El Nino Southern Oscillation
During normal, or non-El Niño conditions, the trade winds blow west across the Pacific. These winds pile up warm surface water in the west Pacific so that the sea surface is about one-half meter higher around Indonesia than around Ecuador. Ocean upwelling occurs off the coasts of Peru and Ecuador bringing nutrient-rich cold water to the surface and increasing fishing stocks. The western side of the equatorial Pacific is characterized by warm, wet, low-pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms.
During an ENSO event, there is a rise in air pressure over the Indian Ocean, Indonesia, and Australia, and a fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean. Trade winds in the south Pacific weaken or head east, and warm water spreads eastward from the west Pacific and Indian Ocean to the east Pacific (animations and graphics of El Niño). This leads to extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific.
While El Niño is characterized by unusually warm ocean temperatures in the central to eastern equatorial Pacific, La Niña is characterized by unusually cold ocean temperatures in this region, but warm waters in the western Pacific. In most years, the warming lasts only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Niño conditions last for many months, more extensive ocean warming occurs and its economic impact on local fishing for an international market can be serious.
This diagram shows a model of surface temperatures, winds, areas of rising air, and the thermocline (blue surface) in the tropical Pacific during El Niño, normal, and La Niña conditions. Click to enlarge. Source: NOAA/PMEL/TAO Project Office, Dr. Michael J. McPhaden, Director
Projections of ENSO
ENSO events are a natural process and have been present for thousands if not millions of years. ENSO events are not caused by climate change, they are caused by the interaction between the surface layers of the ocean and the overlying atmosphere in the tropical Pacific. However, it is certainly possible that global warming will change the way the El Niño cycle behaves.
Since the mid-1970s, there have been more frequent and persistent El Niño episodes than La Niña episodes. Changes in precipitation over the tropical Pacific are related to this change in the ENSO, which has also affected the pattern and magnitude of sea-surface temperatures. However, it is unclear as to whether this change in the ENSO cycle is due to normal variation or is related to global warming.
Although some scientists hypothesize that warmer global sea surface temperatures might lead to an increase in El Niño events, whether the occurrence of El Niño changes with climate change is still an active area of research. Studies of historical data suggest that recent El Niño variation is most likely linked to global warming. ref In contrast, a more recent study suggests that climate change is not expected to affect the extent or frequency of ENSO over the 21st century, but could worsen its impacts. ref
Predicting ENSO Events
Scientists are unsure what changes will happen to ENSO in the future, and climate modelers make different projections. ref More frequent and stronger El Niño events may occur only in the initial phases of global warming and then such events may become weaker. Or El Niño events may continue to strengthen and increase into the future. Due to the major impact on climate patterns caused by El Niño and La Niña, it is critical to be able to predict when these events will occur.
Many tools are available for monitoring, research, and forecasting ENSO events, including satellite and in-ocean observations that provide near real-time data on surface winds, ocean temperatures, currents, and other parameters. Currently, seasonal predictions are generally accurate on average, but individual events are challenging to predict. Experts suggest that El Niño or La Niña predictions that are greater than 9 months into the future may not be accurate. Better predictions can be made by examining several models rather than just one model (see predictions of El Niño and La Niña).
Impacts on Coral Reef Ecosystems
El Niño and La Niña can both have severe impacts on coral reef ecosystems, and particularly on coral reefs. Globally, ENSO generated massive bleaching and coral mortality during 1982-1983, ref 1997-1998, ref 2002-2003, ref 2005, ref and 2010, and contributed to the likely extinction of a coral species. ref
Global observations of coral bleaching occurrences over the last 50 years (as of May 2015). Click to enlarge. Source: Reefbase
El Niño Events
- Mass bleaching observed in Panama ref
- Warm sea surface temperatures (SSTs) associated with the El Niño event were identified as the cause of death for over 50% of corals in Panama and over 99% of the corals in the Galapagos ref
- Resulted in unprecedented coral bleaching and coral death globally ref
- About 70-80% of all shallow water corals were killed on many Indo-Pacific reefs and the Great Barrier Reef
- Reefs in the Florida Keys experienced mild to severe bleaching ref
- Much of the bleaching coincided with a large El Niño event, immediately switching over to a strong La Niña
- High ocean temperatures in the tropical Atlantic and Caribbean resulted in one of the worst bleaching events ever recorded in the region
- Thermal stress during the 2005 event exceeded any observed from the Caribbean in the prior 20 years, and regionally-averaged temperatures were the warmest in over 150 years ref
- One of the worst years for coral bleaching since the 1997-1998 El Niño event
- Resulted in extensive bleaching and mortality across the Indian Ocean and Southeast Asia (bleaching extended from the Seychelles in the west to Sulawesi and the Philippines in the east and included reefs in Sri Lanka, Burma, Thailand, Malaysia, Singapore, and many sites in western and eastern Indonesia) and also severe bleaching occurred in parts of the Caribbean (e.g., Venezuela and Panama)
While there are correlations between the widespread coral bleaching in 1997-1998 and the ENSO system, the patterns are unclear. For example, during the 1997-1998 event, observed bleaching in the eastern Pacific correlated with the El Niño event. By contrast, the bleaching in Southeast Asia coincided with the subsequent 1998-1999 strong La Niña, which brought warm waters to the western Pacific. The Indian Ocean bleaching at that time corresponded to warming during the El Niño, while bleaching in parts of the Caribbean followed a typical pattern of bleaching in the summer following the El Niño. Large-scale bleaching events, however, do not necessarily occur in conjunction with major El Niño or La Niña events. The largest bleaching event recorded in the Caribbean occurred in 2005, following a mild El Niño, and was poorly connected to El Niño climate patterns.ref Despite the limitations in knowledge of how El Niño and La Niña affect coral bleaching events, scientists are concerned that increases in SSTs globally, and potential increases in El Niño events, threaten the survival of coral reefs. In October 2015, NOAA declared the third (and worst) global bleaching event in history was underway due to a combo of a strong El Nino and global warming. Since the El Niño began in 2014, bleaching has been documented in the Pacific Ocean, Indian Ocean, Caribbean, Australia, Hawaii, and Florida Keys. Scientists say 93% of the Great Barrier Reef is now bleached (as of April 2016).