Napier flood indirectly linked to atmospheric river phenomena 11 Mar, 2021 05:02 PM 3 minutes to read Two atmospheric rivers in the leadup to November 9 laid the foundation for the heavy rainfall that caused the Napier Flood. Photo / File Two atmospheric rivers in the leadup to November 9 laid the foundation for the heavy rainfall that caused the Napier Flood. Photo / File New research points to New Zealand's costliest floods coinciding with "atmospheric rivers", but Niwa says November's flood in Napier was not directly triggered by the phenomena. Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that move huge columns of water vapour in the direction of preceding weather.