El Nino and La Nina are two phases of a semi-regular temperature cycle in the tropical Pacific Ocean: El Nino is characterized by warmer-than-normal ocean surface temperatures in the eastern Pacific, while La Nina occurs when the ocean is cooler than normal.

These fluctuations in water temperature affect the air pressure above the ocean and have a dramatic impact on the weather around the Pacific Ocean and the world.

"El Nino" is Spanish for "the boy" — specifically, the baby Jesus — and is so-named because of its appearance near South America around Christmas. It is a phenomenon that has been repeated many times and has been known and observed for generations, at least by the fishing communities affected by the phenomenon.

The widely accepted name for its cold counterpart, "La Nina," means "little girl," but the phenomenon has also been called "El Viejo," meaning "the old man."

To our eyes, the movement of the oceans appears to take place on the surface, in the form of waves or storms. But in the inky depths there is powerful activity that we are only just beginning to understand.

Currents are not merely surface water flows. In truth, they are immensely powerful systems existing in three dimensions.

The famous Gulf Stream, for example, forms a closed loop thousands of kilometres long, with warm water on the surface flowing roughly north before cooling off in the Arctic, sinking and returning along its entire length to be warmed again in southern waters.

Pump creates weather

Not only do the ocean's current affect the temperature of the seawater, but they affect the atmosphere, too. A warm water current heats the air above it. As that warm air rises, cool air is sucked in beneath it.

That circulating motion is commonly referred to as a "pump." It behaves like a vast, atmospheric machine, with weather as its product.

This pumping action creates much of the world's weather and plays an important role in regulating the planet's temperature. It pushes warm air from the equatorial regions to cooler areas closer to the poles, and in return pulls cool air closer to the equator to be heated.

These pumps are a crucial part of the Earth's ecosystem because they regulate and diffuse temperature. The equatorial regions receive much more energy from the sun than the rest of the planet, and the ocean-current pumps serve to cool off the hot regions and warm up the cold ones.

Without them, much of the planet would be uninhabitable for human beings.

El Nino is a periodic adjustment of one of these crucial pumps, occurring on average once every five years, but can occur at intervals from two years to seven years. Ordinarily, a large region of warm water rests in the area of the Pacific Ocean adjoining Indonesia and Australia. During an El Nino, that body of warm water moves east and sits off the coast of South America.

Thus the huge — but intricate — weather systems caused by the current are completely thrown off.

Farming suffers

Dry areas experience floods. Moist areas, such as Borneo, are hit with droughts. Canada may experience a warmer, more damp winter, resulting in terrible ice storms. Agriculture around the world suffers.

Its effect can be felt an ocean away and El Nino has historically been associated with less active hurricane seasons in the Atlantic. However, there have been recent changes in El Nino that aren't fully understood.

Forecasters predicted lower-than-average hurricane activity in 2004, which was expected to be an El Nino year. But the season ended up being unusually high — a total of 15 tropical cyclones developed in the North Atlantic, of which 12 were named storms.

"This could be part of a natural oscillation of El Nino," said Peter Webster, a professor at Georgia Tech's School of Earth and Atmospheric Sciences. "Or it could be El Nino's response to a warming atmosphere. There are hints that the trade winds of the Pacific have become weaker with time and this may lead to the warming occurring further to the west. We need more data before we know for sure."

Weather isn't alone in being affected by El Nino. With such a large part of the Earth's ecosystem in flux, other systems are also affected.

For example, in areas receiving a higher-than-normal rainfall, elevated water levels facilitate the growth of more insects and disease. As well, mould, usually present but invisible, also increases, intensifying allergies for many people. Fish migrate differently, destroying the livelihood of many fishermen. Meanwhile, forest fires rage across the globe as normally steamy rainforests become dry.

Forest fires elevate the already high levels of greenhouse gases in the atmosphere and during a recent El Nino many cities in Southeast Asia were enveloped in a smoky haze from the fires. Thousands died in China from flooding that also ruined crops. Russia suffered a disastrous harvest that threatened that nation's economic and political stability that winter.

On and on goes the list of hardships caused by the great flows of water and air that, until recently, had been virtually unknown to most of the world's people.