Professor Peter Stott:

It is a weather phenomenon and it occurs in both hemispheres. I was involved in a study that examined extremely heavy rainfall and flooding in New Zealand. New Zealand has atmospheric rivers as well. Basically, it is what happens when there is a plume of air moving up from the subtropics, which brings air that is more normally found much further south in the northern hemisphere. It brings that air, heavily laden with moisture, in a steady plume from much warmer ocean temperatures than we are used to here, transporting it many thousands of kilometres up to our latitudes. The issue is that due to how the atmosphere works this stream of air can be quite steady for many hours, even for several days, and basically dumps rain in a particular locality. It is almost like bringing somebody else's weather to our shores. When it happens it shows our vulnerability to that type of weather event. The issue with climate change, as the study from Maynooth University has shown, is that with warmer ocean temperatures - I spoke about 6% or 7% more moisture in the atmosphere for 1° Celsius of warming - that is loading the atmosphere even more and the impact when it happens is even greater than it would have been without climate change.