In mid January, much of the US was suffering under the influence of a large, arctic air mass that brought unseasonably cold temperatures and low wind chills. In Nebraska and South Dakota, nighttime low temperatures were well below zero several days in a row. Now, during this time, I was actually down in New Orleans attending the Annual Meeting of the American Meteorological Society. All of the local people that I met there were complaining about the cold weather there, too. Now, Louisiana was also under the same cold air mass that the upper-midwest was, but by the time the air mass had reached New Orleans, the air mass had “modified”—nighttime low temperature in New Orleans were running right around 32 degrees during that cold air outbreak. To folks in Nebraska and South Dakota, that might not seem so cold, but that is far below normal in southern Louisiana. In TX and LA, these cold air outbreaks are called “northers”.
It’s often the case that a cold air mass slipping south across North America will stall right along the Gulf Coast. Most of the time, meteorological conditions prevent a cold air mass from continuing south across the Gulf of Mexico—that’s part of the reason why the islands of the Gulf and the Caribbean are nice places to take vacations this time of year. However, sometimes the processes that keep cold air masses from sliding out in the Gulf of Mexico aren’t in place, and the atmosphere is set up for an interesting weather phenomenon called a tehuantepecer.
Cold air masses are relatively dense. This dense, heavy air is hard to lift, so it tends to stay trapped right near the surface. It resists being lifted up to go over a mountain range, for example. Now, when a cold air mass moves out over the warm waters of the Gulf of Mexico, the process of modification continues and even accelerates; the cold air is touching the warm surface waters, so heat from the gulf is transferred up into the cold air mass, increasing its temperature. A cold air mass over the Gulf of Mexico is certainly far warmer than it was when it crossed NE and SD, but it is still unseasonably cold for these tropical and subtropical locations.
As the cold air mass presses farther and farther to the south, it is hemmed in on the west by the mountain ranges of Central America. These countries are very mountainous, and the mountain ranges act like a wall, keeping the cold air mass in the Gulf of Mexico and the Caribbean from crossing Central America and pushing westward into the Pacific Ocean. The cold, dense, arctic air mass piles up against this wall of mountains all along the east coast of Central America.
These mountain ranges form a very effective barrier between surface weather in on the Atlantic side of Central America and the Pacific side, except in one location: Chivela Pass in Mexico. This is a narrow pass through the mountain range. It’s only about 25 miles wide, but it’s a little gap in the mountain ranges of Central America—in fact, it’s one of the ONLY such gaps. Cold, dense, arctic air masses that have been piled up against the wall of mountain ranges in Mexico find this narrow gap, and the air begins to race through Chivela Pass, much like air races out of a hole in a tire. Winds accelerate through the Pass and reach speeds of up to 100 miles per hour as they exit the Pass on the Pacific side. The body of water on the Pacific coast of Chivela Pass is called the Gulf of Tehuantepec, so these extreme wind events are known as “tehuantepecers”.
During a tehuantepecer, the Chivela Pass is like a nozzle squirting relatively dense, cold air of arctic origin from the Atlantic side of Central America to the Pacific side. These winds don’t just stop at the coast; rather, they race out to side with surprising speed, where they can be a serious hazard to unsuspecting ships. There are many reports of ships being sandblasted by these winds and the dust and sand that they picked up on the Pacific coast of Mexico.
Tehuantepers typically happen a few times each year, and they have important effects on the Pacific ocean and the environment. On tomorrow’s episode of Weather Break, we’ll talk more about tehuantepers and what they mean for fishing and sea life in the Gulf of Tehuantepec.
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