- Marine heatwave could "energise" low-pressure system bound for NZ
- Last month's Tasman Sea temperatures were warmest on record for December
- Warmer water brought packed beaches, suspected effects on sealife
The "marine heatwave" that's engulfed New Zealand this summer made the Tasman Sea the warmest it ever had been for December.
And now meteorologists say it could energise a storm forecast to hit the country early next week.
At its peak, the La Nina-fuelled marine heatwave pushed sea surface temperatures to more than 6C above normal in some parts of the Tasman.
New data from the Australian Bureau of Meteorology has revealed the warmth recorded over December were the highest since records began more than a century ago.
Surf lifeguards have reported packed beaches and bluebottle jellyfish washing up on beaches earlier than normal, while there had also been anecdotal reports of snapper spawning earlier than usual last year.
While some of the heat had since been taken out of the heatwave, meteorologists say it was still persisting, and could contribute to a low-pressure system forecast to soon bring heavy rain and gales to parts of the country.
Potentially adding to the mix was a Category One but quickly de-powering Tropical Cyclone Joyce, currently located to the northwest of Australia.
"What this means for New Zealand is, as we go into next week, we'll have this plume of moisture, elongated across the Australian continent and spewing into the Tasman sea," Niwa meteorologist Ben Noll said.
"And when that moisture from the cyclone goes in the Tasman Sea, it taps into those warmer waters, and we'd see, possibly by late Monday or Tuesday, the development of a pretty intense area of low pressure, potentially in the Tasman and directly over the marine heatwave area."
MetService forecaster Tom Adams said the system could begin hitting the South Island on Tuesday, before spreading to other parts of the country.
"With that, we are talking about northeasterly gales and also heavy rain, especially in places on the west coast, but also places north and east of the North Island that are exposed to that north-easterly direction."
THE LA NINA EFFECT
The big driver of the marine heatwave had been a La Nina climate system in the Pacific Ocean, which, when in action, tended to bring more high-pressure systems to our part of the world over November in December.
As the La Nina-driven highs meant less wind and storminess, oceans hadn't been churned up as much as usual, allowing the top 10 micrometres on the ocean, or sea surface, to be heated more effectively.
"However, in 2017, the average kicked up a couple of notches compared to previous La Nina years," Noll said.
"The extent of the anomalies, and how unusual it was, was certainly more extreme than we have seen previously."
The Australian Bureau of Meteorology figures indicated December's was the biggest sea surface temperature anomaly since records began in 1900, averaging at 1C above normal.
"And in December over the entire Tasman Sea between Australia and New Zealand, these were a massive 2C - by far the warmest ever," added climate scientist Dr Jim Salinger.
What's called a positive Southern Annular Mode (SAM) also had a big impact.
"In a positive SAM event, the belt of strong westerly winds across the southern oceans contracts towards Antarctica," Salinger explained.
"This results in weaker than normal westerly winds and many anticyclones over the South Island and to the east, and shuts down the commando raids of cold air up from the Southern Oceans, especially east of the South Island.
"The light winds in the Tasman and east of the South Island allowed sea surface temperatures to warm up."
The SAM had been in the positive phase since September - at the same time the current La Nina event hit.
"The two have reinforced more anticyclones across the South Island and weaker westerlies."
IMPACT OF A HEATWAVE
Fishers have told the Herald of snapper spawning much earlier in the season than normal, while in Otago Harbour, warmer waters were suspected to have been behind an increase in stingray sightings.
Other effects may have gone unnoticed beneath the surface, as high temperature events were known to combine with other stresses causing major changes.
University of Auckland marine scientist Simon Thrush told the Herald last month it was possible the heatwave, if combined with disease and overfishing, could fuel shellfish die-offs and "sick-looking kelp forests".
National Surf Life Saving NZ manager Allan Mundy said one benefit had been the early arrival of bluebottle jellyfish in November, before the time beaches were at their busiest.
But the unusually warm water had also meant lifeguards around the country had begun patrolling a week earlier than usual.
"Overall, we are seeing massive numbers, and since Christmas, we have seen very little swell on the eastern seaboard - that's been a good thing for us, because we would have been in for complete carnage if we'd had the swells mixed with warm water.
"People would have been staying in that risk environment a lot longer."
HEATWAVE TO 'DAMPEN DOWN' NEXT MONTH
MetService meteorologist Lisa Murray said some the summer had began with waters in some places around New Zealand being 4C warmer than normal for that time of year.
It had also had a big impact on some of the weather we'd seen so far in the season, she said.
"Put simply; warmer water evaporates more which increases the amounts of water particles into the air, so when the water particles condense there is more latent heat released."
This latent heat was a key ingredient to lows deepening rapidly - or even explosively.
"An example of sea surface temperatures contributing to the development of on intense low, was the low which caused flooding and coastal inundation in many areas including Auckland, Coromandel, Thames, Christchurch and Wellington in the first few days of this year."
But Noll said rough weather events like that one had been helping to break the back of the marine heatwave.
"At the moment, we are now seeing the warmest sea surface temperatures in the Central Tasman Sea, at about a 1C to 0.5C above average."
What has taken the heat out of it?
"We had that pretty potent storm track through the region last week, and that has effectively churned up some of the deeper, cooler water and brought that up to the surface."
Salinger said the Australian Bureau of Meteorology's seasonal climate prediction model kept the marine heatwave going during throughout this month, before it "dampened down" next month in the Tasman Sea.
Global warming simulations suggested these extreme weather events would become more frequent over coming decades.