As rumours of gumboot shortages, washed-out highways and damp cricket matches drifted down the island, it finally dawned on sun-dazzled Wellingtonians that the North was not enjoying the glorious summer weather that they were.
Among the many expressions of sympathy that flowed north was the suggestion that the 1990 Commonwealth Games be transferred to Wellington, and support was offered for an Auckland bid for the Winter Olympics!
Why was Northland’s summer so wet when Canterbury was suffering from drought and Wellington was doing nicely on a touch less than average rainfall?
The short answer is that the airflow was often northeasterly over the North Island during the summer, bringing moist tropical air over Auckland. At the latitude of Christchurch, on the other hand, there was frequently an anticyclone bringing dry weather.
During much of January the weather map showed a northeast airstream stretching from the Kermadec Islands to Auckland and Northland. As warm air moved away from the tropics towards New Zealand it was cooled from below by the ocean. This caused some of the invisible watervapour in the air to condense to the tiny liquid droplets that clouds are made of.
The area of low pressure (called a depression, or simply a “low”) just north of Cape Reinga caused the air nearby to rise, which cooled it further, leading to more cloud droplets and the formation of rain as the droplets coalesced.
The rain in eastern Bay of Plenty and Coromandel was intensified by the hills. As the wind was easterly, it blew from the sea towards the land. The hills forced the air to rise rapidly, leading to rapid cooling of the air, and rapid production of rain from the condensing water vapour.
The presence of the anticyclone meant that the South Island and Wellington were dry, apart from some brief coastal drizzle in Canterbury and Otago.
And why were there more northeasterlies than normal this summer? Aucklanders can blame them on the extremely positive phase of the Southern Oscillation, a giant weather system which affects the whole of the Southern Hemisphere.
It has been noticed that when air pressure between Tahiti and South America is lower than average there is a tendency for higher than average pressure around the New Guinea/ Darwin area. Conversely, when pressures around Darwin are lower than average, then Tahiti’s pressure tends to be higher than average.
This see-sawing of pressure between the east and west of the Pacific is known as the Southern Oscillation. When Darwin is up and Tahiti is down the Southern Oscillation is in a negative phase, sometimes called El Nifio; when Darwin is down and Tahiti is up it is in a positive phase,sometimes called La “Nina.
The useful thing about the Southern Oscillation when it comes to forecasting is that if it is strongly positive or negative in the spring, then it tends to stay that way throughout the summer.
A strong positive Southern Oscillation correlates with more frequent northeasterly weather over the North Island and anticyclones in South Island latitudes, which is what happened in the summer of 1988/89. It has also been blamed for last year’s drought in the American mid-west and severe flooding in Bangladesh.
On the other hand, a strong negative Southern Oscillation correlates with southwesterlies over New Zealand and a ridge of high pressure to the north. It is also blamed for drought in Australia and Southeast Asia, for floods in California, Peru and Ecuador, and for tropical cyclones reaching as far east as Hawaii and Tahiti.
All of which happened in the summer of 1982/83. In New Zealand that summer the wind never seemed to stop blowing. Southwesterlies brought more rain than normal to Buller, Westland, Fiordland, Otago and Southland.
Elsewhere there was less rain than normal, especially in Gisborne and Hawke’s Bay, where there were drought conditions.
In eastern districts there was less rain because the wind blew mostly from land to sea and they were in the rain-shadow of the mountains. This also applied to Manawatu, Wellington and Nelson, which were sheltered by the Tasman Mountains to the west of Nelson.
Further north, even though the wind blew from the sea to the land in Taranaki and Auckland, there was less rain than normal because the ridge of high pressure nearby prevented the air from rising and cooling. Also the air had come from the subantarctic area and was warmed as it moved north and this made it relatively dry.
Normally the New Zealand Meteorological Service does not issue forecasts for more than five days ahead. However, in light of knowledge about the Southern Oscillation, successful forecasts were made months in advance for the weather during last summer and also the summer of 1982/83.
There has been speculation that this summer’s weather has been a further sign of the global warming expected from the enhanced greenhouse effect. This is not so. In fact, the global average temperature may well drop a little due to the cold sea surface temperatures in the Tropics associated with La Nifia.
It is possible that there will be more frequent summers like this in the future, but on the other hand it is also expected that at times we will have summers like 1982/83, when the negative Southern Oscillation brought cooler than average temperatures to the whole country.