Before I dive back into our discussion about El Nińo, did anyone really think we would get through this winter without seeing regular good old-fashioned cold weather? I think we all knew that the amazing period of mild weather would eventually come to an end, but you know, I secretly think there are a lot of people who are actually enjoying this colder weather (even though they would never admit to it). The big question now is, will we see a sustained period of cold weather or will we see a switch back to a more typical El Nińo winter?
Back to our discussion about global weather patterns and just what exactly El Nińo and La Nińa are.
If you remember back to last article on El Nińo, we discussed how the Pacific Ocean is kind of like a giant heat battery, storing the sun’s energy and then releasing it in the form of heat and moisture. We also discussed (very generally) how the atmosphere likes to try to even out areas that are warm with areas that are cold, and this results in a general pattern of winds around the world (easterly winds in the tropics, westerly in the middle latitudes, and easterly winds in the Arctic).
If we are going to understand El Nińo and the cold or opposite version known as La Nińa, we need to keep these wind patterns in mind, and go back to the Pacific Ocean to examine the effect of these winds. For anyone who has watched the effect that wind can have on a body of water, you’ll probably recall seeing how wind can push water around. If you have ever spent any time at one of Manitoba’s larger lakes (take Lake Winnipeg for example) you will know that if you get a strong wind coming from the north, the lake level at the south end of the lake will rise. Have a strong south wind and the water level in the south end will fall, while that in the north rises. This occurs because the wind is actually pushing the water in the direction that it is blowing.
Now, transfer this idea to the Pacific Ocean, except instead of having a water body a few hundred kilometres long, we are now talking about thousands of kilometres long. In tropical regions, the winds in the Pacific are almost always blowing from east to west. This pushes the water away from the west coast of North and South America and piles it up on the far side of the Pacific. What does this have to do with El Nińo and La Nińa?
As the easterly winds in the tropics blow offshore of the Americas, they push the water with it. Now, you just can’t push all this water away from the shore and not have water want to come in and replace it. Since the wind is pushing the surface water, the replacement water can’t come from there, at least not directly. Where the replacement water comes from is from below. This is known as upwelling, and for anyone who has ever gone swimming in a lake; you already know that the temperature of the water is way colder down near the bottom of the lake than it is at the top. So this water rising up to replace the water being pushed away by the 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. Now, 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, whereas warm water will. This means that we don’t see as much in the way of clouds and precipitation in the area with cold upwelling as we do in the area of warm water on the far side of the Pacific. This is the general setup that would be considered “normal” across the Pacific Ocean.
When scientists say an El Nińo or La Nińa event is occurring, this normal pattern of winds and ocean temperature is changing. For an El Nińo, we see an unusual warming of the Pacific in areas that are usually cool, and during a La Nińa event we see a cooling of the Pacific over areas that are usually warm.
Next issue, we will take a break from El Nińo and do our yearly look at Christmas weather.