Understanding El Niño and La Niña

Glacier FarmMedia – We are into an El Niño period and there is high probability it will continue through the winter.

What is El Niño, and why can it have such a profound impact on our weather? Simply put, El Niño is a change in ocean surface temperatures across the tropical and subtropical Pacific Ocean. This creates a change in the weather patterns across the Pacific Ocean.

The Pacific Ocean covers half of our planet, so any large-scale change is bound to have an impact elsewhere. That doesn’t fully explain why changes to the ocean affect weather here. To understand why, we must remember that it takes a lot of energy to warm up water, and conversely, water releases a lot of energy when it cools. In essence, water is like a battery that stores heat, and the Pacific is a big heat battery.

Weather is the atmosphere’s attempt to equal out heat imbalances. If there is too much heat in one place or too much cold in another, the atmosphere tries to make things equal by sending cold air southward and warm air northward.

The tropical areas of our planet rarely, if ever, see or feel cold air moving southward. If you live in arctic regions, you rarely, if ever, see really warm air moving northward. Those of us in the middle constantly feel and see this movement, and we call it “weather.”

To understand El Niño and the cold or opposite version known as La Niña, we need to keep these wind patterns in mind and consider their effect on the Pacific Ocean. Those who have watched the effect of wind on a body of water can likely recall seeing wind push water around. For example, a strong wind from the north will raise the level of Lake Winnipeg at the south end, and vice-versa.

Transfer this idea to the Pacific Ocean, which is thousands of kilometres in size. In tropical regions, winds in the Pacific almost always blow from east to west. This pushes water away from the west coast of North and South America and piles it up on the far side of the Pacific.

As easterly winds in the tropics blow offshore of the Americas, they push surface water. Replacement water can’t come from there — at least not directly. Instead, replacement water comes from below. This is known as upwelling.

Those who have gone swimming in a lake know the water temperature is much colder near the bottom than at the top. In the Pacific, water rising to replace the water being pushed away by wind is cold.

OK, we now have cold water along the west coast of North and South America and the surface water is being pushed across the Pacific and is piling up on the far side. As the surface water travels across the Pacific, it is heated under the tropical sun, so the pile of water on the far side is very warm.

Cold water will usually keep the air above it cooler, while warm water will keep the air above it warmer. Also, cold water won’t evaporate as easily as warm water. This means we don’t see as many clouds or precipitation in areas with cold upwelling as we do warm water areas on the far side of the Pacific. This is the general situation considered “normal” across the Pacific Ocean.

Winds of the world

When scientists say an El Niño or La Niña event is occurring, this normal pattern of winds and ocean temperature is changing. For an El Niño, we see an unusual warming of the Pacific in areas that are usually cool, and during a La Niña event we see a cooling of the Pacific over areas that are usually warm.

During an El Niño, the extra energy or heat stored and released into the atmosphere must go somewhere. Some of it simply warms the air, but then the atmosphere wants to equal out that warm air.

A big chunk of the heat energy coming out of the Pacific goes into developing clouds, precipitation and storm systems. These systems are an efficient way for the atmosphere to move heat around and equalize it, because more energy can be moved in warm water than in warm air. Most of the energy is stored in the form of latent heat that will be released when the water vapour condenses.

This overall movement of warm air and storm systems over the Pacific Ocean creates a general pattern of winds around the world.

Under normal temperature conditions across the Pacific, the general flow of the atmosphere follows the general pattern of tropical easterlies, mid-latitude westerlies and polar easterlies. Since we live in the mid-latitudes, a large portion of our weather comes off the Pacific.

If we change the amount of heat over a large portion of the Pacific, either by warming up the Pacific Ocean (El Niño) or cooling it down (La Niña), this disrupts the general flow of air across this region and can impact air flows.

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the University of Winnipeg. He operates a computerized weather station near Birds Hill Park, Manitoba. His article was originally published at the Manitoba Co-operator.