Polar vortex helping forecasters predict New Zealand weather

Thursday, 14 March 2019

MetService meteorologist Georgina Griffiths talking about 'SAM' - the Southern Annular Mode or polar vortex, which plays an increasingly important role in determining our weather.

Move over El Nino, and make room for SAM.

While attention over the summer focused on the much-promised but yet to arrive El Nino, SAM - or the Southern Annular Mode, to give it its proper name - has been working away quietly in the background determining our weather.

Forecasters are becoming increasingly enamoured with SAM for the valuable guidance it gives of likely weather conditions up to two weeks ahead.

Gale-force winds and a high tide combine to make Balaena Bay in Wellington difficult for motorists and pedestrians.

SAM measures the strength of the 'polar vortex', the ring of westerly winds which encircle the Antarctic.

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MetService meteorologist Georgina Griffiths says 'SAM' - the Southern Annular Mode - is increasingly recognised for its role in determining our weather. Pictured at MetService's Kelburn headquarters.

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When SAM is positive - which it has been for most of the past four months except for the second half of last month - it shows the westerly wind belt is in place south of New Zealand, trapping cold Antarctic air and the stormiest southern ocean air behind it.

Daniel Godden clearing morning snow and ice from his van windscreen in Sumner after the heavy June 2012 snowstorm.

The positive SAM has been a factor in the extended hot, dry spells and recent record-breaking summers in many parts of the country. It typically encourages large areas of high pressure to block to the east and drags warm, sub-tropical northerly winds across both islands.

A positive SAM will also bring a mild winter that is largely free of storms and drier than average for most places, except Gisborne and Hawke's Bay and the South Island West Coast.

The positive flips (red) and negative flops (blue) of the Southern Annular Mode (SAM) can be clearly seen in this MetService plot running from January 2018 at left until this week.

But when it flips negative, take cover. The westerlies below New Zealand fall weaker than normal, allowing those storms and polar southerlies to break through the vortex and blast north on to the country.

Some of the coldest, stormiest, snowiest winters in recent years had a strongly negative SAM. Cold front after cold front, deep depression after deep depression, brought unusually frigid weather in July 1995, through much of 2002, and the extremely snowy winter of 2011, when snow lay on the Wellington hills in August and even fell in parts of Auckland.

A lone walker struggles against gale-force winds on the Whangaparaoa Peninsula.

MetService meteorologist Georgina Griffiths said the good news this week was SAM was likely to remain mostly positive for the next couple of weeks. It was difficult to get a clear signal of Sam's intentions beyond that.

'I really rate SAM for forecasting. It's a very useful tool in our toolbox when we are looking long-range, 14 days out.

A wet day on Oriental Pde in Wellington.

'It's a key driver of our weather. SAM explains a whole lot about what is going on.'

She estimated the fickleness of New Zealand's weather was about 30 per cent due to SAM, 30 per cent due to either an El Nino or La Nina event 30 per cent caused by what was going on in the tropics and 10 per cent down to what happened in the Tasman Sea.

The positive Sam affected both islands to a similar extent but negative SAM was worse the further south you lived, she said.

'The South Island is more in the firing line for the colder, nastier weather. But by the time you go up to the North Island, the north of the North Island, the effect of Sam is not as bad.'

The head of Victoria University of Wellington's geography, environment and earth sciences school, Professor James Renwick, said the impact of SAM was not always straightforward.

'Negative SAM is weaker westerlies overall, but they come farther north as well, and bring the storms with them.

'So over New Zealand, the westerlies actually get stronger, even though the peak winds in the southern hemisphere decrease a bit. But because the storm track is locked to where the winds are strongest, we get stormier and more unsettled weather.'

As for the no-show El Nino event which some had been talking about since last spring, Griffiths said it was possible there might be a 'round two' El Nino this winter or spring.

However, it appeared unlikely, at this stage, to have a major influence on New Zealand's weather.

This summer the El Nino had not become any stronger than 'marginal to weak', she said.

WHAT IS SAM?

The Southern Annular Mode (SAM) measures the strength of the westerly winds, or the polar vortex, which circle the southern hemisphere south of New Zealand.

When SAM is good the weather is settled across much of New Zealand, with long spells of hot, dry weather in summer and milder winters than normal.

When SAM goes bad, watch out. Frequent cold outbreaks and wet and stormy weather wash north across the country at any time of year.