Why is the Atacama so dry?The Atacama Desert is a fascinating place. It is one of, if not the driest

Why is the Atacama so dry?The Atacama Desert is a fascinating place. It is one of, if not the driest place on the planet Earth, with some locations never having recorded precipitation. It sits in northern Chile and perhaps into some surrounding countries, and is a very unique place for geoscientists to work.Given these properties, what makes the Atacama Desert so dry? That’s a really interesting question combining atmospherics, geology, and ocean science.When we discussed the polar vortex a few weeks ago (https://www.facebook.com/TheEarthStory/posts/626004947460553), we described how air, heated by the sun, rises near the equator and comes back down to the north and to the south, a circulation known as the Hadley Cell pattern. Typically, air in this pattern descends close to 30 degrees north and south of the equator – very close to where the Atacama sits.This sinking air creates deserts worldwide at these latitudes – the Sahara, the Mojave, and the Australian Outback also sit at similar latitudes north and south of the Equator.When air rises near the equator, it cools and moves to lower pressure, causing water to condense out and rain – making the areas near the equator some of the rainiest on Earth. By the time that air starts to sink, it has mostly dried out. Sinking, dry air is one component of why the Atacama is so dry.On top of the atmospheric circulation, there is a big contribution from ocean circulation. All of the Earth’s major oceans circulate in big, hemisphere-sized patterns called gyres. These patterns are driven by strong winds known as the trade winds near the equator. They blow consistently west and take water near the equator with them.Warm equatorial waters are driven west – making places like the Western Pacific or the Caribbean warm. Those waters then are pushed up the coasts, moving in giant circles – west across the equator, north/south along the continents, and then back towards the equator.In the South Pacific the waters flowing along the coast of South America are coming from Antarctica as a consequence of this global circulation. Cold water doesn’t evaporate easily, so the ocean offshore of the Atacama isn’t contributing much moisture. That’s the second ingredient in this desert.Finally, topography plays a role. The Atacama is a basin sandwiched in-between mountains. To the east of the Atacama sits the mighty Andes mountain range, with peaks in excess of 6000 meters. Moisture carried over South America from the Atlantic runs into this giant mountain range and rains out in the Amazon basin. The eastern side of the Andes stays wet, the western side winds up as dry as any place in the world.The western side of the Atacama doesn’t help either. The Andes are created by subduction taking place offshore. Right along the shoreline, stresses caused by the subduction zone are pushing rocks upwards as well, creating a mountain range called the Coastal Cordillera. It’s not nearly as tall as the Andes, but in places it can also be over 1000 meters in height. So, whatever water does evaporate from the cold Pacific Ocean offshore runs into this first topographic wall and is kept out of the Atacama.Pinned in on both sides, at the perfect latitude for a desert, and close to a very cold ocean, the end result is hyper-arid conditions – conditions so dry that plant life simply can’t live there.-JBBImage credit: Terry Feuerborn (creative commons license)http://www.flickr.com/photos/travfotos/6799007936/ -- source link

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