Rivers in the sky drive extreme NZ weather
Have you ever been absolutely drenched by rain? You may have experienced an ‘‘atmospheric river’’. These are weather episodes that produce many heavy and extreme rainfall events in AotearoaNew Zealand, especially on mountainous coasts.
As storms, they are typically five times longer than they are wide – that is, long and thin. Think of rivers of water in the sky, bucketing rain onto your head.
They usually come with weather fronts or cyclones.
‘‘Interestingly, atmospheric river storms are found to produce more than two times more daily rainfall than non-AR storms at most [rainfall weather] stations, and significantly more than three times for the west side of mountainous areas and northern New Zealand,’’ say researchers Assad Shamseldin, Evan Weller and Jingxiang Shu, of the University of Auckland.
They play an essential role in regional water resources and are responsible for many extreme rainfall events on the western side of mountainous areas and northern New Zealand.
‘‘Depending on the season in these areas, 40 per cent to 86 per cent of the rainfall totals and 50 per cent to 98 per cent of extreme rainfall events are shown to be associated with atmospheric rivers,’’ the researchers found.
One of the largest atmospheric rivers detected in New Zealand was 13,236 kilometres long and 1216km wide. It stretched from southern Indonesia to well east of the Chatham Islands and made landfall on an island in Cook Strait in late October 2010.
Available internet records from Cook Strait from that week do not show particularly wet weather, but this river pattern does not always produce rain over land. In some places, such as mountainous Fiordland, atmospheric rivers ‘‘largely determine the water availability’’.
Niwa said it was an atmospheric river that bashed the West Coast in March 2019, memorably taking out the Waiho Bridge south of Franz Josef.
‘‘This study is a major step toward understanding the hydrology and extreme rainfall events in New Zealand,’’ the Auckland researchers wrote.
They detected a ‘‘potential connection’’ between floods, droughts and atmospheric rivers in northern New Zealand.
Worryingly, climate change is suggested to result in a 60 per cent increase in the frequency of atmospheric rivers and a 20 per cent increase in their strength in the southern mid-latitudes, which include all of New Zealand.
Climate change will increase the frequency and intensity of extreme rainfall, leading to more floods.
‘‘Climate change is expected to also lead to increases in the frequency of droughts in most of the North Island, including regions where atmospheric rivers are the major contributor to water resources,’’ they wrote.
Further work is needed to understand how these phenomenons interact, especially in a changing climate.
The researchers analysed two datasets containing New Zealand weather information collected by the European Centre for Medium-Range Weather Forecasts for the 39 years between 1979 and 2018.
They also got data from Niwa on 654 rainfall gauges.
An algorithm detected the atmospheric rivers making landfall and the rainfall gauge data told them how much rain fell.
Atmospheric rivers were first described in 1992 and have become highly researched and discussed since. The Auckland researchers noted ‘‘several global studies’’ have looked at atmospheric rivers in New Zealand, but not with the ‘‘local scale’’ needed to understand what was happening.
Climate change is suggested to result in a 60 per cent increase in the frequency of atmospheric rivers.