Weather wakes up to wintry potential

Whatever is gingering up August’s weather has gingered it up so well it could stay unsettled into September.

The kind of major change to the weather New Zealanders have experienced this month is rarely the result of a single factor.

In this case, it is a bothersome trio of jetstreams, the Southern Annular Mode (SAM) and a variant of El Nin˜ o which have brought frequent biting southerlies, snow and severe thunderstorms.

Forecasters believe low-pressure systems south of the country, accompanied by a large area of cold air, will continue to threaten the country with volatile westerly and southerly weather for the rest of the month and into early spring.

SAM measures the strength of the ‘‘polar vortex’’, the ring of westerly winds that encircle the Antarctic. When SAM is positive, the westerly wind belt is in place south of New Zealand, containing the coldest Antarctic and stormiest southern ocean air behind it. But in its negative phase, that belt is weaker than normal, and storms and polar southerlies can break through.

MetService meteorologist Lewis Ferris said the negative value of SAM was a player in the current energetic weather.

The position and intensity of the subtropical jetstream had enhanced the temperature gradient through the atmosphere, creating a large area of very cold temperatures well above the surface and the damaging thunderstorms in parts of the North Island this week.

National Institute of Water and Atmospheric Science meteorologist Ben Noll said a likely third protagonist was an El Nin˜ o Modoki, a variant of El Nin˜ o in which unusually warm sea-surface temperatures were in the central equatorial Pacific Ocean instead of their typical location closer to South America. Modoki is a Japanese word meaning ‘‘same but different’’.

A similar Modoki in 2004 brought a cool/cold and unsettled late winter-early spring, Noll said.