Unseasonal beginning

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If ever New Zealand wanted a sunny morning, it was on January 1, when our much-vaunted “first light” festivities would be viewed by the world. Alas, blue skies were not to be. The new millennium dawned cloudy over the whole country—although many areas along the east coast of both main islands enjoyed a brief consola­tory glimpse of the sun rising above the horizon as it shone through a gap in the stratocumulus.

Back in the hills there was no such luck. Cloud enveloped the mountaintops and drizzled on those souls who had hoped for a high-altitude dawn display. The contest for first-light honours between Mount Hikurangi in the Raukumara Ranges behind East Cape and Tapuae-o-Uenuku in the Kaikoura Ranges remained unresolved.

Cloudy weather in the east is not unusual for a La Nina summer, when north­east winds prevail. On January 1 a front crossing the country in the morning brought the cloud and rain, and once it had moved away to the east, the skies cleared and the sun beat down. But more drama was to come. The coolest air, which typically lags some distance behind a front, did not cross the country until mid­afternoon, by which time the sunlight had heated the ground to its maximum temperature for the day. It was a recipe for thunder­storms.

An easterly sea breeze over the north of the North Island ran into a strengthening north-westerly, fueling the brew. The result was a line of towering cumulonimbus clouds that unleashed a short burst of heavy rain as it crossed Northland and Auckland. These clouds then moved away to the south-east, taking their rain to the Bay of Plenty. At the same time, the line of cumulonimbus grew at its southern end, extending into the heart of the North Island.

About 4 P.M., radar at MetService in Wellington picked up the strong echoes of one thunderstorm moving out of the Ruahine Ranges and crossing southern Hawkes Bay. As it happened, this storm passed right over a farm in Waipawa where my brother was working. He later told me that when he first saw the black clouds looming he thought the rain would miss him. But the clouds spread out quickly, and when he felt the first heavy drops he headed for his car. Within seconds the rain changed to marble-sized hail and he broke into a run, accompanied by his dog, loud in its disapproval of the weather.

The downburst of wind that accompanied the thunderstorm was so strong that it snapped off nearby poplar trees that were 25 cm in diameter. When he reached his car, my brother was concerned the wind might rip off the doors, so he opened the boot instead and tumbled in with the dog. Meanwhile, all the cattle in the paddock, terrified by the hail and noise, charged down to the fence at one end, then turned and stampeded back up, passing on both sides of the car.

The rain was torrential, if brief, with 45 mm falling in 15 minutes. Even higher rainfalls were reported elsewhere in Hawkes Bay, with about 75 mm recorded on one farm in less than half an hour. Although the hail flattened crops and damaged fruit trees, the rain was beneficial to those farms in the path of the thunder­storm, and they enjoyed improved pasture growth for months afterwards.

Dramatic weather was not restricted to the North Island. A couple of days later, a cold snap brought snow as low as 500 m in Otago. At Coronet Peak, skiers and snowboarders took advantage of 20 cm of fresh snow to enjoy a full day of winter excitement in the middle of summer. Skis were also out at the nearby Waiorau Snow Farm, where snow lay as deep as 70 cm in some drifts.

Heavy rain affected low-lying areas, causing minor flooding but bringing delight to drought-stricken farmers. Normal holiday activities were disrupted. In Alexandra, where the temperature did not rise above 10°C on January 3, the rain caused a Shell Cup cricket game between Auckland and Otago to be postponed 3 for a day—a first for the normally warm and sunny Otago town. The a cold, wet weather also inconvenienced campers in many areas. One family in a Christchurch campground finally decided to up stakes and head for home when ducks invaded their tent to shelter from the rain!

Although good weather soon followed, January ended up colder than average over much of the country. Summer recovered its composure in February, with most places recording near-average temperatures. However, averaging all three summer months, New Zealand’s temperature was 16.2°C, 0.4°C below the 1961-90 summer average. If it felt colder than this, it was probably because the temperature was almost 2°C below the average for the previous summer.

What was surprising about these cooler tempera­tures was that they happened during a La Nina summer, when temperatures are usually above average.

According to Jim Renwick of the National Institute of Water and Atmospheric Research there were two likely reasons for the cool La Nina. First, the country ended up with more south-easterlies than normal, rather than north-easterlies, so air was reaching New Zealand from cooler southern latitudes more often than usual. Second, after a long period of warmer-than-normal sea-surface temperatures in the Tasman Sea we experienced a change to a cooler-than­normal ocean surface. This lowered the temperature of air reaching New Zealand from the west.

Just why these two things occurred is hard to fathom, but it is interesting to note that other parts of the Southern Hemisphere have had the kind of weather associated with a typical La Nina. For example, the northern part of Australia had heavy rainfall and widespread flooding. In the last week of February about a third of Queensland was under water. Many people were evacuated and hun­dreds of animals drowned.

Heavy rain also brought flooding to Madagascar and nearby parts of Africa. In Mozambique the results were catastrophic. Thou­sands of people are believed to have drowned and around one million were made homeless. Helicopters rescued thousands of people from treetops or the roofs of buildings where they had been perched for days. One woman gave birth in a tree before being rescued.

Over New Zealand, the rainfall pattern for the summer was mostly along the lines of La Nina expecta­tions, although there were some notable exceptions. Northland and Auckland had below-average precipitation, when in most La Ninos they are wetter than normal. Most of Wairarapa and Gisborne had above-average rain but parts of Hawkes Bay extending inland from Napier bucked the trend and had over 50 per cent less rain than normal.

These sorts of variations within regions are impossible to predict, and are a re­minder that El Nino-La Nina influences explain only 20 per cent of the year-to-year variations in rainfall.

What of the future? We have not had two La Ninos in a row for more than 20 years, so this may be a sign of a major change in the behaviour of the El Nino-Southern Oscillation (ENSO) phenomenon.

Palaeoclimatological indicators, such as tree-ring widths, suggest a change every two or three decades between periods dominated by El Ninos, such as we have had since the mid-1970s, and periods when La Ninos and El Ninos occur with about the same frequency. If we have moved into a period when El Nitios are less frequent than they have been, this should cause a rise in temperatures over New Zealand.

The global average temperature climbed steadily last century, with 1999 being about 0.7°C warmer than the end of the 19th century. A new study by NIWA in conjunction with Britain’s Hadley Centre and the University of East Anglia’s climate research unit has shown that the 1990s were the warmest decade of the century and probably of the millennium. Seven of the ten warmest years of the century occurred in the 1990s, and 1999 was the 21st year in a row with above-normal global surface temperatures.

So, despite a slightly cool start to the millennium, be prepared for things to continue heating up. Rainfall, however, is a little harder to pick. But if we are in for fewer El Ninos, then droughts should be less frequent in eastern areas of New Zealand than during the last 20 years.

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