What is El Nino

Every several years, local weathermen talk extensively about El Nino.  Responsible for affecting weather patterns on a large scale, El Nino is responsible for unseasonably heavy rain and flooding in some areas, and devastating drought in others.  Despite its common reference by meteorologists, the average person often has a poor understanding of what El Nino actually is.  By understanding the interplay of trade winds and sea surface temperatures, you can gain an appreciation for how El Nino affects global weather patterns. 

The term El Nino was first used by Peruvian fishermen to describe an unusually warm wind that began to blow around Christmas time.  It was named El Nino, or “the child”, referring to the Christ, since it usually began around the Christmas holiday.  Gradually, the term El Nino came to refer only to the strongest occurrences of this warm wind.  In the 1960s, scientists began to understand that El Nino’s effects spread far beyond Peru to the entire Pacific region and beyond. 

The El Nino phenomenon refers to a pattern where warmer than average sea-surface temperatures (SSTs) affect the flow of the trade winds, which in turn affect the SSTs.  The sun heats the equatorial region of the globe more intensely than the subtropical regions, so the surface temperature of water is substantially higher closer to the equator. 

As the planet spins on its axis, the Coriolis effect causes winds to turn to clockwise in the in the northern hemisphere and counter clockwise in the southern hemisphere, moving air towards the warmer water at the equator in the Pacific.  Blowing over the water increases the air’s temperature, which causes the air to rise.  Cooler air then rushes in to take its place, forming the trade winds. 

Due to the directionality of the Coriolis effect, the trade winds blow steadily westward across the equatorial region in a normal atmospheric year.  Generally, this also means that the warmer Pacific surface water has blown westward too.  Consequently, the sea level is about half a meter higher in Indonesia than it is in South America, and water temperatures are up to 8°C higher in the west.  With the warmer water flowing westward, nutrient-rich cold water in the east rises to the surface, helping to support the diversity of life and ecosystems off the coast of South America. 

In an El Nino year, however, the trade winds are relaxed.  The large area of high SST water spreads throughout the Pacific, rather than being funneled westward.  This prevents the cooler water in the Eastern Pacific from rising to the surface.  Since warmer water temperatures are responsible for standard patterns of rainfall, climates in Indonesia that depend on heavy rain are left relatively dry, causing droughts and brush fires.  Moreover, the increased rainfall in the Eastern Pacific leads to torrential rain and flooding.  These anomalous weather patterns continue to spread across the continents on both sides of the Pacific, leaving normally wet regions relatively dry, and dry regions wet.