An "atmospheric river" delivering widespread and heavy rain to the upper North Island. Image / Niwa
An "atmospheric river" delivering widespread and heavy rain to the upper North Island. Image / Niwa
EDITORIAL
“Atmospheric rivers” funnel warmth and moisture from the subtropics, thousands of kilometres above New Zealand.
In simple terms, they are airborne currents of warm water. But how much impact they have on us is a more complex question, depending on a series of factors.
The biggest question facing meteorologistsis just how and where the moisture-packed lines of convection fall. And fall, this river did. Auckland received last month the equivalent of 769 per cent of its normal January rainfall, and nearly 40 per cent of what the region would normally receive in an entire year.
One of the key features of last Friday’s cataclysmic event was a “low-level jet” - a ribbon of strong wind in the atmosphere, positioned not far above the surface, extending from the system’s flank to siphon warmth and tropical moisture straight across Auckland.
The jet was orientated almost due north to south - or straight through Auckland, described by Niwa Weather forecaster Ben Noll as “probably the worst-case scenario, as it can cover more of the geographic area at once”.
Niwa explained in a social media post on Sunday night that the moisture in an atmospheric river fuels the weather pattern but “energy” is still needed to convert water vapour into rain.
While moisture from an atmospheric river is fuel, we still need 'energy' to convert water vapour into rain.
A low-level jet (LLJ), or strong winds about 1500 m ABOVE the ground, will produce winds of 110+ km/h.
A low-level jet wind, or strong winds about 1500 metres above the ground, will produce winds of 110+ km/h. In the Auckland event, the jet wind was helped by a “squeeze” due to a high in the southeast and a low in the northwest.
WeatherWatch described the system as one packed inside a “squash zone” between two pressure systems.
Another factor heaped on Auckland’s woes was the presence of thunder. In this case, a long band of thunderstorms was squeezed within the moisture-charged system to the point of being unable to take on more water, unleashing torrents of rain on the city below.
Ultimately, several factors combined to drive this atmospheric river from the tropics precisely to the isthmus of Tāmaki Makaurau and then emptied it hard and fast. A similar phenomenon occurred again yesterday but it did not have the same level of commitment from accomplices.
Friday’s episode also followed an unusually rainy summer. Auckland was wet before the airborne river cascaded down.
Illustration / Daron Parton
Atmospheric rivers occur at other times and we take little notice. It’s one thing to get these intense events out in the Pacific Ocean, for example, but it’s another to have them right over the most populated part of the country.
Some - including Auckland Mayor Wayne Brown, before changing his mind - have queried the role of climate change. Average air temperatures have warmed by slightly more than 1C over the past century. A warmer atmosphere can hold more moisture, increasing the likelihood of extreme heavy rainfall events such as this.
Sea surface temperatures around New Zealand are also warmer than average, with the potential to amplify the magnitude of such weather events.
Several unique features conspired to deluge Auckland but more of us will now be more informed and vigilant about what atmospheric rivers are up to.
