Transcript of ENSO El Niño Southern Oscillation

[music] ENSO, the El Niño Southern Oscillation, is a cyclic climate phenomenon that has a big  impact on California. The “oscillation” of this   phenomenon is between two end member climate  states – El Niño and La Niña – which swing back   and forth with a varying interval and varying  intensity. Imagine being on a seesaw that goes   back and forth – with a somewhat erratic frequency  – and raises you a varying amount of height each   time. The middle or normal state was discussed  in previous air circulation and current videos:   the trade winds blow across the equatorial Pacific  from a high pressure system in the east towards a   low pressure in the west. These winds drag on the  surface water and create the equatorial currents:   water that moves westward and piles up off  the eastern coasts of northern Australia and   Indonesia, supporting some of the largest  distribution of coral reefs in the world’s   oceans. Most of this dome of water is funneled  north and south as part of the western boundary   currents and continues its movement through the  world’s surface gyres. However, some water slides   back eastward and creates a small trickle of warm  water called an equatorial countercurrent that   runs between the main northern and southern gyre  equatorial currents. Under normal circumstances,   this countercurrent is weak and small in  comparison to the equatorial currents. What’s   happening in the eastern equatorial Pacific  as a result of all this water being pushed   west? Because so much surface water is removed,  deep water wells up to take its place and brings   with it nutrients and cold temperatures.  We end up with one of the biggest zones of   upwelling in the world’s oceans, and those high  nutrients support a large ecosystem and highly   productive fisheries. The high pressure air  system that drives the trade winds sits over   the eastern equatorial Pacific, and with the  low water content of this high-pressure air,   the coastlines of northern Peru and Ecuador  and Central America are deserts. During their   journey across the Pacific, these same trade  winds pick up lots of moisture; and by the   time they reach the islands and continents in  the west, these air masses are warm and wet;   they collide with the rainforests of Indonesia and  Northern Australia and dump their water there.   So that’s the normal situation. One end-member of  the oscillation happens when the pressure systems   that produce the Trade Winds increase;  the strength of the winds increase;   and the rains are greater in the west; and the  land is drier in the east; and the upwelling is   even more intense, and we call that a La Niña. An El Niño occurs when the pressure systems that   cause the Trade Winds weaken or reverse. The  normal trickle of a countercurrent intensifies   as the entire dome of water in the west slides  back over the water in the east. Air masses now   move eastward, picking up moisture, and dumping  it in the desert regions of the Eastern Pacific.   These winds and storms increase wave energy and  coastal erosion. Upwelling stops and with it the   nutrients that sustain the food web in this area.  Without the autotrophs at the base of the chain,   the heterotrophs migrate to find new food  sources or die. In the western Pacific,   the rainforests lose much of their  water and suffer droughts and fires.   ENSO is the regular oscillation back  and forth between the El Niño and La   Niña states. A quick way to determine what  state we are in at any given time is to look   at the sea surface temperatures in the eastern  equatorial Pacific. During El Niño the eastern   equatorial Pacific shows warm surface waters as  upwelling has stopped. During La Niña the eastern   equatorial Pacific has even colder surface  water than normal as upwelling intensifies.   How frequent is this oscillation? This  graph of past records of ENSO events   shows that each state can last 1 to 7  years and be of varying intensities.   How does ENSO affect California? This image  shows the sea-surface temperature off of   the Los Angeles Basin during January 1982, on the  left, and January 1983, on the right – an extreme   El Niño year, as indicated by the much warmer  surface water. This is water that moved up the   coast northward from the equatorial regions where  upwelling had stopped. The region from Los Angeles   to San Francisco sits just on the northern edge of  the desert belts of normal or La Niña conditions.   Our normal condition therefore is to have limited  rainwater and colder ocean surface currents coming   from the poles. During El Niño, that changes,  and the weakened or reversed trade winds bring   us increased rain storms along with warm water  that pushes eastward along the equator and funnels   north and south up the coasts. That water changes  the fish populations we find off our coast. Bird,   sea lion, and seal populations diminish with  the fish. During the 1983 El Nino event,   California experienced high mortality rates  for marine mammals and significant coastal   erosion and flooding. Pause now. [music] [music]