What are La Nina and El Nino?
There is no mistaking some weather events. It is hard to miss a thunderstorm, or a bout of hail. Similarly, a flood or snow dump are so obvious when they occur that we cannot escape them.
The El Nino and La Nina weather systems, however, are phenomena which can take months to develop, and even when in full swing, can be difficult to recognise.
El Nino got its name about 100 years ago, when Peruvian anchovy fishermen noticed differences in the seas around one Christmas time. El Nino, or boy child, was a colloquialism for Jesus, and the name has stuck to describe the event they experienced. More recently, the opposite weather system has been coined La Nina.
NIWA principal scientist James Renwick says while the El Nino/La Nina weather systems operate in and are confined to the tropical Pacific Ocean, their effects are felt almost around the world.
An El Nino tends to "get going" around May or June, with the system really kicking in by August or September and reaching its peak around Christmas time.
The phenomena sees a departure from normal conditions in the Pacific, when trade winds blow from east to west, or from South America to Indonesia and Australia. The warmer water tends to be confined to the western Pacific, where sea temperatures can be about eight degrees warmer than in the east.
In an El Nino year, Dr Renwick says the trade winds start to weaken, reducing the temperature difference between the east and the west.
"El Nino is where that delicate balance breaks down."
As a result, the warm water in the west is able to move eastwards. This causes a drop in atmospheric pressure, which in turn reduces the trade winds even more and again reduces the temperature difference.
"It's a spiral. As soon as you redcue trade winds you reduce the difference in temperature, and on and on and on. And that's El Nino."
The process usually takes three to four months to happen and does not have to start with reducing trade winds. If the difference in ocean temperatures drops, that reduces the winds and there you go again, Dr Renwick says.
A La Nina sees the opposite occur.
"Instead of the trade winds weakening and breaking down, things wind up and become a super version of normal."
The gap between sea temperatures in the east and western Pacific can rise to about 11 degrees, and as the trade winds become stronger, the warmer water is pushed further west.
"In the tropical Pacific it's a fine balance. A push in either direction can cause either an El Nino or La Nina," Dr Renwick says.
The impact of the weather systems is felt most in the tropics, although there are effects on New Zealand.
In an El Nino, the west winds that normally blow over the country become stronger. They tend to also become a little more southwesterly so conditions can be cooler.
The system sees eastern regions become particularly dry and the west coast especially wet. Wellington, not surprisingly, can get even windier because of the westerlies. Summer too stays windy and feels cool.
In the opposite phenomena, a La Nina, rainfall is pushed further west. The westerly winds decrease but the country is exposed to storms from the north. Those places that look to the north tend to be quite wet, while from Christchurch south, the system brings dry weather.
Overall, there is more wind from the north and that tends to be warm.
In Wellington, again the most noticeable effect of the system is wind-related. The weather is relatively calm and we get more days than normal without much wind.
The 2011/2012 summer was the second year in a row in which the La Nina phenomena was experienced. While the first year brought a strong weather system with it - perhaps most noticeable for its increased sea temperatures for summer swimmers - the second was far more moderate.
*For more on El Nino and La Nina weather systems, Dr Renwick recommends the National Oceanic and Atmospheric Administration (NOOA) website.
The Dominion Post