The El Niño Southern Oscillation, or ENSO, is one of the most common phrases in the global climate system. This is because of its global impact, being one of the primary climate drivers. The ENSO is a reiterating climate pattern that involves the change in the sea surface temperature over the tropical Pacific. In the beginning, South American fishermen noticed abnormal warming of coastal waters, and fish production was massively affected, which occurred so often around Christmas. They named the event “El Niño” (niño means boy child in Spanish, believed to be Baby Christ, a local religious reference).
During the 1920s, Sir Gilbert Walker, a mathematician working as the Director-General of the meteorological observatories of the British colonies, was located in India for the first time. He worked on the problem of India’s monsoon forecast following the frequent drought situations affecting the country’s economy. This led him to discover the Southern Oscillation, i.e the change in sea level pressure over Indonesia and the tropical Pacific, which is the atmospheric component of the ENSO phenomena. Next, the phrase “Walker Circulation” was coined by Bjerknes in 1969 to describe the whole process as a large-scale atmospheric flow comprising the sinking air in the eastern Pacific and rise in the western Pacific employing feedback exchange between the surface winds and sea surface temperatures. Usually, the Sea Level Pressure (SLP) over the south-central Pacific (Tahiti) is relatively higher than in the northern part of Australia (Darwin), resulting in the surface trade winds blowing from east to west at the low latitudes. However, the SLP difference between Tahiti and Darwin weakens every few years of periodicity, which further affects the net transport of equatorial trade winds. This SLP difference between Tahiti and Darwin has been coined the “Southern Oscillation Index (SOI)” by Bjerknes (1969). Hence, the pressure gradient, abnormal sea surface heating, and trade winds' subsequent transport trigger the ENSO phenomenon.
This ENSO event has two phases: El Niño (Warm or positive phase) and La Niña (Cold or negative phase). El Niño is an abnormal warming of the tropical eastern Pacific, and the trade winds weaken, and the coastal upwelling off Peru's coast diminishes.
Figure #1: ENSO Schematic (Source: NOAA Pacific Marine Environmental Laboratory )
In contrast, La Niña is the opposite of El Niño, associated with the eastern Pacific's anomalous cooling, easterly trade winds' strengthening, and more upwelling over the South American coast. As displayed in the schematic, the Walker circulation shifts eastward during El Niño and westward during La Niña. The thermocline becomes shallower in the western Pacific during El Niño with minimal steepness. In contrast, the thermocline becomes deeper during La Niña.
ENSO Impacts:
The ENSO plays a significant role in impacting large-scale global ocean-atmospheric circulations that include the Indian Summer Monsoon. ENSO brings cooler weather conditions and more precipitation to the southern part of North America. Studies also suggested that the ENSO impacts agriculture production in various regions, including India. A study by Adams et al.(1999) assessed the impact of ENSO on the United States (US) agro-economic sector. They found an estimated $1.5 to $1.7 billion loss during El Niño and a $2.2 to $6.5 billion loss due to La Niña.
Figure #2: Climatological global precipitation patterns during El Niño Years (Source: FOA, UN)
The FAO (UN) schematic clearly shows the kind of global impact ENSO has in terms of precipitation patterns. Australia, Indonesia, India, Southern Africa and Tropical South America experience a dry spell during a positive phase of ENSO. On the other hand, regions such as Middle Asia and the southern states of North America receive anomalous surplus rainfall. From extensive studies, It is evident that El Niño causes significantly less precipitation developing drought conditions over the Indian and Australian monsoon regions simultaneously. Studies also suggest that ENSO affects tropical cyclone activity in various basins. For example, cyclogenesis frequency has significantly increased over the Atlantic, associated with El Niño events, with most landfalls along the Gulf of Mexico and the central part of the US. However, ENSO has a special bond with Indian Summer Monsoon.
ENSO And Indian Summer Monsoon:
Earlier studies suggest that many El Niño events have caused drought in the Indian region, almost 60% in the last 100 years. Usually, an El Niño is associated with a deficit in ISMR, and La Niña with a surplus. Various aspects of ISM and ENSO relationships have been studied extensively by many earlier studies.
Figure #3: The composites of anomalous ISMR (in mm/day) during El Niño and La Niña years (Plotted)
What happens? The Walker Circulation shifts to the equatorial Indian Ocean during El Niños. At the same time, there is a westward shift of the Walker Circulation with the amplification of equatorial convergence over the equatorial Indian Ocean. This anomalous equatorial convergence is also responsible for the cross-equatorial meridional circulations and further divergence over India, which causes dry conditions over India. According to a recent study, the relationship between ISMR and ENSO has been weakening in the past two decades. In a changing climate, it is found in a recent study (CMIP5) suggests a possible shift in the relationship between ENSO and monsoon rainfall from 3–5 years of lasting duration to 2.5–3 years in future.
A ENSO Variant: ENSO Modoki
El Niño Modoki is different from the typical El Niño. The associated anomalous warming in the equatorial central Pacific and anomalous cooling in the adjacent equatorial western and eastern Pacific characterizes El Niño Modoki. This variant of El Niño identified decades ago (Congbin et al.,1986).
Figure #4: Side by Side comparison of ENSO & ENSO Modoki Schematic
(Source: Ashok and Yamagata, The El Niño with a difference. Nature 461, 481–484 (2009))
This El Niño type has been referred to as El Niño Modoki by Ashok et al.(2007) and Warm Pool El Niño by Kug et al.(2009). It also has a significant impact on the global climate system, and studies suggest that El Niño Modoki events are associated with below-average monsoon rainfall over the peninsula of India and a surplus rainfall along the monsoon trough region.
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