It usually refers to a funnel like shape in the airflow north of NZ, focusing southwards into one narrow point that gets heavy rain as it comes into NZ. The term is overused in our view, but it’s something Niwa/Earth Sciences use with frequency to describe slow moving rain fed by sub-tropical air. “River” potentially implies widespread water, but often they can be patchy with narrow areas that have the heavy rain issues. Our video today talks a bit about it and the upper end and lower end potential of it all.
– PD

Hi Sylvia, high pressure zones are bit like bubbles floating on the surface of water and they generally are guided by the current high up – called the jetstream (high altitude winds north and south of NZ usually). When the polar jet stream sinks south, the high pressure zones go south too – when the jet stream goes north, so do the highs. In spring we get a lot of ups and downs like this and is unrelated to La Nina, although LN can see the tropical jetstream sink southwards bringing more low pressure with it towards NZ (and more easterlies).

Our hottest weather in NZ usually comes from a nor’wester out of Australia’s desert (or NSW/Qld area) and for that to happen high pressure is usually north of NZ. Spring will continue to see the highs all mixed up (chaotic) and usually in summer they join up more often bringing longer spells of calm/dry (although with NZ being so small compared to high pressure zones we can always buck the trend here).
– Phil D