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Weather man Bob McDavitt explains the La Nina summer

This is a La Nina summer. To understand the varying impact of La Nina on winds around New Zealand, let us first look at the three normal weather zones in our part of the world. First, to the north we have the tropics with trade winds that mostly blow from the east. Second, to the south and in the “roaring 40s” we have a zone of disturbed westerly winds and the low-pressure systems of the Southern Ocean. Third, in between, we have the latitude zone which we see cells of high-pressure take as they migrate from west to east across our weather map- let us call this the “sub tropical ridge”.

These weather zones tend to follow the sun. The strong cool westerlies dominate our winter and spring when the sun is overhead in the northern hemisphere, and then recede to the south when the sun is seen to get higher in our sky during our summer. Click on the animation above and watch how the orange belt, which marks the subtropical ridge, shifts from the Australian Bight / NZ latitude belt in summer, to the Australian Desert/north of NZ zone in winter. This annual solar-driven cycle is so dependable we use it to name our seasons and we track time in years.

The second strongest weather cycle is called the El Nino-Southern Oscillation cycle (ENSO) At present it is in the La Nina phase and this occurs when the sea temperatures along the eastern equatorial Pacific Ocean go through an episode of being cooler than normal. This can last for a year or so and the current event has already been going for six months and should last at least until autumn.

La Nina deflects the normal weather zones southwards around New Zealand.

The subtropical ridge shifted southwards across New Zealand during late spring, much earlier than normal this year, and that helps explains the dry period since Labour weekend and the late November “heat wave”. This zone is likely to stay in the south until March so that the anticyclones may take a path along 45 South, as shown by the pink arrow in the weather map. Individual highs may linger around southern New Zealand and the Chatham Islands, producing extended periods of dry sunny weather with light winds around the southwest of the South Island.

Watch the highs on the weather maps and - when one of them peals off to east of Chathams Island, as shown above, note that northern New Zealand then becomes vulnerable to anything that more form in the tropics to the north. If a low-pressure system forms north of New Zealand at this time then winds around it may combine with winds around the high like the wheels of an eggbeater, and focus their fury onto a localised area. This weather map can produce a day or two of wind driven rain and heavy surf, and eastern places between Kerikeri and Gisborne are the most likely targets.

During the coming summer the windscape is, in general (but NOT all the time) likely to favour more than normal wind from between north and east- mainly affecting places northern and eastern parts of the North Island. With the subtropical ridge lingering over the South Island that is the place where there may be extended periods of sunny dry weather with light winds.

The subtropical ridge should make its way north again around Easter.

I hope these ideas help your thinking . for more info read our seasonal weather outlook on our rural page ,

at http://www.metservice.com/rural/seasonal-forecast-north-island

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