How can temperatures get cooler during global warming ?

When speaking of climate change, is often brought up the subject of the planet’s warming and the rise of global temperatures. However, some years are colder than others and global temperatures don’t go up regularly year by year.
To avoid confusions and misunderstanding, the term “global warming” has been switched to “climate change”. Indeed, for several reasons “climate change” does not necessarily and only mean fast rising temperatures all over the planet. One explanation to this is the existence of the natural climate phenomenon El Niño and La Niña.

What are these phenomena ?

El Niño and La Niña refer to opposite phases of the ENSO (El Niño-Southern Oscillation) cycle. They have large-scale impact on the weather and climate all over the globe.

Australian Government – Bureau of Meteorology : Pacific Ocean (1)

El Niño can be described as the periodic warming phase of sea surface temperatures across the central and east-central Equatorial Pacific.
It comes with high air surface pressure in the tropical western Pacific. 
El Niño generally brings milder weather to Northern U.S. areas and to western and central Canada. Wetter conditions are also expected in the South of North America. This warm phase also affects Australia and southeast Asia with drier conditions, and Pacific coastal South America with wetter conditions. 

On the contrary, La Niña refers to the cooling phase of the ENSO cycle, accompanied by low air surface pressure, and results in below-average sea surface temperatures across the east-central Equatorial Pacific.
This phase tends to have the opposite effects of the El Niño phase. 
Indeed, winter temperatures are generally warmer than average in the Southeast and cooler in the Northwest of the United States. Rainstorms and warm waters are driven towards the western equatorial Pacific over Indonesia.

These two periods last typically 9 to 12 months each and normally occur every 3 to 7 years. (2) It is also important to note that El Niño occurs more frequently than La Niña. (3)

Global Annual Temperature Anomalies showing El Niño and La Niña years, by the NOOA. (4)

A little history
El Niño means “The Little Boy” in Spanish. The original name, “El Niño de Navidad” in reference to the newborn christ, traces its origin centuries back to Peruvian fishermen. These used the term to describe the appearance of a warm ocean current off the South American coast around Christmas.
La Niña, chosen as the « opposite » of El Niño, refers to « The Little Girl ». (1)

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References

http://www.bom.gov.au/climate/enso/history/ln-2010-12/ENSO-what.shtml (1)

https://www.americangeosciences.org/critical-issues/faq/what-are-el-nino-and-la-nina (2)

https://oceanservice.noaa.gov/facts/ninonina.html (3)

https://www.ncdc.noaa.gov/sotc/global/201213 (4)

https://en.wikipedia.org/wiki/El_Niño–Southern_Oscillation


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