The Atacama Desert, a vast plateau located along the Pacific coast of South America, is the driest non-polar desert on Earth, with some areas recording virtually no rainfall for centuries. This extreme aridity has even led to its use as a testing ground for Mars expedition simulations due to its striking resemblance to the Martian environment. Its extreme dryness results from a complex interplay of geographical and atmospheric phenomena.
The Andes Mountains’ Rain Shadow
The towering Andes Mountains block moisture from reaching the Atacama Desert. Situated to the east of the desert, these high peaks act as a formidable barrier to moisture-laden air originating from the Amazon basin and the Atlantic Ocean. As humid air masses approach the Andes from the east, they are forced to rise over the mountain range. This ascent causes the air to cool, leading to the condensation of water vapor and the formation of clouds.
Most of the precipitation is then released on the eastern, windward slopes of the Andes, supporting lush ecosystems like the Amazon rainforest. By the time the air descends on the western, leeward side, where the Atacama Desert is located, it has lost nearly all its moisture. This phenomenon, known as a rain shadow effect, leaves the air that reaches the Atacama exceptionally dry.
The Pacific Ocean’s Cooling Influence
The Pacific Ocean, specifically the cold Humboldt Current, also known as the Peru Current, exerts a profound drying influence on the Atacama Desert. This cold, low-salinity ocean current flows northward along the western coast of South America, originating from higher latitudes. As the Humboldt Current moves along the coast, it significantly cools the air above the ocean surface.
This cooling effect contributes to the formation of a stable, dense air mass over the ocean. The presence of this cold air mass is associated with a persistent high-pressure system known as the Pacific Anticyclone. This anticyclone enhances stable and dry conditions in the region. The cold, dry air originating from this oceanic influence then sweeps inland towards the desert.
Atmospheric Stability and Temperature Inversions
Extreme atmospheric stability, influenced by cold Pacific waters and the high-pressure system, is a defining characteristic of the Atacama’s climate. The cold, dense air near the ocean surface, coupled with the subsiding air from the Pacific Anticyclone, creates a temperature inversion layer. This inversion occurs when a layer of warmer air sits above a layer of cooler air closer to the ground, effectively trapping the cooler air beneath it.
This stable stratification prevents the vertical movement of air, which is necessary for the formation of rain-producing clouds. Moist air cannot rise, cool, and condense into precipitation, thereby suppressing convection and rainfall over the desert. While coastal areas of the Atacama may experience a dense marine fog known as camanchaca, which provides some moisture for local flora, these fog banks rarely lead to significant rainfall. These atmospheric conditions largely inhibit precipitation.