Editorial: What’s in the weather?

Having been a mountaineer for most of my life, I have always retained an obsession with the weather. After all the success of trips into the mountains hinges on the coming weather forecast. I’m now an adventure motorcyclist and with this sport, weather also plays a large part. Riding a motorcycle through the Nevis Valley into a southerly, up at about 2000 metres, can be a freezing, torrid experience, as I discovered recently.

So how is December weather looking for Wānaka? I took a troll through the National Institute of Weather and Atmosphere (NIWA) website to try and find out. So here goes.

First of all: “Annual precipitation in Otago typically decreases with increasing distance from the western ranges and the east coast.”

 We all know that 

“Indeed, Central Otago is the driest region of New Zealand, receiving less than 400 mm of rainfall annually. Dry spells of more than two weeks occur relatively frequently in Central Otago, but less so elsewhere. Temperatures are on average lower than over the rest of the country with frosts and snowfalls occurring relatively frequently each year.”

 I didn’t know that; I was always under the assumption we were the hottest part of the country.

The key factor in the weather for the coming month is “La Nina”.

El Niño and La Niña are opposite phases of a naturally occurring global climate cycle known as the El Niño Southern Oscillation (ENSO). ENSO influences rainfall, temperature, and wind patterns around the world, including New Zealand. El Niño and La Niña episodes occur on average every few years and last up to around a year or two. This year we are in a La Nina cycle apparently.

During a La Niña event, ocean water from off the coast of South America to the central tropical Pacific cools to below average temperatures. This cooling occurs because of stronger than normal easterly trade winds, which churn cooler, deeper sea water up to the ocean’s surface. Sea temperatures can warm above average in the far western Pacific when this happens. This change impacts weather patterns around the world, but in a different way than El Niño does.

North-easterly winds tend to become more common during La Niña events, bringing moist, rainy conditions to north-eastern areas of the North Island and reduced rainfall to the lower and western South Island. That’s us. Warmer than average air and sea temperatures can occur around New Zealand during La Niña.

NIWA says that for December we will experience near normal rainfall. There will be above average temperatures. There will be lots of those horrible nor ’westers. So, after all that, I’d say it’s business as usual.

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