2. Analysis
1) ENSO (El Niño Southern Oscillation)
- Refer following video. (Understanding ENSO, https://www.youtube.com/watch?v=dzat16LMtQk)
El Niño/Southern Oscillation (ENSO) is the interaction between the atmosphere and ocean in the tropical Pacific that results in a somewhat periodic variation between below-normal and above-normal sea surface temperatures and dry and wet conditions over the course of a few years. While the tropical ocean affects the atmosphere above it, so too does the atmosphere influence the ocean below it. One layer of the Pacific Ocean that is influenced by ENSO is the thermocline.
The thermocline marks the transition between the warm upper water and the cold deep water in the Pacific Ocean. The upward currents along the equator (or upwelling) are strongest across this transition zone. The depth of the thermocline has a direct relationship with water surface temperatures. When the thermocline is closer to the water surface, upwelling of cold, nutrient rich deep-water is transported up from the bottom layers, leading to cooler temperatures at the water surface.
The interaction of the atmosphere and ocean is an essential part of El Niño and La Niña events. During an El Niño, sea level pressure tends to be lower in the eastern Pacific and higher in the western Pacific while the opposite tends to occur during a La Niña. This see-saw in atmospheric pressure between the eastern and western tropical Pacific is called the Southern Oscillation, often abbreviated as simply the SO.
Since El Niño and the Southern Oscillation are related, the two terms are often combined into a single phrase, the El Niño-Southern Oscillation, or ENSO. Often the term ENSO Warm Phase is used to describe El Niño and ENSO Cold Phase to describe La Niña.
2) MEI (Multivariate ENSO Index)
The multivariate ENSO index, abbreviated as MEI, is one of methods used to describe the intensity of an ENSO event. This model is regarded as the most comprehensive index for monitoring ENSO since it combines ocean-atmosphere phenomenon to cause global climate variability on inter-annual time scales. There are six main observed variables over the tropical Pacific. These six variables are: sea-level pressure (P), zonal (U) and meridional (V) components of the surface wind, sea surface temperature (S), surface air temperature (A), and total cloudiness fraction of the sky (C). These observations have been collected and published in ICOADS (International Comprehensive Ocean-Atmosphere Data Set) for many years.
In following graph, negative values of the MEI represent the cold ENSO phase (La Niña), while positive MEI values represent the warm ENSO phase (El Niño).
- Refer following video. (Understanding ENSO, https://www.youtube.com/watch?v=dzat16LMtQk)
El Niño/Southern Oscillation (ENSO) is the interaction between the atmosphere and ocean in the tropical Pacific that results in a somewhat periodic variation between below-normal and above-normal sea surface temperatures and dry and wet conditions over the course of a few years. While the tropical ocean affects the atmosphere above it, so too does the atmosphere influence the ocean below it. One layer of the Pacific Ocean that is influenced by ENSO is the thermocline.
The thermocline marks the transition between the warm upper water and the cold deep water in the Pacific Ocean. The upward currents along the equator (or upwelling) are strongest across this transition zone. The depth of the thermocline has a direct relationship with water surface temperatures. When the thermocline is closer to the water surface, upwelling of cold, nutrient rich deep-water is transported up from the bottom layers, leading to cooler temperatures at the water surface.
The interaction of the atmosphere and ocean is an essential part of El Niño and La Niña events. During an El Niño, sea level pressure tends to be lower in the eastern Pacific and higher in the western Pacific while the opposite tends to occur during a La Niña. This see-saw in atmospheric pressure between the eastern and western tropical Pacific is called the Southern Oscillation, often abbreviated as simply the SO.
Since El Niño and the Southern Oscillation are related, the two terms are often combined into a single phrase, the El Niño-Southern Oscillation, or ENSO. Often the term ENSO Warm Phase is used to describe El Niño and ENSO Cold Phase to describe La Niña.
2) MEI (Multivariate ENSO Index)
The multivariate ENSO index, abbreviated as MEI, is one of methods used to describe the intensity of an ENSO event. This model is regarded as the most comprehensive index for monitoring ENSO since it combines ocean-atmosphere phenomenon to cause global climate variability on inter-annual time scales. There are six main observed variables over the tropical Pacific. These six variables are: sea-level pressure (P), zonal (U) and meridional (V) components of the surface wind, sea surface temperature (S), surface air temperature (A), and total cloudiness fraction of the sky (C). These observations have been collected and published in ICOADS (International Comprehensive Ocean-Atmosphere Data Set) for many years.
In following graph, negative values of the MEI represent the cold ENSO phase (La Niña), while positive MEI values represent the warm ENSO phase (El Niño).
(Images from http://www.esrl.noaa.gov/psd/enso/mei/, MEI)
The MEI is computed separately for each of twelve sliding bi-monthly seasons (Dec/Jan, Jan/Feb... Nov/Dec). After spatially filtering the individual fields into clusters (Wolter, 1987), the MEI is calculated as the first un-rotated Principal Component (PC) of all six observed fields combined. This is accomplished by normalizing the total variance of each field first, and then performing the extraction of the first PC on the co-variance matrix of the combined fields (Wolter and Timlin, 1993). In order to keep the MEI comparable, all seasonal values are standardized with respect to each season and to the 1950-93 reference period.
Following graph shows a comparison figure with strong El Niño conditions from April-May 2015 through April-May 2016 and the classic set of strong El Niño events during the MEI period of record.
The MEI is computed separately for each of twelve sliding bi-monthly seasons (Dec/Jan, Jan/Feb... Nov/Dec). After spatially filtering the individual fields into clusters (Wolter, 1987), the MEI is calculated as the first un-rotated Principal Component (PC) of all six observed fields combined. This is accomplished by normalizing the total variance of each field first, and then performing the extraction of the first PC on the co-variance matrix of the combined fields (Wolter and Timlin, 1993). In order to keep the MEI comparable, all seasonal values are standardized with respect to each season and to the 1950-93 reference period.
Following graph shows a comparison figure with strong El Niño conditions from April-May 2015 through April-May 2016 and the classic set of strong El Niño events during the MEI period of record.
(Images from http://www.esrl.noaa.gov/psd/enso/mei/, MEI for historic El Niño conditions)
Compared to last month, the updated (September-October) MEI has dropped further to -.38 (down by -.28 in just one month), which translates into low ENSO-neutral rankings. The nine-month run in the Top-3 from May-June 2015 through January-February 2016 is tied with 1982-83 for its duration, while 1997-98 kept this level going for a full 12 months. No other El Niño since 1950 even exceeded three months at that level. The August-September 2015 MEI of +2.53 represents the peak of the 2015-16 event, and was exceeded only during the 1982-83 and 1997-98 events. The overall evolution of the 2015-16 El Niño was most similar to 1997-98, as monitored by the MEI, except that the latter had already transitioned to weak La Niña conditions by September 1998.
Compared to last month, the updated (September-October) MEI has dropped further to -.38 (down by -.28 in just one month), which translates into low ENSO-neutral rankings. The nine-month run in the Top-3 from May-June 2015 through January-February 2016 is tied with 1982-83 for its duration, while 1997-98 kept this level going for a full 12 months. No other El Niño since 1950 even exceeded three months at that level. The August-September 2015 MEI of +2.53 represents the peak of the 2015-16 event, and was exceeded only during the 1982-83 and 1997-98 events. The overall evolution of the 2015-16 El Niño was most similar to 1997-98, as monitored by the MEI, except that the latter had already transitioned to weak La Niña conditions by September 1998.