The neutral winter of variability

By Daniel Corbett

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This winter saw a good mix of weather types, ranging from a brief polar blast in June, to blocked southwesters, anticyclones and stuck Tasman lows.

The winter may not have felt so cold and perhaps appeared back to front, with the season opening with a polar blast and snow, then milder weather later on.

This mix of weather systems was typical of the passing neutral phase in the El Nino-Southern Oscillation (ENSO) cycle. It is moving away from last summer's La Nina, without reaching the other extreme of El Nino.

In the neutral phase, other factors can influence the season's weather, including sea temperature anomalies, the Southern Annular Mode (Sam) and winter blocking.

The other interesting thing about the winter was the source regions for the air moving across New Zealand at various times. Source regions influence how cold, or warm, the air feels in weather systems. In the first part of the winter, the source region was the Southern Ocean.

During the second half, air was more often from the milder Tasman Sea.

This winter was coldest early on, when the Southern Ocean opened up and began to feed polar-chilled air toward New Zealand.

On June 6, some of this air collided with a moist Tasman Sea low, bringing significant snow to parts of the South Island. Christchurch received 15cm and some higher elevations 50-100cm.

In mid-June, the Southern Ocean continued to be the main airflow source, resulting in colder than normal daytime highs, barely above 8°C in some parts of central and southern regions. A blocked, or stuck, weather pattern kept a bone-chilling southerly flow in place with wintry showers peppering southern and eastern coasts for more than a week.

Stubborn lows

In late July, the upper wind flow changed to a more blocking regime, causing many lows from the Tasman Sea to get stuck and linger. This led to excessive rain and flooding, particularly along northern and eastern areas.

A stubborn low spent nearly a week spinning off the west coast of the North Island in the final days of July. The system began as an active low, moving in from the Tasman Sea. The rain spread across the north of the North Island during July 30. By the following morning, more than 100mm had fallen in the Coromandel ranges, with Kopu recording 115mm. The low remained for several days, bringing wet weather to much of New Zealand.

The blocking eased in mid-August. A more typical winter westerly flow returned with a procession of rain-bearing fronts interspersed with milder anticyclones.

Another curious event was early September's wintry blast which brought snow to sea level across parts of the South Island and low levels over southern and eastern parts of the North Island. The polar-chilled air behind the system was some of the coldest air over winter, although it happened in the first few days of spring.

What is to come?

Westerly flows will be a major feature during the next several weeks, bringing spells of rain with active troughs. Looking further into spring, do not be surprised at continued variability, especially once the weak El Nino settles into the driver's seat for the ride toward summer. This weak signal El Nino could also allow other factors to play a part in our weather patterns.

Expect settled periods, with clear sunny days and frosty, or foggy mornings, associated with passing anticyclones.

Fronts and troughs rolling in from the Tasman may be followed by episodes lasting several days of cool southwest winds across the whole country. Occasionally, a low-pressure centre may move on to the country from the north, preceded by an easterly flow with some heavy rain for north-eastern areas. Watch for a blocking pattern during the next several weeks where the weather patterns get stuck, causing some anomalies.

- Hamilton News

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