On yesterday’s episode of Weather Break, we talked about a weather phenomenon called “tehuantepecers”. Tehuantepecers are extreme wind events in Central America, when cold air masses from the arctic slide south all the way across the United States into the Gulf of Mexico. The cold, dense air is trapped on the Atlantic side of Central America by the fairly high, continuous mountain ranges found there. These mountain ranges make a very effective wall that keeps this air from crossing Central America and reaching the Pacific side, except in the area of Chivela Pass, which is a narrow gap in the mountains in Mexico. The Chivela Pass acts like a nozzle, squirting the cold, dense air from the Atlantic side to the Pacific side. The air accelerates through the Pass, sometimes reaching speeds of 100 miles per hours as it crosses the Pacific coast of Mexico and heads out to sea. Tehuantepecers are a particularly spectacular example of what meteorologists call “gap flow”, where air masses accelerate as they are squeezed through narrow gaps in mountain ranges.
Tehuantepecers get their name from the fact that the part of the coast of Mexico where they occur is called the Gulf of Tehuantepec. The Gulf of Tehuantepec is a particularly important fishing area; it’s where much of the world’s tuna catch comes from, for example. The Gulf of Tehuantepec is such a productive part of the ocean because of the upwelling of cold, nutrient-rich water right along the coast of Central America. These nutrients support various forms of plankton in the Gulf of Tehuantepec, which are the food source for small fish. The small fish are eaten by bigger fish like tuna, which, in turn, we eat.
During a tehuantepecer, strong surface winds of as much as 100 miles per hour race out over the Gulf of Tehuantepec. These strong winds generate huge waves that cause strong mixing in the ocean. The mixing of surface water with deep, cold, nutrient-rich waters is one of the hallmarks of a tehuantepecer. Sea surface temperatures in the Gulf of Tehuantepec can fall by as much as 15 degrees Fahrenheit in a matter of hours. Satellites in orbit can actually detect sea surface temperatures from space, and monitoring and forecasting the dramatic changes in water temperature is the work of synoptic oceanographers.
When the nutrient-rich water gets to the surface of the ocean, plankton in the ocean take advantage of the food and the sunlight, and they reproduce wildly. A so-called “plankton bloom” sets up for a matter of days or weeks, when the surface waters of the Gulf of Tehuantepec suddenly teem with many times more plankton than usual. The increased numbers of plankton form the base of the food chain in the region, so more plankton means more small fish, and maybe eventually more tuna.
Because the population of plankton is so important to life in the ocean, scientists have developed ways to monitor plankton worldwide. Plankton change the color of the ocean slightly, so special satellites have been developed that can detect subtle changes in the color of the ocean and, from that information, determine how large the plankton population of a region is. One such satellite is called Sea-WIFS. Scientists can actually watch the growth and development of plankton blooms after tehuantepecers using Sea-WIFS imagery; we’ll have examples of what can be seen using Sea-WIFS on our website.
Tehuantepecers don’t happen every time a cold air mass slips down from the arctic, but they do typically happen a few times each year. One particularly famous example of a tehuantepecer happened in March of 1993 in association with the so-called Storm of the Century, which, in addition to causing an impressive tehuantepecer and plankton bloom, also brought as much as 4 feet of snow in the eastern US.
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