BOX 3-1
The El Niño–Southern Oscillation Phenomenon

The El Niño–Southern Oscillation (ENSO) phenomenon is a multi-year cycle marked by changes in relative atmospheric pressures at sea level across the tropical Pacific Ocean and changes in the strength of the Pacific trade winds and the temperature of the ocean’s surface in the central and eastern Pacific. This mode of climate fluctuation has been linked globally to devastating droughts, extreme rainfall events, and the suppression of hurricanes in the Atlantic Ocean, among other interannual climatic changes, and therefore has a significant impact on global society. Recent studies have also linked ENSO to the natural variability of the carbon cycle (Jones et al., 2001). An El Niño event (the warm phase of the cycle, as indicated by sea surface temperature) is characterized by a weakening of the Pacific trade winds, a warming of the ocean’s surface in the central and eastern Pacific, and smaller differences in tropical sea level pressures between the eastern and western tropical Pacific Ocean. Opposite conditions occur in the cold phase, or La Niña. The warm events often last 12 to 18 months and historically occur every 2 to 7 years, although recent anthropogenic climate changes may possibly contribute to more frequent and intense El Niño events. The term ENSO is used to describe the full range of coupled ocean–atmosphere climate variability in the tropical Pacific Ocean.

Over the course of many decades of research, great strides have been made in understanding, monitoring, and predicting ENSO and its effects on climate. During an El Niño event, the warmest waters of the ocean, usually west of the international dateline, spread eastward into the eastern tropical Pacific, a distance one-third the circumference of the planet, and significantly affect the global atmospheric circulation. Via a process known as teleconnection, ENSO is often linked to global extreme precipitation events, either drought or flooding, in many far-flung places. ENSO also appears to play a large role in yearly and multi-year variations in sea level rise, while larger ENSO events can affect decadal trends in sea level by shifting the distribution of rainfall between land and the ocean. ENSO also affects marine and terrestrial ecosystems. During a warm ENSO phase the primary productivity in the eastern tropical Pacific is severely reduced because of weaker upwelling off the coast of Peru. The reduction in productivity affects the population and location of marine mammals, sea birds, and commercial fishing. Year-to-year variability in global atmospheric carbon concentrations is dominated by the ENSO cycle (Rayner et al., 1999). During El Niño, equatorial upwelling decreases in the eastern and central Pacific, significantly reducing the supply of carbon dioxide to the surface (Feely et al., 2006). As a result, the global increase in atmospheric carbon dioxide noticeably slows down during the early stages of an El Niño. Coastal regions are particularly affected by ENSO variability.

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