while this week heavy gust of rain in Auckland could the deluges from the storm on january 27 have not subsided, is this a sign that they are becoming more common?
Auckland Council data shows that around 77mm of rain fell in 2 hours in Henderson on Tuesday morning, and almost 70mm of rain fell in about 2 1/2 hours in Whenuapai, while Glendene fell in around 66 mm in about 2 hours.
University of Auckland School of Environment Honorary Associate Professor Anthony Fowler calculated Tuesday's rainfall event to be about 1 in 5 years by the amount of rain that fell in 1 hour and in 2 hours.
In contrast, the January 27 event had return periods of hundreds of years.
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Fowler also looked at how often Albert Park, in central Auckland, got the same or more rain in a day than the 82mm that fell on Tuesday. The record goes back to 1863, with some gaps.
This chart by Anthony Fowler, Honorary Associate Professor at the University of Auckland's School of Environment, shows the return periods, over several hours, for the January 27 and Tuesday rain events.
He found three times when there were two such events in one month: August 1965, January 2011, and January 2023. Only once, in 1979, were there three such events in one calendar year.
It appeared that so far in 2023 it had happened four times, which was definitely unusual, Fowler said. But he cautioned that that was "a bit behind the envelope," with a long-term analysis yet to be completed.
Niwa meteorologist Seth Carrier said Tuesday's heavy rain across the upper half of the North Island was linked to the arrival of a much drier, cooler air mass from the Southern Ocean, to replace the warm, moist air. that had been around for a few days.
Precipitation occurred primarily along the transition zone between warm northerly and cold southerly winds.
Some places in Auckland had 24-hour rainfall totals of over 100mm, which was unusual, but still less than half the 265mm recorded at Mangere on 27 January.
Jason Dorday/Things
Flooding during heavy rain in Mangere, Auckland on Tuesday.
Even though January 27th was much wetter, the events had some similarities.
Both involved what was called training rain or thunderstorms. โThe training just means heavy rain moving over the same spot repeatedly,โ Carrier said.
The strong cold front along which Tuesday's heavy rain fell was oriented approximately north-south, and had been moving slowly eastward.
Associate Professor Asaad Shamseldin, from the University of Auckland's school of engineering, said although Tuesday's rain caused less of a problem than on January 27, some places had flooded again.
โThe main point is that these short, heavy rains, I think are going to increase with climate change,โ Shamseldin said. โEspecially in urban areas they will cause a lot of damage.โ
So 100mm over a day might not cause problems. "But if it's a bit slow, then you have maybe 50mm in a very short period of time, then the stormwater network can't cope."
STUFF
Video and images from 27 and 28 January 2023 show the extent of flood damage after a severe weather event in Auckland.
While improving the pipeline network was part of the solution, the problem had to be comprehensively analyzed in each basin.
โYou need to tailor the solution to particular locations, because you don't want just pipes everywhere,โ Shamseldin said.
In some heavily built-up areas, enlarging pipes may be the only solution, but that shouldn't be the general rule.
With larger pipes, stormwater would travel faster and that could lead to other problems, such as damage to the health of rivers.
Climatologist Dr Kevin Trenberth, an honorary scholar in the physics department at the University of Auckland, said the heavy rains had a climate change aspect, linked to unusually warm sea temperatures.
Sea surface temperatures north of New Zealand were 1-1ยฝC above normal, and the amount of moisture in the atmosphere had been about double what was expected for this time of year.
Tuesday's rain was of tropical origin, with moisture coming in from north of 20 degrees south, Trenberth said.
Heavy rain and flooding on the North Auckland motorway on Tuesday morning.
"The high rain rates certainly had a big effect in this case, but the volumes were not as great as those that occurred on January 27."
Amounts of around 65mm over 2ยฝ hours seen on Tuesday were unusual for Auckland, but the weather that produced that shower was not unusual, although the slow speed at which it moved likely was.
โThere was a large, organized line coming very slowly through the Auckland area, so the amount of rain was compounded by the relatively slow movement of this frontal system,โ Trenberth said.
โJanuary 27th was a huge big dropโฆ There were tons of moisture in the atmosphere, and it just sat and spilled on us.โ
Over the past year, more than a dozen subtropical lows have descended from the north towards New Zealand, while "rain bombs" have fallen on the east coast of Australia.
It was part of a much larger weather pattern related to La Niรฑa.
La Niรฑa is a weather pattern associated with the cooling of ocean surface waters along the tropical west coast of South America. It is considered to be the counterpart to El Niรฑo, which is characterized by unusually warm ocean temperatures in the equatorial region of the Pacific Ocean.
La Niรฑa has faded in recent months, but it had affected global weather for the past three years, an unusually long, though not unprecedented, amount of time.
