El Niño is a disruption of the ocean-atmosphere system in the tropical Pacific with important consequences for weather around the globe. During "normal", non-El Niño conditions, the trade winds blow toward the west across the tropical Pacific. These westward-blowing winds pile up warm surface water in the western Pacific, so that the sea surface is higher and sea-surface temperatures warmer near Indonesia (western Pacific) than near Ecuador (eastern Pacific). Cool sea-surface temperatures prevail off South America (eastern Pacific), both because the warm surface waters are moved toward the west and because divergence of surface waters causes an upwelling of cold water from deeper levels (see Part I: upwelling currents). This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Rainfall is abundant where air rises strongly over the warmest water in the western Pacific, and the eastern Pacific is relatively dry.
During El Niño, trade winds relax in the central and western Pacific, causing the depth of the thermocline to lower in the eastern Pacific, and to rise in the western Pacific. This wind relaxation reduces the efficiency of upwelling currents in the eastern Pacific and decreases the supply of nutrient-rich thermocline water to the surface zone. The result is a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affects higher levels of the food chain, including commercial fisheries in the region.
NOTE: Remember that, from an oceanic point of view, the western Pacific refers to the area along the eastern coast of Asia, Indonesia, and Australia, and that the eastern Pacific refers to the area along the western coast of North and South America. To provide further confusion, the International Date Line runs through the middle of the Pacific Ocean, and west longitudes occur in the eastern part of the Pacific, while east longitudes occur in the western part of the Pacific. Refer to a world map to clarify these relationships before proceeding further.
Briefly contrast the following ocean characteristics during normal and El Niño conditions and on the eastern and western sides of the ocean:
i. sea-surface temperatures (SSTs)
ii. winds
iii. areas of rising air
iv. thermocline
The first diagram shows a map view of normal conditions at the top and El Niño conditions on the bottom. Red is warm water, pink arrows are winds with longer arrows indicating stronger winds, and white lines indicate areas of clouds and rising air.
The second diagram shows a profile view of the two climatic situations
along the equator.
Red indicates warm water and blue indicates cold water. Relative wind strength
and areas of clouds and heavy rain are also shown.
(c.) The full name of the tropical disturbance is ENSO: El Niño Southern Oscillation. El Niño refers specifically to the warmer sea-surface temperatures in the eastern Pacific that were first observed and named by fisherman in coastal Peru. The Southern Oscillation refers to observed changes in sea-level pressure across the tropical Pacific that correlate to changes in wind and sea-surface temperatures.
The diagram below shows observations from 1970 to 1988 at a location in the east-central Pacific. Yellow highlights indicate years when El Niño conditions were observed.
What happens to sea-level pressure in the eastern Pacific during El Niño years? (The converse prevails in the western Pacific.)
