 2017;Prince et al., 2021;Ralph et al., 2019). Given the association between ARs and precipitation, the occurrence of ARs brings the potential for substantial environmental and socioeconomic impacts (Corringham et al., 2019). On the West Coast of the U.S., landfalling ARs are the primary cause of flooding with ∼90% of all floods occurring during ARs (Dettinger et al., 2011;Paltan et al., 2017). The occurrence of these hydrological extremes often results in damage to property and infrastructure, a noteworthy event being the damage to the Oroville Dam in northern California resulting in mass evacuations and financial damages exceeding USD$1 billion