The extraordinary flood event that Auckland experienced on the night of January 27, the eve of the city’s anniversary weekend, was caused by downpours that were literally off the charts.
Over 24 hours, 249mm of rain fell, well above the previous record of 161.8mm. HAS a state of emergency was declared late at night.
A terrible toll has been taken in Aucklanders, with two people Report dead and two more missing. Damage to homes, cars, roads, and infrastructure will run into the many millions of dollars.
Seeing the images on social media on Friday night, I thought I had seen this type of image before. But usually they are from North America or Asia, or maybe from Europe. But this was the largest city in New Zealand. No place is safe from extreme weather these days.
as a step
The torrential rain came from a storm in the northern Tasman Sea linked to a source of moisture from the tropics. This is what meteorologists call an “atmospheric river.”
The storm was moving rather slowly because it was cradled to the south by a huge anticyclone (a high) that prevented it from moving quickly across the country.
Embedded in the main rain band, severe thunderstorms developed in unstable air over the Auckland region. These generated the heaviest rainfall, and MetService figures show that Auckland Airport received its average monthly rainfall for January in less than an hour.
However, the type of storm that caused the chaos was not especially notable. Many similar storms have passed through Auckland. But as the climate continues to warm, the amount of water vapor in the air increases.
I’m sure climate change contributed significantly to the incredible volume of rain that fell so quickly in Auckland this time.
Warmer air means more water
There will be careful analysis of historical records and many simulations with climate models to nail down the return period of this flood (probably in the hundreds of years at least, in terms of our past climate).
The amount of climate change that contributed to the total precipitation will be part of those calculations. But it’s obvious to me that this event is exactly what we expect as a result of climate change.
One degree of warming in the air translates, on average, into about 7% more water vapor in that air The globe and New Zealand have experienced a little more than a degree of warming in the last century, and we have measured the increasing content of water vapor.
But when a storm hits, it can translate to much more than a 7% increase in precipitation. Air “converges” (is pulled) near the Earth’s surface in a storm system. So, all that humid air is collected and then “squeezed” to generate rain.
A severe storm is the same on a smaller scale. Air is sucked in at ground level, rises, and rapidly cools, losing much of its moisture in the process.
While the atmosphere now contains 7% more water vapor, this convergence of air masses means that rain gusts can be 10% or even 20% more intense.
Beyond the capacity of stormwater systems
The National Institute of Water and the Atmosphere (NIWA) dear that over Auckland, one degree of warming translates to a 20% increase in precipitation in an hour, for a one-in-50-year event.
The longer we keep warming the weather, the heavier the torrential rains will be.
Given what we’ve already seen, how do we adapt? Floods occur when stormwater cannot drain fast enough. So what we need are bigger drains, bigger stormwater pipes, and stormwater systems that can handle such extremes.
The country’s stormwater drainage system was designed for the climate we used to have, 50 years or more ago. What we need is a stormwater system designed for the climate we have now and the one we will have in 50 years.
Another part of the response may be a “softening” of the urban environment. Tar and concrete sealing surfaces force water to remain on the surface, pool and flow.
If we can re-expose some of the streams that have been diverted into sewers, restore some wetlands between built-up areas, we can create a more spongy surface environment more naturally able to cope with heavy rain. These are the answers we need to think about and act on now.
We must also stop burning fossil fuels and reduce global emissions of carbon dioxide and other greenhouse gases as quickly as possible. New Zealand has a emissions reduction plan – we need to see it have an effect starting this year. And each country must follow continues.
As I said at the beginning, no community is immune to these extremes and we all need to work together.