FORECAST FOR

Move over El Nino, there’s a new wild weather kid in town. And, writes Paul Gorman, it’s name is Sam.

OUT of the way El Nino, and make room for Sam.

While attention over the summer focused on the muchpromised but yet-to-arrive El Nino, Sam – or the Southern Annular Mode, to give it its proper name – has worked away quietly in the background determining our weather.

Forecasters are becoming increasingly enamoured with Sam for the valuable guidance it gives of 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 recordbreaking 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, Hawke’s Bay and the 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.

Some of the coldest,