La Nina only partly to blame for rotten weather

Photo / Dean Purcell

The "little girl" who controls New Zealand's weather is a fickle, deceptive creature.

After last summer's heatwaves and long, dry spells, the La Nina (Spanish for "little girl") weather cycle became synonymous with golden weather.

So this summer's relentless, heavy rainfall has come as a shock for North Islanders, many of whom believed another La Nina cycle would again generate warm, fine weather.

La Nina usually means above-average rainfall and storms for the North Island and dry spells in the South Island. But Metservice weather ambassador Bob McDavitt stresses that "no two La Nina events really follow each other exactly".

He says La Nina is a strong influence, but not the only influence on our weather.

Forecasters look at three key factors in predicting the summer outlook: the direction in which warm water sloshes across the Pacific (the La Nina/El Nino effect), the influence of westerly winds in the Southern Hemisphere (the Southern Annular Mode or SAM) and the daily highs and lows that pass over New Zealand.

National Institute of Water and Atmospheric Research (Niwa) principal scientist James Renwick says subtle changes in any of these three factors can be the difference between an Indian summer and a soggy, miserable season.

Dr Renwick said last summer the SAM went through a strongly positive phase, which meant storms and strong westerly winds swept over the Southern Ocean, while New Zealand experienced light winds and relatively settled weather.

In the lead-up to this summer, the SAM has mostly been in the negative phase, which means the opposite has occurred - westerlies sweeping over New Zealand, unsettling the weather and increasing storm activity, while the Southern Ocean has experienced settled conditions.

Another reason for New Zealand's contrasting summers is that last year the large highs that typically sat to the east of New Zealand moved over the country, creating long stretches of balmy weather.

This summer, they have remained offshore, allowing storms to drift down from the Tasman Sea and over the North Island and upper South Island, dumping the heavy rain which drowned Nelson in December.

The "little girl" has also been more shy this year. A weaker La Nina event has seen milder conditions, fewer sunshine hours and more overcast skies. That's not a bad thing, say meteorologists, because we are more likely to be spared another La Nina curse - tropical cyclones.

Summer of 2010/11

Very strong La Nina + consistently positive SAM (westerly winds) + persistent highs = heatwaves, dry weather, high number of tropical cyclones

Summer of 2011/12

Weak La Nina + mostly negative SAM + stubborn lows = heavy rainfall, moister conditions, storms