The multivariate ENSO index, abbreviated as MEI, is a method used to characterize the intensity of an

El Niño Southern Oscillation EL, El or el may refer to: Religion * El (deity), a Semitic word for "God" People * EL (rapper) (born 1983), stage name of Elorm Adablah, a Ghanaian rapper and sound engineer * El DeBarge, music artist * El Franco Lee (1949–2016), American ...

(ENSO) event. Given that ENSO arises from a complex interaction of a variety of climate systems, MEI is regarded as the most comprehensive index for monitoring ENSO since it combines analysis of multiple meteorological and

oceanographic Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamics ...

components.Mazzarella, A., A. Giuliacci, and N. Scafetta
"Quantifying the Multivariate ENSO Index (MEI) Coupling to CO2 Concentration and to the Length of Day Variations."
Theoretical and Applied Climatology 111.3 (2013): 601-7.

Overview

MEI is determined as the first principal component of six different parameters:

sea level pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars ...

, zonal and meridional components of the

surface wind A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...

sea surface temperature Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air ma ...

surface air temperature Temperature measurement (also known as thermometry) describes the process of measuring a current local temperature for immediate or later evaluation. Datasets consisting of repeated standardized measurements can be used to assess temperature tren ...

and

cloudiness Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud c ...

using data from the

International Comprehensive Ocean-Atmosphere Data Set The International Comprehensive Ocean-Atmosphere Data Set (ICOADS) is a digital database of 261 million weather observations made by ships, weather ships, and weather buoys spanning the years 1662 to 2007. The database was initially constru ...

(ICOADS ).Wolter, Klaus
"Multivariate ENSO Index (MEI)."
NOAA Earth System Research Laboratory - Physical Sciences Division. National Atmospheric and Oceanic Administration, n.d. Web. 24 Oct. 2013. MEI is calculated twelve times per year for each “sliding bi-monthly season”, characterized as January–February, February–March, March–April, and so on. Large positive MEI values indicate the occurrence of

El Niño El Niño (; ; ) is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (approximately between the International Date ...

conditions, while large negative MEI values indicate the occurrence of

La Niña La Niña (; ) is an oceanic and atmospheric phenomenon that is the colder counterpart of as part of the broader El Niño–Southern Oscillation (ENSO) climate pattern. The name ''La Niña'' originates from Spanish for "the girl", by ...

conditions.

Extended MEI

While the

National Oceanic and Atmospheric Administration The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditi ...

(NOAA) has recorded MEI values from 1950 to the present, various researchers have cited the need for data before 1950 in order to better characterize typical ENSO behavior versus unusual occurrences that may be a result of climate change. According to some sources, it is inadvisable to attempt to calculate MEI before 1950 given that environmental measurements were unreliable during the World Wars and there was a revolution in virtually all meteorological measurement methods on ships during the 1940s.Wolter, Klaus, and Michael S. Timlin
"Measuring the Strength of ENSO Events: How Does 1997/98 Rank?"
Weather 53.9 (1998): 315-24. However, a module known as “Extended MEI” or “MEI.ext” has been created that estimates MEI values from as far back as 1871.Wolter, Klaus, and Michael S. Timlin
"El Niño/Southern Oscillation Behaviour since 1871 as Diagnosed in an Extended Multivariate ENSO Index (MEI.ext)."
International Journal of Climatology 31.7 (2011): 1074-87. This was accomplished by using reconstructed data on sea level pressure and sea surface temperature, the two components thought to be most influential to determining MEI. Plots comparing MEI and MEI.ext values have shown that data from both methods are highly correlated, supporting the accuracy and effectiveness of MEI.ext.

Alternate indexes

Southern Oscillation Index

The Southern Oscillation Index (SOI) is calculated based on the sea level pressure difference between

Tahiti Tahiti (; Tahitian ; ; previously also known as Otaheite) is the largest island of the Windward group of the Society Islands in French Polynesia. It is located in the central part of the Pacific Ocean and the nearest major landmass is Aust ...

and

Darwin, Australia Darwin ( ; Larrakia: ) is the capital city of the Northern Territory, Australia. With an estimated population of 147,255 as of 2019, the city contains the majority of the residents of the sparsely populated Northern Territory. It is the smalle ...

. Despite being used frequently in ENSO studies, it is not considered as reliable as MEI given that it only takes into account one environmental variable.

Niño 3.4 SST

Similar to SOI, Niño 3.4 SST uses one parameter – sea surface temperature – to characterize ENSO. The Niño 3.4 SST region (overlapping regions 3 and 4, see map) consists of temperature measurements from between 5° N – 5° S and 120° – 170° W.Gergis, Joëlle L., and Anthony M. Fowler
"Classification of Synchronous Oceanic and Atmospheric El Niño-Southern Oscillation (ENSO) Events for Palaeoclimate Reconstruction."
International Journal of Climatology 25 (2005): 1541-65

Coupled ENSO Index

The Coupled ENSO Index (CEI) uses a combination of both the SOI and Niño 3.4 SST to account for both an atmospheric and oceanic component.

Proxy-based ENSO Index

Developed by Braganza, et al., 2009, this index uses coral, tree ring and ice core data to characterize ENSO events from 1525 – 1982. The proxy ENSO index covers a wide area across the Pacific, and includes data from the western and central Pacific, New Zealand, and subtropical North America. Although the proxy ENSO index covers a large time scale of over four centuries, it shows high correlation (> 40%) with the SOI, Niño 3.4 SST and CEI, indicating its accuracy and utility.Braganza, Karl, Joelle L. Gergis, Scott B. Power, James S. Risbey, and Anthony M. Fowler
"A Multiproxy Index of the El Nin ̃o–Southern Oscillation, A.D. 1525– 1982."
Journal of Geophysical Research 114 (2009): 1-17.

References

{{reflist Tropical meteorology Regional climate effects