Context:
- Any discussion on Indian monsoon these days invariably has references to the El Nino phenomenon. It is almost as if the fate of the Indian monsoon depends on the abnormalities in sea surface temperatures in far-away Pacific Ocean.
What is El Nino?
- This year’s monsoon is also progressing under the cloud of an El Nino in the Pacific Ocean.
- El Nino, as is commonly known, refers to an abnormal warming of surface waters in equatorial Pacific Ocean. It is known to suppress monsoon rainfall.
- The opposite phase, La Nina, which is the abnormal cooling of sea surface waters in the same region, is known to aid rainfall over India.
- There is a third, neutral phase, as well in which the sea surface temperatures remain roughly in line with long-term averages.
- Together, these three phases in the Pacific Ocean are referred to as El Nino Southern Oscillation, or ENSO.
Reason:
- But why do the surface waters in equatorial Pacific Ocean get abnormally warm or cold? And, why do these warm or cold phases have any bearing on Indian monsoon? Not everything about ENSO and Indian monsoon is very well understood, but scientists do have a fair idea of how this entire system works.
- El Nino phenomenon was first noticed by the scientists in the 1920s, though local populations in Peru and Ecuador were aware of the periodic warming much earlier. The La Nina phenomenon, on the other hand, was discovered only in the 1980s.
Ocean-Atmosphere system
- Though ENSO is mostly discussed in terms of temperature abnormalities of sea surface waters, it is important to understand that it is not just an ocean system. ENSO actually is an interaction of ocean and atmospheric conditions. In fact, the ‘southern oscillation’ part in the term ENSO refers to a specific atmospheric condition that is a measure of the difference in sea-level air pressure over western and eastern side of the Pacific Ocean. Another atmospheric condition that plays a key role in ENSO is the strength and direction of winds.
- Just the abnormal warming or cooling of surface waters in Pacific Ocean does not result in an El Nino or La Nina event. The associated atmospheric conditions also have to be synchronized.
- The ocean part of the ENSO is measured by what is known as the Oceanic Nino Index or ONI. The atmospheric part is monitored through Southern Oscillation Index, or SOI.
- The ocean and atmospheric conditions in El Nino or La Nina tend to reinforce each other, producing a cyclic process. That means that the warming of the sea surface waters during an El Nino event influences atmospheric conditions in a way that these, in turn, result in further warming of the waters. Similar processes happen during La Nina event as well. The linkages between waters and wind were unveiled in the 1960s.
- The playground of this entire system is the equatorial region in the Pacific Ocean. On the east are Ecuador and Peru in northwestern South America and on the west are the islands of Philippines and Indonesia. Between them is nearly 17,000 km of uninterrupted ocean. This region receives the most sunlight anywhere on earth, a lot of which is stored as heat in the ocean.
ENSO Neutral condition
- To understand the processes that lead to abnormal warming or cooling, it would help to know what happens during a normal year, when ENSO neutral conditions prevail. The tropical regions, that is the area immediately above and below the equator, is home to a permanent wind system called trade winds that move from east to west at quite low altitudes.
- Because of the exposure to sunlight, the sea surface in the Pacific Ocean is quite warm. When the trade winds move over the Pacific Ocean, they push these relatively warm waters, which also become lighter, in the westward direction. So, the surface waters in the eastern Pacific Ocean, that is near the South American coast, get pushed towards the west. It is replaced by the relatively cooler waters from below. The warmer surface waters continue to get pushed till they encounter a landmass in the islands at Philippines and Indonesia. They cannot be pushed any further. The result of this process is the accumulation of relatively warm waters near Indonesia, called the Western Pacific Warm Pool, and relatively cold waters near Ecuador and Peru. This sweeping of surface waters and its accumulation also results in a relative rise in sea levels near Indonesia. The sea levels on the eastern coast of Indonesia happen to be about half a meter higher than the western coast of Ecuador and Peru.
- The warmer surface waters near Indonesia create a region of low-pressure area, causing the air to rise upwards. This also results in formation of clouds and heavy rainfall. The air flow also helps in building up the monsoon system which brings rainfall over India.
- At higher altitudes, this air starts to move towards eastern Pacific Ocean, that is, in direction opposite to the trade winds that flow at lower altitudes. This wind system, east to west near the surface, and west to east at higher altitudes, creates a loop, and reinforces the temperature gradient between the east and west Pacific Ocean.
The Abnormal behavior
- In some years, for reasons that are not fully understood, the trade winds get weakened. It affects the ability of the trade winds to push warmer surface waters towards the Indonesian coast. Not enough warmer water is swept towards western Pacific Ocean. That means the central and eastern Pacific Ocean, off the coasts of Ecuador and Peru, becoming warmer than normal. This is the El Nino phase. Because the sea-level at the Indonesian coast is higher, and the trade winds are not very strong to resist the motion, some accumulated warm water begins to flow backwards towards the South American coast under the influence of gravity. This further adds to the warming in the eastern Pacific Ocean.
- The air circulation loop also gets affected, as a result. That, in turn, reduces the amount of precipitation over Indonesia and neighboring regions, and impacts the Indian monsoon as well.
- Exactly the opposite happens during a La Nina event. The trade winds become stronger than usual, pushing warmer waters towards the Indonesian coast, and making the eastern Pacific Ocean colder than normal.
- The amount of energy transferred during the movement of waters and winds in the different phases of ENSO system is massive. Though the most profound impacts of ENSO events are seen in the tropical regions, weather patterns across the world get affected.
- Both El Nino and La Nina usually begin to develop in the March to June season, reach their peak strength in the winters and then begin to dissipate in the post winter season. Both these phases typically last for a year, though La Nina, on an average, lasts longer than El Nino. While these phases alternate over a period of two to seven years, with the neutral phase thrown in between, it is possible for two consecutive episodes of El Nino or La Nina to occur.
ENSO and Climate Change
- In general, El Nino has a warming effect on the planet, while La Nina tends to cool it down. The warmest years in a decade are usually the El Nino years. The warmest-ever year on record, 2016, was part of one of the longest and strongest El Nino episodes ever, dubbed the Godzilla El Nino.
- It is important to note that the warming over the planet accounts only for the near-surface temperatures. It does not account for the massive amount of heat trapped in the oceans. El Nino or La Nina years do not alter the overall heat in the system, but these do influence how much of it gets sunk in the ocean. During the La Nina phase, for example, a larger than normal amount of warm surface water of Pacific Ocean is pushed towards the Indonesian coast. Here the entire column of ocean, several hundred metres deep comprises of relatively warm water. On the other side of the Pacific Ocean, relatively colder water from the deep emerges on the top. A large area over the eastern Pacific Ocean thus contains colder water. This has the ability to absorb some of the heat from the atmosphere, making the atmosphere slightly cooler. This is how La Nina produces a cooling effect.
- El Nino works in exactly the opposite direction, and produces a heating effect. El Nino thus accentuates the global warming phenomenon, and contributes to climate change. The reverse effect, that of climate change on ENSO – the kind of impacts climate change is having in the Pacific — is not very clear, however.
Source: Indian Express