El Nino and La Nina

By | Updated Sep 9, 2016 3:10 PM UTC

Here’s what can happen when the surface of the equatorial Pacific gets just a little warmer: Thousands of people die as the weather changes from India to Florida. Some economies lose billions of dollars; others enjoy respite from weather-related losses. Prices of commodities ranging from nickel to coffee jolt skyward. Then when the waters cool, patterns shift, with areas previously spared often experiencing calamitous hurricanes, floods or drought, and others getting a break from such buffeting forces. The whole cycle is known as El Nino-Southern Oscillation. It is made up of El Nino, the Pacific's warm phase; La Nina, the cold side; and a neutral phase in between. The whole thing tends to play itself out every two to seven years.

The Situation

The El Nino that the U.S. declared in March 2015 was pronounced over in June 2016. In September, U.S. authorities said the odds of a La Nina forming had declined, while Japanese officials announced that one was underway. The two countries' weather agencies use different criteria and look at different areas of the Pacific to make their determinations. The latest El Nino was the strongest since the record event of 1997-98. It has been blamed for making 2015 the hottest year on record, and officials expect another record this year. El Nino reduced rainfall in the Indian monsoon, parching farmlands, and curbed production of cocoa in Ivory Coast, rice in Thailand and coffee in Indonesia. A La Nina could ameliorate that dryness. Among the changes it can bring are colder winters in the northern U.S. and more rain across Australia. 

Sources: National Oceanic and Atmospheric Administration, ESRI

The Background

Peruvian fishermen named El Nino for the Christ child in the 1600s when they noticed the tropical Pacific warming around Christmas some years. El Ninos occur when, for reasons unknown, there’s a weakening in the trade winds that push the sun-warmed waters of the equatorial Pacific into a mound in the west. Some of that water flows back east, making the eastern Pacific hotter. The phenomenon affects larger wind currents, shifting moisture-bearing storms away from some places, such as Indonesia and Africa, and toward others, including Argentina and Japan. When the heat from El Nino is spent, the ocean begins to cool. Initially, the Pacific falls into a state between the extremes called its neutral phase. If the cooling continues and sea surface temperatures fall below normal, La Nina occurs. The term gained prominence only in the 1980s. When there's been a strong El Nino, the Pacific often snaps into La Nina after a short neutral phase. The Atlantic and Indian oceans have similar events but theirs don’t have the far-reaching impact of those in the immense Pacific. By slightly slowing the earth’s rotation, a strong El Nino can increase the length of a day by about a millisecond. The 1997-98 El Nino is blamed for an estimated 23,000 deaths and $33 billion in property damage. Researchers argued in a 2011 paper that the El Nino-Southern Oscillation cycle may have played a role in a fifth of all civil conflicts from 1950 to 2004.

Sources: National Oceanic and Atmospheric Administration, ESRI

The Argument

Scientists have debated since the late 1990s whether global warming is affecting the frequency or intensity of El Ninos, but there is still no consensus. In 2013, researchers published evidence based on coral skeletons that El Ninos had become stronger and tentatively concluded that climate change played a role. Later that year, a study predicted a doubling of extreme El Ninos in response to global warming. U.S. government meteorologist Tom Di Liberto recently noted that there was evidence to support the idea that El Ninos would grow more common and stronger as well as to support the opposite view. There are so many variables influencing El Nino and its ramifications, he said, isolating the role of global warming may not be possible. If El Ninos do become stronger, the impact on humans could be mitigated by improvements in weather forecasting and better disaster preparations.

The Reference Shelf

  • Animations produced by the U.S. National Center for Atmospheric Research show changes in sea-surface temperatures for the current and 1997-98 historic El Ninos.
  • The U.S. and Japanese El Nino forecast pages.
  • University Corporation for Atmospheric Research’s guide to the El Nino-Southern Oscillation cycle.
  • J. Madeleine Nash’s book “El Nino: Unlocking the Secrets of the Master Weather-Maker.”

First published July 22, 2015

To contact the writer of this QuickTake:
Brian K Sullivan in Boston at bsullivan10@bloomberg.net

To contact the editor responsible for this QuickTake:
Lisa Beyer at lbeyer3@bloomberg.net