Context 

• It is very likely that El Niño Southern Oscillation rainfall variability, used for defining extreme El Niños and La Niñas, will increase significantly by the second half of the 21st century

ENSO cycle

• The ENSO cycle is a scientific term that describes the fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific.
• La Niña is sometimes referred to as the cold phase of ENSO and El Niño as the warm phase of ENSO. These deviations from normal surface temperatures can have large-scale impacts not only on ocean processes, but also on global weather and climate.

What is El Nino?

• El Nino is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean.
• During El Niño, the surface winds across the entire tropical Pacific are weaker than usual. Ocean temperatures in the central and eastern tropical Pacific Ocean are warmer than average, and rainfall is below average over Indonesia and above average over the central or eastern Pacific.
• Rising air motion (which is linked to storms and rainfall) increases over the central or eastern Pacific, and surface pressure there tends to be lower than average.  Meanwhile, an increase in sinking air motion over Indonesia leads to higher surface pressure and dryness.  
• Since the Pacific covers almost one-third of the earth, changes in its temperature and subsequent alteration of wind patterns disrupt global weather patterns.
• El Niño causes dry, warm winters in Northern U.S. and Canada and increased flooding risk on the U.S. gulf coast and southeastern U.S. It also brings drought to Indonesia and Australia.

La Nina

• La Nina is the “cool phase” of ENSO, a pattern that describes the unusual cooling of the tropical eastern Pacific.
• The surface winds across the entire tropical Pacific are stronger than usual, and most of the tropical Pacific Ocean is cooler than average. Rainfall increases over Indonesia (where waters remain warm) and decreases over the central tropical Pacific (which is cool). 
• Over Indonesia, there is more rising air motion and lower surface pressure whereas there is more sinking air motion over the cooler waters of the central and eastern Pacific.
• La Nina has also been associated with heavy floods in Australia. Two successive La Niña events in the last two years caused intense flooding in Australia, resulting in significant damage.

Duration and frequency

• Episodes of El Nino and La Nina typically last nine to 12 months, but can sometimes last for years. 
• While their frequency can be quite irregular, El Nino and La Nina events occur every two to seven years, on average.
• Generally, El Nino occurs more frequently than La Nina.

Key Findings

• The IPCC Sixth Assessment Report indicates that “it is very likely that ENSO rainfall variability, used for defining extreme El Niños and La Niñas, will increase significantly.
• This would be regardless of amplitude changes in ENSO SST (sea surface temperature) variability, by the second half of the 21st century” in most emission scenarios.
• Rainfall variability over the east-central tropical Pacific will increase significantly. Therefore, we might expect El Niño in the future to be wetter in this region and La Niña to be drier.

Conclusion

• Although we have a good understanding of past El Niño interconnection and precipitation impacts, the complexity of interactions between ENSO and climate change require more climate research and investigations.