ENSO El Niño—Southern Oscillation

 El Niño (Spanish for “the Child” in reference to baby Jesus) = warm surface current in equatorial eastern Pacific that occurs periodically around December.  Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El Niño.  ENSO describes a combined oceanic-atmospheric disturbance. El Niño-Southern Oscillation (ENSO)

 Occurs when a change in the direction of tropical winds warms coastal surface water, suppresses upwellings, and alters much of earth’s weather.  La Niña is the reverse of this effect. But what is it exactly?

 The exact initiator of this pattern is still not well understood.  What is known is that every so often the prevailing tropical trade winds blowing westward weaken or reverse direction.  This warms up the surface water along the South and North American coasts, which suppresses normal upwellings of cold, nutrient-rich water.  This results in a flattening of the thermocline. What causes it?

Normal Pacific Ocean Conditions

El Niño Pacific Ocean Conditions

La Niña Pacific Ocean Conditions

 With less cold water upwellings, waters can become nutrient deficient causing a reduction in primary productivity, thus a reduction in fish populations.  A strong ENSO can trigger extreme weather changes globally.  El Nino conditions means warmer temperatures in Canada and in the west and north US. The Gulf experiences wetter than usual conditions, but drier conditions persist in the Pacific northwest.  La Nina conditions means more Atlantic Ocean hurricanes, colder winters in Canada and the northeast US. Warmer and drier winters in the southeast and southwest US. The Pacific northwest gets wetter winters. What are the consequences of these changes in sea temperature and atmospheric pressure?

What do you think are some secondary effects of El Nino?

 Anomaly Maps  Anomaly (a = without, nomos = law) maps show the difference from normal conditions.  Sea Surface Temperature (SST) anomaly maps are useful for identifying unusually warm or cool water:  Positive SST anomaly values = water warmer than normal  Negative SST anomaly values = water cooler than normal Which correspond to El Niño or La Niña? How do we track and predict ENSO?

January 1998 SST Anomaly Map

January 2000 SST Anomaly Map

El Niño Recurrence Interval  Typical recurrence interval for El Niños = 2-12 years  Pacific has alternated between El Niño and La Niña events since 1950