El Nino and La Nina Ocean_4_ENSO Lecture

Where in the world are we? El Nino- Occurs in tropical waters (Equatorial Pacific) El Nino means “The Baby” (not necessarily “the Nino,” but close) because it usually happens around Christmas (connected to the baby Jesus)

First, a review of Solar Intensity and Latitude At low latitudes (near the equator) the sun’s rays are nearly vertical. At high latitudes (near the poles) the sun’s rays are much less intense because the sun remains close to the horizon From http://capone.mtsu.edu/cdharris/GEOL100/weather/wthr-sum.htm

What causes the seasons? Earth is tilted by 23.5 degrees The northern and southern hemispheres each tilt toward the sun for half the year Hence we have seasons The equator is unaffected by this

Equal amounts of sunlight are spread over a greater area in polar latitudes than in temperate or tropical latitudes. Tropics temperate polar From http://capone.mtsu.edu/cdharris/GEOL100/weather/wthr-sum.htm

The take-home message Tropical latitudes are the warmest Air rises there This causes convection cells (Hadley Cells) that constantly move air throughout the atmosphere http://minerva.union.edu/failinge/earths_convection_cells.html

Because the Coriolis Effect also moves air masses Clockwise in the N Hemisphere (and CCW in the S Hem.), ... Winds move generally Westward off of South America’s W. Coast Now back to El Nino...

Normal conditions in Pacific Trade winds blow off the west coast of S. America (movement from high pressure to low pressure) The movement of warm water into western Pacific forces cold deep water towards the eastern Pacific Causes area of upwelling on South American coast. Water is cold and nutrient-rich; supports strong fishing industry by providing nutrients for the basis of food chains

What do normal Pacific conditions look like?

El Nino Trade winds that move warm water to western Pacific lessen, allowing warm water to move towards eastern Pacific

Effects of El Nino Coastal upwelling halts in eastern Pacific ocean More rainfall with movement of warm water mass to eastern Pacific Droughts in western Pacific

La Nina Also known as the “anti-El Nino” or “El Viejo” Cold phase of the climate pattern Higher pressure and trade winds increase towards the western Pacific Causes more upwelling in eastern Pacific and more rain in western

Visual comparison between El Nino and La Nina

Duration and impacts of El nino Occurs every 2-7 years and lasts 12 to 18 months Triggers weather extremes worldwide High sea surface temperatures trigger thunderstorms/hurricanes Coastal fisheries in S. America are devastated without cold upwelling of nutrients Flooding in typically dry areas ex. S. California Drought and fire in typically humid areas ex. Indonesia