# El Niño Phenomenon A guided diorama of the equatorial Pacific comparing normal conditions with El Niño\. By Valentin Mermet\-Bouvier Topic: Earth & Geography [Open the interactive Jig](https://ask.jig.com/j/jWAdLBf8CZSL) ## Transcript ### 1. El Niño Phenomenon This is a slice of the Pacific Ocean along the equator, from Indonesia on the left to South America on the right, about 15,000 kilometres across\. Above it is the air, and on its cut face you can see the top 300 metres of ocean, coloured by temperature\. Drag anywhere to turn it around\. Heights and depths are hugely stretched so you can see them\. The real ocean slice would be thinner than a sheet of paper at this width\. ### 2. Atmosphere: the Walker circulation Normally, strong easterly trade winds blow west along the surface\. Warm, moist air rises over Indonesia and builds huge rain clouds, flows back east high up, and sinks as dry air over South America\. In El Niño the trades weaken or even reverse, and the rising air and rain shift east to the central and eastern Pacific\. The loop is named after Gilbert Walker, who found the see\-saw in air pressure between the two sides of the Pacific in the 1920s\. ### 3. Ocean surface temperature Normally the trade winds push sun\-warmed surface water west, so a warm pool above 28 °C collects near Indonesia, while cool water wells up along South America\. In El Niño that warm water sloshes back east\. Sea surface temperatures climb far above average in the central and eastern Pacific\. El Niño is declared from the anomaly, not the temperature itself: at least 0\.5 °C above average in the central Pacific for several months in a row\. ### 4. Sea level Wind pushing on the water piles it up in the west\. Normally the sea near Indonesia stands about 50 centimetres higher than off South America\. When the trades weaken in El Niño, the pile slumps back east: the slope flattens, sea level drops in the west and rises in the east\. Fifty centimetres over 15,000 kilometres is a tiny slope, so here sea level is drawn 120 times taller than the ocean depths\. ### 5. Thermocline and upwelling The thermocline is the sharp boundary between warm surface water and cold deep water\. Normally it tilts steeply: about 150 to 200 metres deep in the west but only 30 to 50 metres in the east, so cold, nutrient\-rich water easily wells up off Peru and Ecuador\. In El Niño the thermocline flattens and the upwelling is suppressed\. Upwelling does not stop completely, but it now lifts warm water with few nutrients, so plankton and fish such as anchovies decline\. ### 6. How an El Niño begins The parts feed each other\. Weaker trades let warm water drift east, a warmer east weakens the air pressure difference that drives the trades, and the trades weaken further\. Weaken the trades and watch a wave of warm water travel east below the surface, followed by the clouds\. The eastward pulse is a Kelvin wave\. Scientists call this loop of ocean and air feeding each other the Bjerknes feedback\. ### 7. Why it matters Moving the Pacific rain belt changes weather worldwide\. During El Niño, Indonesia and Australia often face drought and wildfires, while Peru and Ecuador get heavy rain and floods, and their fisheries suffer\. Events recur every two to seven years and usually peak around December\. La Niña is the opposite phase: even stronger trades, a steeper thermocline and a colder eastern Pacific\.