What’s going on with the weather?
A spring so dry it led to water restrictions across the country followed by a summer that is both much wetter and warmer than normal. Is this a sign of things to come?
The short answer is: yes. The longer answer is a bit more complicated.
Niwa meteorologist Ben Noll said one of the factors for the wet and warm January across the top of the south and much of New Zealand were the easterly winds which typified the La Nin˜a weather pattern.
‘‘La Nin˜a brings a lot more easterly winds than normal to New Zealand, and for the top of the South Island that exposes you to warm winds, and moist winds,’’ Noll said.
But Lisa Murray, communications meteorologist for MetService, said the weather this summer had been ‘‘extremely unusual’’, even taking into account the effects of La Nin˜a.
The very hot, dry late spring followed by the muggy and wet summer were both caused by a high pressure system which started in mid-November and stayed in effect for three-and-a-half weeks.
‘‘One of the forecasters has been working [at MetService] for 40 years, and he said he’s never seen a high stick around for so long in his entire career, so that shows just how unusual it is,’’ Murray said.
High pressure systems lead to warmer sea surface temperatures.
TO SUBSCRIBE PHONE 0800 800515
Around the country, sea surface temperatures are averaging three degrees warmer than usual, with the Tasman Sea reaching temperatures 5 to 6 degrees above normal.
‘‘Sea surface temperatures being warmer will end up with more moisture in the atmosphere, more moisture means heavier rainfall, and we’re all seeing a lot of humidity as well,’’ Murray said.
The warmer water is also a contributing factor to rapidly deepening lows, which can lead to heavier rainfall, stronger winds, and strong swells.
Murray said: ‘‘You can’t pin climate change on one event, it’s a really broad thing, but what I’ve noticed is, the climate change forecasters said ‘we’re going to see warmer sea-surface temperatures,’ and we’re seeing that, and ‘we’re going to see more extreme events,’ and we’re seeing that.’’
The return of the sun across the country this week, with temperatures scorching past 30 degrees Celsius, is thanks to another high pressure system.
Many parts of the country, including Nelson, are heading for record or near-record mean January temperatures, several degrees above normal.
Murray said the current high ‘‘isn’t going to stick around for three weeks’’, but had much of the same ingredients as the November-December high – warm seas, warm land, and warm, light winds.
Dr James Renwick, who researches climate variability, said a lot of what’s been happening could be put down to La Nin˜a, but some aspects are a little less clearcut.
Renwick said on one level the weather this summer has been a single event, but on a wider view the extremely dry conditions and recent extreme rainfall are ‘‘very much a signature of climate change’’.
‘‘The way the climate changes just allows some events to happen more easily. So extreme rainfalls become more likely over time, and a warmer atmosphere means evaporation is more efficient, so soil dries out faster and dry-spells tend to lock in for longer.’’
This spring and summer, with water restrictions in place right up until an extreme rainfall, typifies the extremes predicted. ‘‘ What we’ve seen this summer is very consistent with climate change,’’ Renwick said.
Renwick said the best way to think of climate change and its effects on weather was as a combination of normal weather variation, with ‘‘an imprint of climate change on top’’.
‘‘The climate is already different from how it was 50 years ago. The day-to-day weather continues, but it’s gradually shifting in certain directions; towards warmer temperatures, heavier rainfall events when they happen, drier conditions when it’s dry.’’
That means even if annual rainfall stays the same, but comes in shorter, heavier bursts, there is a potential for more regular water restrictions, or even droughts. Because New Zealand’s weather has a lot of natural variation, it can be very hard to tell if any event in particular had been affected by climate change – with the exception of sea level rises.
‘‘The chances of getting coastal inundation or flooding is very sensitive to sea level, so you raise it by 10 centimetres and that triples the chance of getting a one-ina-hundred year coastal inundation, so it becomes one every 35 years. Increase by another ten centimetres and it triples again, so it’s once every 10 years, and so on,’’ Renwick said.
Modelling studies had shown the Takaka flood of 2009 was far more likely to occur in climate change circumstances. ‘‘We’ve already had about 20cm of sea-level rise in the last century, we’re bound to get at least another 30 this century, actually in the next 30 to 40 years.’’
Renwick said coastal flooding events were likely to become much more common, and more severe, ‘‘before our eyes’’.
For Nelson, that meant flooding of roads and carparks at spring tides could become more frequent, and potentially more damaging, as deeper floodwaters reached further.