A fickle summer, westerlies and tropics

By Daniel Corbett, Metservice weather ambassador


The relaxing of the current La Nina over the next few weeks should continue to allow anticyclones to take a path across central and northern parts of the country rather than southern New Zealand.

This summer was certainly quite different to the summer of 2010-11. In some cases you might say it was rather fickle. It was wetter and cloudier than normal in many places.

This was mainly due to La Nina, but other factors played their part too.

The westerlies certainly made their presence known across New Zealand during late February and much of March.

You could say they came in with a bang as a result of the strong upper level dynamics, helping bring about early March's weather bomb.

There were brief periods of blocking, but some more welcome visitors came in the form of large anticyclones tracking across New Zealand for the first time in weeks.

Later in the month, upper level troughing helped feed in moisture rich air from the Coral Sea, bringing heavy rain and flooding to parts of Northland and other parts of New Zealand.

Ocean

The moderate strength La Nina is still showing in the ocean as cooler-than-normal water across the central equatorial Pacific, but these waters have continued to warm and are now about average in the eastern equatorial Pacific.

This is a further indication the La Nina is on the way out. A warmer zone of water, part of a larger horseshoe shape of warmer water over the western Pacific, still exists from the Coral Sea down to the Kermadec group.

Waters around New Zealand have been about to just below normal over the past couple of weeks. Global models are picking this La Nina episode to end in the next month or so.

Atmosphere

The Southern Oscillation Index is a parameter taken from weather maps that is positive during a La Nina. This parameter is continuing to relax and is now hovering around neutral from its past high in December.

This La Nina is behaving in a similar way to the weather patterns of early 2009 and early 2000.

Anyone with local weather recordings from these years may be able to use them as a possible indicator of the impact of the coming season.

La Nina coming to an end

A La Nina episode in the tropics tends to encourage high pressure systems to track a little further south than normal around New Zealand.

This is consistent with the large highs that blocked well east of New Zealand.

The relaxing of the current La Nina over the next few weeks should continue to allow anticyclones to take a path across central and northern parts of the country rather than southern New Zealand.

If this trend continues, we are likely to see west and northwest airflows blow from Australia to parts of New Zealand, bringing episodes of warm dry air.

Between the anticyclones, fronts and troughs are expected to cross the country, increasing rainfall in south-western areas. The strengthening westerlies could make some approaching fronts quite active, preceded by heavy rain and strong winds, followed by cooler southerly winds.

Northern parts have been open to easterly episodes and may still receive a few of these when a sub-tropical disturbance forms north of the country. This trend should slowly ease over the next couple of weeks. There are other factors at work as well, which could add variety and some extremes to the weather over the next few months.

The tropical moisture descending southwards towards New Zealand, along with the likelihood of further tropical cyclone formations, will now drop off smartly in the next few weeks. It looks like we could be in store for an active autumn.

- Hamilton News

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