This study is motivated by the potential improvement in water supply reliability and better forecasts of extreme rainfall and floods linked to Atmospheric rivers (ARs) in New Zealand. Results indicate that ARs generally dominate monthly rain amounts and wet and dry conditions for the western side of mountainous regions and the north part of the country. Precursor soil moisture conditions and their relationship to runoff are also evaluated. In these regions, ARs are more likely to replenish soil moisture or lead to flash floods in warm-season months but generate severe floods in cool-season months due to low soil moisture deficit. This relationship is further confirmed by the seasonality of the monthly maximum streamflow of the Whanganui River (a major river in the North Island of New Zealand). Additionally, ARs generally lead to high streamflow all year round and are responsible for most top 10% streamflow.

Atmospheric Rivers (ARs) are filamentary channels of strong poleward water vapour transport in the midlatitudes. Recent studies have demonstrated the significant role of ARs in New Zealand’s water resources and extreme precipitation events. Motivated by a recently proposed AR-impact ranking scale in the USA to enhance the communication between scientific communities and water sectors, here the characteristics of AR events with peak daily rainfall greater than 100 mm over 5 divided sectors are further investigated, and the AR-impact ranking scale is evaluated for the applicability for such events in New Zealand. Previous studies have found that the windward side along coastlines favours locally high rainfall. As such, we show that the strength and duration of those AR events also vary with event direction, and NW-AR events are normally stronger and longer than those of other directions throughout the country. However, over the eastern areas, most of those events are easterly directed and produce anomalously high rainfall, despite being ranked as “Weak AR” events based on the current AR-impact ranking scale. It is found that easterly directed ARs originating from the west make landfall along the eastern coastline from the ocean (i.e., from the east) or simply the onshore flow. Therefore, our results suggest that localised ranking scale or considering more parameters, such as AR over-land direction, might help improve the AR-impact ranking scale applicability over eastern regions in New Zealand.

Atmospheric Rivers (ARs) are filamentary channels of strong poleward water vapour transport in the midlatitudes. Recent studies have found that ARs are northwesterly and northeasterly orientated and can make landfall in all directions over New Zealand. In this study, we further investigate the characteristics, in particular orientation and landfall direction, of detected landfalling ARs based on two atmospheric reanalysis datasets over 35 years. Daily rainfall records from 655 rain gauges between 1979 to 2018 were used to investigate the spatial variability of the AR contribution to annual rainfall and extreme rainfall linked with AR events with different orientations and mean landfall directions. A modified AR impact ranking scale was then evaluated regarding AR-event orientation and mean landfall direction, different “AR impact” sectors, and peak daily AR-event rainfall. We found that landfalling ARs (events) with a northwesterly orientation and northwesterly landfall direction (NW-NW ARs) are the most frequent and relatively stronger, more coherent and concentrated over the country. As a result, NW-NW ARs are major contributors to annual rainfall and extreme rainfall for the country’s West Coast. Generally, the windward side experiences anomalously high rainfall as ARs reach the country from different directions, and the spatial distribution of AR-event heavy rainfall is shown to vary with an AR’s orientation and landfall direction. Moreover, the AR impact ranking scale performs well for NW-NW ARs over the West Coast. However, more factors need to be considered to improve the applicability of the scale on the East Coast.