Connection of Atmospheric Rivers to water availability and extreme rainfall

ARs are large flowing amounts of vapour water that occur in the atmosphere of mid-latitude zones, where New Zealand sits. They are a major contributor to the water cycle, severe floods, droughts breaking, and strong winds.

Understanding the seasonality and characteristics of ARs can contribute to water resources management and can help in deciding whether to store or release a certain amount of floodwater in advance, particularly for dam areas. A lack of ARs can also be linked to times of limited rainfall, which Auckland has been experiencing over the last year.

By using global atmospheric reanalysis datasets, an automated AR detection algorithm, and New Zealand's national climate database, Associate Professor Asaad Shamseldin and his team were able to detect ARs approach the country, understand AR patterns, and evaluate their impact on the surface.

To do so, daily rainfall records from 654 rain gauges across the country, the data, combined with global atmospheric reanalysis datasets, makes the connection between ARs over New Zealand  and rainfall events from 1979 to 2018 were investigated.   

Results indicate that ARs play a vital and key role in regional water resources and are responsible for many extreme rainfall events on the western side of mountainous areas and northern New Zealand.  

In these regions, depending on the season, 40–86 percent of the rainfall totals and 50 to 98 per cent of extreme rainfall events are shown to be associated with ARs, with the largest contributions predominantly occurring during the summer (December to February). The median daily rainfall associated with ARs is 2–3 times that associated with other storms, and in some cases, it can be over 20 times.

“There is a potential link between the drought occurrence and the frequency of days that ARs present. For example, during the severe historical drought event in 1993-1994, the frequency of ARs in the Auckland region was 41% below the average; Auckland's water supply reservoirs dropped to 32% in May 1994,” said Professor Assad Shamseldin.

“On the other hand, ARs can cause floods. One case was from 26 to 28 December 2010.  Flood flows in the Aorere and Anatoki Rivers in northwest Nelson were at or above 100-year return period levels and high flows occurred in several rivers draining the Southern Alps' central portion, notably in the Hokitika and Rakaia.”

The Government is making a $536,000 contribution to recovery costs from severe flooding on the West Coast in December 2010.

More research is planned to categorise ARs in terms of risk level, how ARs respond to climate change and to investigate the relation between AR seasonality and global circulation features in New Zealand.

The research is published in Scientific Reports.