Four key reasons to explain New Zealand's sodden 2023
ANALYSIS: Intense rain has once put more peoples lives at risk, flooded homes, caused traffic chaos and closed schools. After months of thunderstorms, cyclones and atmospheric rivers, some Northland and Auckland residents might be wondering if its time to pack up and leave for drier ground. Climate change is increasingly likely to play a role in every damaging storm. But its not the only factor, experts say. Stuff asked climate scientist James Renwick about whether this is simply a wet year, or a new reality for the upper North Island. Before considering the long-term, lets take a look at exactly what caused this weeks flooding. MetService forecaster Mmathapelo Makgabutlane says the weeks weather had produced a perfect recipe for thunderstorms. The last week or so brought in so much warm, moist air from the tropical regions, she says. This meant there was a lot more moisture in the system when it arrived over the upper North Island. Partly due to this warm air, temperatures in recent days have been much higher than are typically recorded in early May. Then, a cold front approached the country from the west, forcing damp air higher into the atmosphere. Finally, instability in the upper atmosphere allowed thunderstorms to form, Makgabutlane says. Auckland and Northland, which have borne the brunt of this years record wet weather, experienced intense deluges. More than 113mm of rain fell on Whenuapai, in north-west Auckland, on Tuesday. At Auckland Airport, 32mm fell in just 60 minutes, from midday. Of course, this weeks thunderstorms are part of a spate of wet weather for the upper North Island that began in early January . Victoria Universitys Renwick says there are four key reasons to explain the countrys sodden 2023: The world is more than 1C hotter than the pre-fossil-fuel era. That is already having an impact on many of the storms both large and small. For a start, warmer air is able to carry more water vapour. For every degree the planet heats, the air can hold roughly 7% more moisture. So in our pollution-heated world, storms have the potential to release even more rain which increases the chances of the type of intense bursts that MetService warned about. In the centre of thunderstorms , a snowball effect occurs. Water vapour condenses into clouds and rain as it does, it also releases heat, which encourages even more rain to form. With more water vapour in the mix thanks to climate change, theres more energy to power this vicious cycle, again leading to more intense deluges. Alongside this weeks intense rain, a band of thunderstorms caused the Auckland anniversary weekend floods. Renwick says the recent thunderstorm events are intriguing and worth further research. The extra heat in the atmosphere is also radiating into the ocean. Marine heatwaves have become increasingly common in recent years. Warmer seawater can influence individual weather events, Renwick says, such as tropical cyclones and thunderstorms. Tropical cyclones including Gabrielle form when ocean temperatures are higher than 26C. Gabrielle developed in the Coral Sea, one of the fastest-warming bodies of water . When the storms pass into the colder temperate waters, they expand and lose intensity. But the Pacific Ocean has been unusually warm in recent months. That means storms retain more of their power once they reach Aotearoa. Even as we head into winter, the marine heatwave is lingering. The sea around New Zealand is still warmer than normal, Renwick says. Climate change may be loading the dice, but theres still a large element of chance in the weather. After looking at the weather maps, Renwicks gut instinct is that this is a long run of stormy conditions. We have been unlucky. Weve just happened to get a sequence of these storms following each other quite quickly. Sometimes the atmosphere configures itself so that over the Tasman Sea you get these conditions for storms to develop more often than not. Its just the way things go. Tropical cyclones that get close enough to New Zealand to do the kind of damage that Gabrielles done are rare, Renwick says. It could be 10 years before the next one happens or it could be next week. That is random. Northland, Auckland and Coromandel residents can take solace that this isnt expected to be their new norm. Renwick sees no signal of a permanent change: I dont think New Zealand has gone into a state where were going to get these kinds of things happening continuously. The El Nino and La Nina climate cycles also influence Aotearoas weather. At the start of the year, La Nina was still in force. During this phase, weather typically arrives from the north-east potentially bringing moisture and heat from the tropics to the motu. In the El Nino pattern, the Pacific Ocean is typically warmer. Scientists are concerned a recent spike in ocean temperatures is a sign that a strong El Nino is inbound. Under this pattern, New Zealands weather typically comes from the west. Heavy rainfall events would be more likely to hit western areas while regions including Hawkes Bay and Gisborne can get very dry. You tend to get big high pressure systems over Northland and north of the country, that in a sense protect the North Island, Renwick says. Renwick warns El Nino is no guarantee of dry for any region, however. These big events... push the weather in a certain direction, but you can still get anything happening. Even so, the North Island could experience a comparatively dry second half of the year if El Nino conditions develop, he adds. Concerns about flooding could be replaced by worries about drought. That, unfortunately, is the way the climate is changing when its wet, its very wet and when its dry, its very dry, he says. Makgabutlane also sympathises with Auckland and Northland residents. I know its been a really tough year for everybody, and a tough week as well. Weve had a lot of rain. By Friday, the weather should brighten in the north, she says. There is light at the end of this very rainy tunnel. Our weekly email newsletter, by the Forever Project's Olivia Wannan, rounds up the latest climate events. Sign up here .