KEY POINTS:
El Nino is dead, but New Zealanders may get a La Nina weather pattern in its place, a climate scientist says.
"A transition to La Nina has recently become a substantial possibility," says researcher Jim Salinger of the National Institute of Water and Atmospheric Research (Niwa).
Conditions in the Pacific Ocean are neutral at the moment, but over the next couple of months New Zealand could be set up for a wet winter.
The strong El Nino of 1997-1998 was followed by a prolonged La Nina phase that extended from mid-1998 to early 2001.
A La Nina brings cold water to the tropical Pacific Ocean, but water temperatures around New Zealand are usually warmer than normal.
And there are more northerly and northeasterly winds, which tend to bring moist, rainy conditions to the northeast of the North Island and reduced rainfall to the south and southwest of the South Island.
This means that some areas, such as central Otago and South Canterbury, can experience drought in both El Nino and La Nina years. But a cloudier, wetter summer in the north of the country increases the chance of tropical storms, such as the one that hosed down Northland this week.
The really wet winter of 1998 marked the arrival of New Zealand's last La Nina.
The north and northeast of the Pacific - Tuvalu and western Kiribati - tend to be drier in a La Nina pattern, with wetter conditions in countries such as Fiji, Tonga and New Caledonia.
Dr Salinger said the El Nino climate pattern that dominated our summer weather broke down during February - but not before it brought drier-than-normal conditions across many parts of Australia, Indonesia and Fiji; unusually heavy rains and flooding across parts of eastern Africa; and extended dry spells across many southwestern parts of southern Africa.
Dr Salinger said that predictions for Pacific-wide climate patterns were difficult at this time of year, but several computerised forecast models showed an emerging La Nina over the next few months.
Experts had also noted the presence of a substantial pool of cooler- than-normal water just beneath the surface of the central and eastern Pacific at the Equator. This water was expected to reinforce, over the next few weeks, the already cooler-than-normal waters at the surface. If this did bring a La Nina pattern, it was likely that would persist for the rest of the year.
The patterns start showing up in the oscillation of sea temperatures in the tropical waters of the central and eastern Pacific Ocean.
New Zealand's climate is also affected by other wide-scale influences, including the Interdecadel Pacific Oscillation (IPO), which usually stays in one state for two to three decades before suddenly shifting. In 1998 the IPO brought slightly wetter weather in the north and east of the North Island, and drier conditions in the south and west of the South Island.
New Zealand is also likely to have fewer intense El Ninos in the next couple of decades and more La Ninas.
- NZPA
