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El Niño Southern Oscillation

The most well known global weather pattern is the periodic El Niño Southern Oscillation (ENSO). The ENSO has an oceanic (El Niño/La Niña) and atmospheric (Southern Oscillation) component.

El Niño events are the result of sustained rises in sea surface temperatures (SSTs) in the Pacific. The Pacific basin drives global precipitation patterns.

During El Niño events, rainfall that normally falls as monsoons in India and Indonesia now falls in the Americas. Sub-surface waves carry these warmer surface waters towards the Eastern Pacific where they overwhelm the nutrient-rich Humboldt Current and prevent cooler, nutrient-rich, waters from rising to the surface off the western coast of South America. As a result, primary productivity (boxed text) is seriously reduced in this region during El Niño years. A direct effect of this is the crashing of fish stocks.

La Niña events occur when SSTs drop below average for a significant period. La Niña events produce broadly opposite effects to El Niño events.

The Southern Oscillation is driven by changes in SSTs, which cause changes in atmospheric pressure. Typically, as warm air rises above heated water, surface areas of low pressure are created (air masses, over cooler waters, descend and increase surface pressures). SSTs of the Eastern and Western Pacific vary during a Southern Oscillation, impacting the direction and strength of trade winds and the location and intensity of rainfall. Rising SSTs clearly have the potential to significantly affect ‘global’ weather patterns.

Increasing SSTs also have ‘local’ weather effects. One of these effects is the intensification of open-water storm systems that are themselves the result of rising warm air over warm seas. It is predicted for example that the numbers, durations and intensities of hurricanes in the North Atlantic are likely to increase as a result of increased SSTs. Storm surge and waves that result from open-water storm systems will increase in intensity thereby threatening fragile coral reefs and coastal communities. The most intense ENSO events in recent years were recorded between 1957-58, 1972-73, 1982-83, and 1997-98. Of these, the 1997-98 ENSO event was the most severe, resulting in widespread damage to coral reef ecosystems worldwide.

• El Niño Southern Oscillation
• Global Impact of Carbon Dioxide
• Ocean Acidification


Primary Productivity
At the base of the marine food web are the photosynthetic organisms known as phytoplankton. These organisms use sunlight to drive the incorporation of dissolved nutrients in the ocean into complex, long- chained organic molecules. These long-chained molecules are energy-rich and are used by the phytoplankton to carry out many biological processes. This production of complex organic molecules by phytoplankton is known as ‘primary productivity’. They in turn are consumed by organisms higher up the food chain that need these energy- rich molecules. This process continues to the top of the food tree. These complex organic molecules, produced by countless single-celled phytoplankton, therefore underpin the entire marine food web.

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