El Niño/La Niña

Upwelling Upward movement of water which brings nutrients from deep in the ocean to the surface. (Nutrient-rich water!) This stimulates the growth of marine plants (phytoplankton), which supports fisheries (high biological productivity). This is positive along the whole food chain.

Equatorial Upwelling Tradewinds push water away from the equator. Water from the deep ocean comes up to take the place of the missing water.

A normal (non-El Niño) Year Trade winds blow from the normally high-pressure area over the eastern Pacific (near South America) to the normally stable low-pressure area over the western Pacific (north of Australia) Upwelling happens along the coast of South America

A normal (non-El Niño) Year Also look at figure 9.19 a A normal (non-El Niño) Year

A normal (non-El Niño) Year sea surface is about 1/2 meter higher in the west than in the east sea surface temperature is about 8° C higher in the west, with cool temperatures off South America Rainfall is found in rising air over the warmest water (western Pacific), and the east Pacific is relatively dry Strong upwelling = high primary productivity

A normal (non-El Niño) Year

An El Niño Year – Southern Oscillation Every 3 to 8 years the pressure areas change places (high pressure builds in western Pacific and low pressure dominates the eastern Pacific) The trade winds in the central and western Pacific diminish or reverse This change in atmospheric pressure (and thus wind direction) is called the Southern Oscillation

An El Niño Year Equatorial currents weaken; warm water usually found in western Pacific drifts eastward (rise in sea surface temperature) reduces the efficiency of upwelling and cuts off the supply of nutrient rich bottom water to the surface drastic decline in primary productivity (bad for whole food chain)

figure 9.19 c An El Niño Year

An El Niño Year Sea level rises in eastern Pacific as much as 8” Water temp in eastern Pacific rises up to 7°C increased rainfall across the southern tier of the US and in Peru, which has caused destructive flooding drought in the West Pacific, sometimes associated with devastating brush fires in Australia. Little to no upwelling = low primary productivity

An El Niño Year

ENSO The phenomena of the Southern Oscillation and El Niño are related, so the terms are often combined to form the acronym ENSO (El Niño / Southern Oscillation) Event usually lasts about a year, but some have lasted much longer

La Niña Normal circulation sometimes returns with surprising vigor Strong tradewinds strong surface currents powerful upwelling chilly/stormy conditions along the South American coast.