INTRODUCTION OF ENSO

The El Niño/Southern Oscillation (ENSO) is a naturally occurring phenomenon involving fluctuating ocean temperatures in the central and eastern equatorial Pacific, coupled with changes in the atmosphere. This phenomenon has a major influence on climate patterns in various parts of the world. The El Niño and La Niña weather events are excellent examples of how interconnected the physics of the ocean and atmosphere are when it comes to cooling and heating. Although we think of air and water as discrete bodies however, when changes occur in one it is often reflected in the other. El Niño and La Niña are mutually opposite phenomena, during which an abnormal warming or cooling of sea surface temperatures is observed in the Pacific Ocean along the equator, off the coast of South America. Together they constitute what is known as the El Niño-Southern Oscillation system, or ENSO.

The two phenomena impact weather and climate events across the world, including the Indian monsoon. The warming of the Pacific — El Niño — is known to suppress rainfall over India, especially in the north-western parts; La Niña has been observed to help the monsoon. El Niño and La Niña are the oceanic components while the Southern Oscillation is the atmospheric counterpart, thus giving rise to the term El Niño/Southern Oscillation. The El Niño/Southern Oscillation comprises three phases: El Niño, La Niña and neutral.

EL-NINO

El Nino can further be classified into two types— traditional El Nino which is characterized by strong anomalous warming in the eastern equatorial Pacific and the El Nino Modoki that is associated with strong anomalous warming in the central tropical Pacific and cooling in the eastern and western tropical Pacific.
El Niño, meaning “boy child” in Spanish, was first used in the nineteenth century by fishermen in Peru and Ecuador to refer to the unusually warm waters that reduced their catch just before Christmas. El Niño events often begin in the middle of the year with large-scale warming of surface water in the central and eastern equatorial Pacific Ocean and changes in the tropical atmospheric circulation (i.e. winds, pressure and rainfall). It occurs every two to seven years and can last up to 18 months. Strong and moderate El Niño events have a warming effect on average global surface temperatures.

During an El Niño event, the surface waters in the central and eastern Pacific Ocean (off the coast of South America) become significantly warmer than usual. That change is intimately tied to the atmosphere and to the winds blowing over the vast Pacific. Easterly trade winds (which blow from the Americas toward Asia) falter and can even turn around into westerlies. This allows great masses of warm water to rush from the western Pacific that is off the coast of Australia toward the Americas. It also reduces the upwelling of cooler, nutrient-rich waters /Peru Currents from the deep—shutting down or reversing ocean currents along the equator and along the west coast of South and Central America.

The circulation of the air above the tropical Pacific Ocean responds to this tremendous redistribution of ocean heat. The typically strong high-pressure systems of the eastern Pacific weaken, thus changing the balance of atmospheric pressure across the eastern, central, and western Pacific. While easterly winds tend to be dry and steady, Pacific westerlies tend to come in bursts of warmer, moister air.

IMPACT OF EL-NINO - El Niño is often associated with warm and dry conditions in southern and eastern inland areas of Australia, as well as Indonesia, the Philippines, Malaysia and central Pacific islands such as Fiji, Tonga and Papua New Guinea. During the northern hemisphere summer season, the Indian monsoon rainfall generally tends to be less than normal, especially in the north-west of India. The magnitude and scale of the changes in atmospheric circulation associated with El Niño are such that the effects on regional climate extend beyond the tropical Pacific basin. El Niño tends to lead to more tropical cyclone activity in the central and eastern Pacific basins and less in the Atlantic basin. Over the central and eastern Pacific, increased sea-level atmospheric pressure in the subtropical areas adjacent to the area of ocean warming is associated with weaker upper-level winds and reduced vertical wind shear, which favours more tropical cyclone activity.

LA-NINA

The opposite of El Nino within the ENSO cycle is known as La Nina, which means “little girl” and refers to the large-scale cooling of the ocean surface temperatures in the same region in the equatorial Pacific, coupled with a reversal of the overlying atmospheric conditions of Walker Cell . La Nina occurs when equatorial trade winds become stronger, changing ocean surface currents and drawing cooler deep water up from below. This results in a cooling of the central and eastern tropical Pacific Ocean. The enhanced trade winds also help to pile up warm surface waters in the western Pacific and to the north of Australia. The warming of ocean temperatures in the western Pacific means the area becomes more favourable for rising air, cloud development and rainfall. As a result, heavy rainfall can occur to the north of Australia. Conversely, over the eastern and central tropical Pacific, air descends over the cooler waters, meaning the region is less favourable for cloud and rain. The air rising in the west and descending in the east enhances an atmospheric circulation – called the Walker circulation – which can result in changes to the climate felt across the globe.

IMPACT OF LA-NINA
La-Nina causes drought and dry conditions in the eastern side of the equatorial Pacific as opposed to the western side. The dry spells and droughts are mostly witnessed in the coastal regions of Peru and Chile which may result in poor crop yields. La-Nina leads to increase fish presence along the coast line of Peru and beneficial for Peruvian fishermen . It is associated with the higher than normal rainfall ,often causing catastrophic flooding Australia ,Philippines, Malaysia and Indonesia. La-Nina years are good for Summer Monsoon in India .

Walker Cell
The East–West zonal or longitudinal circulation of tropical winds across the equatorial pacific ocean is called walker circulation .It was discovered by Gilbert Walker . It is not similar to other convective cells like Hadley , ferrel and polar cells in which there is meridional circulation of winds take place . The Walker cell is driven by temperature and pressure gradients. After 2–7 years , this general condition of East-West pressure gradient becomes West-East . Thus , there is an oscillation in pressure gradient and air circulation ,known as Southern Oscillation .

SOUTHERN OSCILLATION
Southern Oscillation is an east–west see-saw-like movement of air masses between the Pacific and the Indo-Australian areas. These changes in atmospheric circulation occur in response to changing ocean surface temperatures and at the same time influence El Niño or La Niña ocean temperature patterns. The negative phase of the Southern Oscillation occurs during El Niño episodes, and refers to the situation when abnormally high air pressure covers Indonesia and abnormally low air pressure covers the central and/or eastern tropical Pacific. In an El Niño event, the low-level surface trade winds, which normally blow from east to west along the equator (“easterly winds”), weaken or in some cases start blowing in the opposite direction. The positive phase of the Southern Oscillation occurs during La Niña episodes, when abnormally low air pressure covers Indonesia and abnormally high air pressure covers the central and/or eastern tropical Pacific.
This difference from average air pressure patterns across the Pacific is traditionally measured using the Southern Oscillation Index (SOI), based on pressure measured by two stations: one in Darwin, Australia (south of Indonesia) and the other in Tahiti (east-central tropical Pacific).