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El Nino – Southern Oscillation (ENSO)

The El Nino – Southern Oscillation (ENSO or just El Nino for short) is one of the most famous climate indices and also one of the oldest in terms of our understanding. In fact, the first recorded history of ENSO dates back to the 1500s and though humans did not know the cause of the odd weather events at the time, it is now clear that people then were experiencing El Nino related monsoon flooding. The modern understanding of this climate index dates to the early 1960s.

The easiest way to define ENSO is as an abnormal warming of surface water (the El-Nino part of the term) in the eastern part of the Pacific Ocean (that would be the west coast of South America) combined with a change in air pressure (the Southern Oscillation part of the term) over the western Pacific. Though they don’t have to be, the warming and pressure changes are usually simultaneous and so the entire event has been dubbed the El Nino-Southern Oscillation or ENSO. The best way to understand just what ENSO is to look at a diagram.


Yellow arrows indicate ocean currents while blue indicate wind movement.

There are two things that happen, from a physical standpoint, when an ENSO event occurs. First, the normal upheaval of cool ocean water that occurs near the west coast of South America stops. This allows warm surface water from the central part of the Pacific to move closer to the coast of South America. This is the El Nino event. Second, the prevailing winds change due to atmosphere pressure anomalies. The result is that rainfall moves from Australia and Indonesia toward South America.  The opposite of these conditions is sometimes called La Nina, which is characterized by cooler than average ocean temperatures near South America. El Nino and La Nina events are part of the ENSO climate index.

Unfortunately, no one knows why these changes take place or why they take place when they do. Significant ENSO events occurred in 1957, 1965, 1972, 1976, 1982, 1983, 1993, 1997 and 2003. ENSO events have an impact on global weather and wildlife.

ENSO Impacts on Wildlife

Before discussing the impact of ENSO on weather, it is worth noting that the phenomenon has a direct impact on wildlife. The cool ocean currents that are normal along the west coast of South America bring nutrients from the floor of the sea to the surface. These nutrients feed plankton (small plant-like organisms that live in the ocean), which in turn feed fish, which in turn feed most of the rest of the ocean wildlife as well as a number of birds. Without the upwelling of nutrients, fish populations decline and so do populations of other ocean and avian species. In essence, ENSO events can be directly related to declines in the populations of a number of ocean animals because they affect the very basis of the ocean food chain.

The significance of ENSO events on ocean life should not be underestimated. The fluctuation reveals a critical lynchpin in the largest ecosystem in the world. Changes of less than half of a degree Celsius are enough to create an ENSO event and disrupt the nutrient supply off of the coast of South America. If these events were to continue for a prolonged period (such as some people predict will result from excessive global warming) there could be a massive kill off in the ocean and a resultant severe impact on food chains not only in the ocean, but on land as well.

ENSO Impacts on Weather

The most obvious effect of an ENSO event is on precipitation. For areas that are normally wet, conditions become drier, and vice versa. Impact tends to be more prominent during winter months and varies with the strength of the ENSO event. Strong events can lead to severe flooding in the wettest areas as well as drought in the driest areas. This can have tremendous impact on agriculture. (Note Winter/Summer are defined for the Northern Hemisphere – also known as boreal winter).

El Nino

  • Africa
    • Winter – Modest impact with drier conditions in central Africa and Madagascar
    • Summer – No impact
  • Asia
    • Winter – Very warm conditions in Japan with somewhat drier conditions in Southeast Asia
    • Summer – Dry conditions in India. Limited impact elsewhere
  • Australia/Indonesia
    • Winter – Much Driver conditions in most of Indonesia, extending to Hawaii. Pocket of increased precipitation south of Hawaii.
    • Summer – Pocket of increased precipitation remains south of Hawaii. Dry conditions spread into continental Australia.
  • North America
    • Winter – Much warmer conditions in Alaska, western Canada, and Northwest United States as well as Northeastern United States.
    • Summer – Very limited impact.
  • South America
    • Winter – Wet conditions on west coast with dry conditions on east coast. Flooding a serious threat.
    • Summer – Dry conditions in northern SouthAmerica, Central America, and the southern parts of the Caribbean.


Image from the National Oceanographic and Atmospheric Association (NOAA) in the United States.

La Nina

  • Africa
    • Winter – Cool along the west coast with increased rainfall in the southeast
    • Summer – Cool along the west coast
  • Asia
    • Winter – Cool conditions in Japan, Korea, and eastern China. Drier conditions in Southeast China.
    • Summer – Wet and Cool in India with cool conditions for most Southeast Asia
  • Australia/Indonesia
    • Winter – Very wet conditions in Indonesia and northern Australia. Dry conditions across the equatorial Pacific Ocean.
    • Summer- Wet and cool in Indonesia. West in southern Australia/Tasmania along with warm conditions for most of the eastern coast of Australia. Dry along the equatorial Pacific Ocean.
  • North America
    • Winter – Cool in western Canada into Alaska with increased precipitation in western Canada and the continental U.S. Wet in the northeast U.S. with dry, warm conditions across much of the southern U.S. and Mexico.
    • Summer – Little effect
  • South America
    • Winter – Dry conditions along the western coast. Wet conditions in the northeast and cool conditions in the southeast.
    • Summer – Very cool along the west coast. Dry in the southeast. Wet, cool conditions in north, extending into Central America and the southern Caribbean.


Image from the National Oceanographic and Atmospheric Association (NOAA) in the United States.