Polar vortex helping forecasters predict New Zealand weather

What is the dominant climate for Oceania, Australia, New Zealand, and Antarctica?

They refer to the ‘no-show El Niño event’ but allow that a weak version could still show up at some point this year, in theory at least.

Move over El Niño, and make room for SAM, says NZ’s Stuff website.

While attention over the summer focused on the much-promised but yet to arrive El Niño, 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.

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

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.

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.

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.

Continued here.