Friday, June 10, 2016

How El Niño Affects Global Temperatures

El Niño was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific Ocean, occurring near the beginning of the year. El Niño means The Little Boy or Christ child in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas.

Scientists have found past El Niño oscillations in the Pacific Ocean may have amplified global climate fluctuations for hundreds of years at a time.

The team uncovered century-scale patterns in Pacific rainfall and temperature, and linked them with global climate changes in the past 2000 years.

For example, northern hemisphere warming and droughts between the years 950 and 1250 corresponded to an El Niño-like state in the Pacific, which switched to a La Niña-like pattern during a cold period between 1350 and 1900.

Wahyoe Hantoro in Liang Luar Cave, Flores.

Credit: Garry K. Smith

The new data will help scientists build more accurate models of future climate, said member of the research team, Alena Kimbrough, from The Australian National University.

"Our work is a significant piece in the grand puzzle. The tropics are a complicated, yet incredibly important region to global climate and it's been great to untangle what's happening," said Ms Kimbrough, a PhD student at the ANU Research School of Earth Sciences.

"The current models struggle to reflect century-scale changes in the El Niño Southern Oscillation (ENSO).

"We've shown ENSO is an important part of the climate system that has influenced global temperatures and rainfall over the past millennium."

The team measured trace elements and stable isotopes in stalagmites from the Indonesian island of Flores to reconstruct ancient rainfall, and compared it with records from East Asia and the central-eastern equatorial Pacific.

The El Niño Southern Oscillation is an irregular variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean. In one extreme it brings high temperatures and drought to eastern Australia and Indonesia, and the opposite extreme, known as La Niña, heavy rainfall and storms.

"In the past decade or so the rise in global temperature had a brief reprieve, the so-called warming hiatus, which can be partly attributed to a persistent La Niña pattern over that period," Ms Kimbrough said.

The new work found periods of predominantly El Niño-like patterns for several hundred years that alternate with La Niña patterns, impacting on global climate over the last 2000 years.

"Until we can model this lower-frequency behaviour in the tropical Pacific, one can only speculate on how the warming will play out over the next few decades," said lead author Dr Michael Griffiths from William Paterson University, in the United States.

The international team of scientists was led by Dr Michael Griffiths of William Patterson University in New Jersey, along with PhD candidate Alena Kimbrough and Dr Michael Gagan at the ANU, Professor Wahyoe Hantoro of the Indonesian Institute of Sciences and colleagues at the University of Melbourne and the University of Arizona.

The research is published in Nature Communications.

El Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which is characterized by unusually cold ocean temperatures in the Equatorial Pacific. El Niño is an oscillation of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe.

Among these consequences are increased rainfall across the southern tier of the US and in Peru, which has caused destructive flooding, and drought in the West Pacific, sometimes associated with devastating brush fires in Australia. Observations of conditions in the tropical Pacific are considered essential for the prediction of short term (a few months to 1 year) climate variations.

In normal, non-El Niño conditions (top diagram) the trade winds blow to the west along the equator from South America towards Asia in the tropical Pacific Ocean. These winds pile up warm surface water off Asia, so that the sea surface is about 1/2 meter (1 1/2 feet) higher at Indonesia than at Ecuador in South America.
Credit: NOAA

During El Niño (bottom schematic diagram), the trade winds relax in the central and western Pacific leading to a flattening of the thermocline (blue band) due to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. The observations at 110ºW show, for example, that during 1982-1983, the 17-degree isotherm dropped to about 150m depth. This reduced the efficiency of upwelling to cool the surface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region.

Contacts and sources:
Alena Kimbrough
The Australian National University.

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