The Wind Trade

There has been speculation that El Niño may return during late winter or early spring. Drought in the east of both Australia and NZ is more likely during El Niño events because of drying winds from the west.

Farmers see the prospect of an El Niño event as bad news. Anything scary from weather bureaus sells news and we haven't had a decent El Nino for a couple of years, so interest is high.

To explain what El Nino really is, imagine a dish of water slowly sloshing from side to side, once every 4.5 years. Imagine that to be the Pacific Ocean.

The normal situation is for cold Antarctic water to travel to the left from Peru towards Australia, pushed by ocean currents in turn controlled by the moon. Because water-surface interfaces air, so the winds also flow westwards.

When I was in Standard 3 these easterlies along the equatorial band used to be called The Trade Winds, and I remember doing a ‘project’ about them.

Finding new names is the mother of all funding, and reinventing The Trade Winds gave the world ‘La Nina’. In this part of the world it means cool sea currents and moist easterlies that bring rain to eastern parts of Australia and NZ, and is the normal situation.

Eventually water driven one way makes the sealevel higher on one side, and when that difference reaches 62cm it tends to briefly - over about a year – try to restabilise by flowing back.

Currents then trek eastwards to Peru, and this reversal, which used to be called the ‘Southern Oscillation Index’ and before that ‘The Humboldt Current’ is now called ‘El Nino’, typified by renewed warm westerlies that bring droughts to eastern coasts.

There are about four complete oscillations in one lunar declination cycle. Whichever way the sea is flowing is measured by the difference in sea surface temperatures between Darwin and Tahiti.

To be called El Niño or La Niña the measured index has to exceed +0.4°C for El Niño or -0.4°C for La Niña, for 6 consecutive months.

It is now near +0.4°C, having jumped up rapidly over a short period. Farmers will tell you that quick to come is also quick to go.

If the index is maintained over the next 6 months, then by next January there might be fledgling El Nino.

Even if we are looking at a dry summer, and this is on the cards, then there may be an El Nino next autumn, well into 2013, if it stays on track. But it probably won’t.

In reality El Nino is only called off on, after a year. Until then it gets called a "condition". If it lasts the year it becomes an "episode".

Usually if it fails to fire, meteorologists still call it an episode to protect their earlier prediction and thus their jobs, and we read of it as being a "fizzer" or "weak". They even have a new category - ‘Modoki El Nino’, which means El Nino-like.

El Nino correlates well to the lunar cycle of currents-reversal which translates to surface winds through the ocean/air interface, which in turn influences pressure zones in the air.

La Nina brings higher pressures in the east which flow to lower pressures in the west, and is the longer part of the overall cycle. El Nino comes with higher pressures in the west which flow to lower pressures in the east, and brings stronger cyclones and storms because around El Nino the moon is averagely closer to earth.

El Nino typically follows a solar minimum, the most powerful about a year after solar minimum, as was the last El Nino in 2010. We are midway between minimums now, with the next likely around 2015, making the next serious El Nino arguably not until then.

It does not mean interim events may not be called El Nino but they will probably be weak fizzers or in sub-categories.

And yet over the next 6 months we could see drier than average conditions mimicking El Nino, followed by a delayed monsoon season at the top of Australia.

Interestingly too, any year following El Nino typically brings more earthquakes.

The official forecasters need to sort whether they think the coming tropical cyclone season will be active. In the current lunar cycle the next tropical cyclones should be on the lighter side and probably late, and the unsettled systems that form should rapidly breakdown into ineffective depressions, as El Nino talk fades.

You may hear new names, because there is money in winds. Plain-talking, the hottest and driest part of summer in NZ should be December and January, with plenty of rain arriving in March. The rest is met-puffery.

Ken Ring of www.predictweather.com is a longrange forecaster for Australia's Channel Seven.