The name ‘ENSO’ is the close interaction between the atmosphere and ocean. It is an irregularly periodic variation in winds and sea-surface temperatures over the tropical eastern pacific ocean, affecting the climate of much of the tropics and subtropics. The warming phase of the sea temperature is known as El-Nino and the cooling phase is known as La-Nina.
The two phases relate to the Walker circulation, which is caused by the pressure gradient force that results from a high pressure area over eastern pacific ocean, and a low pressure system over Indonesia. Weakening or reversal of the walker circulation decreases or eliminates the upwelling of cold deep sea water, thus creating an El-Nino by causing the ocean surface to reach above average temperature. A Strong walker circulation causes, La-Nina, resulting in cooler ocean temperature due to increased upwelling.
The ENSO periodically fluctuations between three phases – Neutral, La-Nina, El-Nino
Neutral phase :-
Normally the Humboldt current brings relatively cold water from the southern ocean northwards along the South American west cost to the tropics, where it is enhanced by upwelling taking place along the cost of Peru. Along the equator, trade winds cause the ocean currents in the eastern pacific to draw water from the deeper ocean to the surface thus cooling the ocean surface further.
Under the influence of trade winds, the cold water flows westward along the equator where it is slowly heated by the sun. As a result sea-surface temperature at the western pacific are generally warmer by about 8-100C than those in the eastern pacific.
This warmer ocean is the source for convection and is associated with cloudiness and rainfall, and the collected moisture is dumped in the form of typhoons and thunderstorms. Ocean surface is some 60cm higher in the western pacific as a result of this motion.
It creates the oceanic upwelling off the costs of Peru and brings nutrient-rich cold water to the surface increasing the fishing in the area.
Warm Phase (El-Nino) :-
When the walker circulation weakens or reverses and the Hadely circulation strengthens an El-Nino occurs causing the ocean surface to be warmer than average, as upwelling of cold water occurs less or not at all. The El-Nino with the bend of warmer ocean temperature that periodically developes off the pacific coast of South America.
It refers toa s christ child because periodic warming in the pacific near South America is usually noticed around christmas.
 Over Indonesia and the far western Pacific, the waters cool slightly, resulting in less rainfall and convection. The trade winds along the equator from the east-to-west weaken.
The warmer waters cause the Pacific jet stream to move south of its neutral position. With this shift, areas in the northern U.S. and Canada are dryer and warmer than usual. But in the U.S. Gulf Coast and Southeast, these periods are wetter than usual and have increased flooding.
Cold phase (La-Nina) :-
An especially strong walker circulation causes La-Nina, resulting in cooler ocean temperature in the central and eastern tropical pacific ocean due to increased upwelling. La-Nina is the counterpart of the El-Nino, and resulting the ocean temperature down to 3-50C than normally. The trade winds that typically blow from east-to-west along the equator become even stronger.Â
This environment supports more marine life and attracts more cold-water species, like squid and salmon, to places like the California coast.
Over the western Pacific, warmer temperatures cause air near the surface to rise, moisten, and become convective, leading to thunderstorms and more rain. Over the cooler eastern Pacific, air begins to sink more and dry out.
These cold waters in the Pacific push the jet stream northward. This tends to lead to drought in the southern U.S. and heavy rains and flooding in the Pacific Northwest and Canada. During a La Niña year, winter temperatures are warmer than normal in the South and cooler than normal in the North. La Niña can also lead to a more severe hurricane season.
Southern Oscillation Index :-
The Southern Oscillation Index (SOI) is a time series used to characterized the large scale sea level pressure pattern in the tropical pacific. The SOI is linked to large scale tropical SST variability and as such is a measure of the “SO” part of the ENSO phenomenon.
The strength of southern oscillation is measured from fluctuations in the surface air pressure differences between Tahiti ( in the Pacific) and Darwin ( in the Indian ocean). El-Nino episodes have -vs SOI meaning there is lower pressure over Tahiti and higher pressure over Darwin, while La-Nina episodes have +ve SOI meaning there is higher pressure over Tahiti and lower pressure over Darwin.
REFERENCE :-
- https://www.climate.gov/news-features/blogs/enso/rise-el-ni%C3%B1o-and-la-ni%C3%B1a
- https://oceanservice.noaa.gov/facts/ninonina.html
- https://climatedataguide.ucar.edu/climate-data/southern-oscillation-indices-signal-noise-and-tahitidarwin-slp-soi
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