Monsoon is a familiar though a little known climatic phenomenon. In India, from agriculture to economic policies to disaster management, a lot depends on the Monsoon.
The Monsoon is a recurring event i.e. it repeats after a certain frequency of time – a year in our case. But, it may not be uniform in every period (year). There are a lot of factors which affect its duration and intensity over India.
The Monsoon is basically a result of the flow of moisture laden winds because of the variation of temperature across the Indian Ocean.
There are a number of climatic phenomena which affect it namely the Indian ocean dipole, El nino, La nina, Equatorial Indian Ocean Oscillation (EQUINOO) etc. These phenomena affect the temperature distribution over the oceans and thus affecting the direction and intensity of flow of the moisture laden winds.
There have been recent reports that El Nino may disturb the Indian Monsoon and play badly with Indian agriculture. This brings us to the discussion of the concepts of El Nino and La Nina.
---------------------------------------------------------------------------------
# Origin :
El Nino and La Nina are opposite phases of what is known as the El Niño-Southern Oscillation (ENSO) cycle. The ENSO cycle is a scientific term that describes the fluctuations in temperature between the ocean and atmosphere in the east-central
Equatorial Pacific. (The area between South America and Australia near the equator )
La Nina is sometimes referred to as the cold phase of ENSO and El Nino as the warm phase of ENSO. These deviations from normal surface temperatures can have large-scale impacts not only on ocean processes, but also on global weather and climate, including India.
--------------------------------------------------------------------------------
Mechanism :
NOW, it is important to understand how these phenomena affect the Monsoon system? To know this, we must first know the pressure and temperature distribution in the region before their onset. (We are assuming here that you are a little bit familiar with the phenomenon of Monsoon.)
For a normal monsoon season, the pressure distribution is as such:
The Peruvian coast has relatively high pressure than the areas near north Australia and South-East Asia.
The Indian Ocean is slightly warmer than the adjoining oceans (West pacific –see diagram) and thus the pressure is low relatively due to the warm seas. This is why the moisture laden winds move from near the west pacific to the Indian Ocean and from there on to the lands.
The pressure on heated Indian land is much lower than that on the Indian Ocean.
This facilitates the movement of monsoon winds from the sea to the Indian land without any significant diversion.
But if for some reason this normal distribution is affected, then there is a change in the way trade winds (or monsoon winds) would blow.
==> This is because of the following reasons (and its effects):
Off the coast of Peru (read in Eastern Pacific and Central Pacific), there is normally cool surface water because of the cold Peruvian current. But El Niño makes it go warm.
When the water becomes warm, the tread winds, which otherwise flow from East to west, either reverse their direction or get lost.
Due to this warm water, the air gets up and surface air pressure above Eastern Pacific gets down. On the other hand, the waters cool off in western pacific and off Asia. This leads to rise in surface pressure over the Indian Ocean, Indonesia, and Australia.
Now as the pressure over the Peruvian coast reduces because of the warm sea water, the flow of moisture laden winds is directed to the Peruvian coasts from the western pacific (the areas near North Australia and South-east Asia – refer to the diagrams above).
Hence, the moisture laden winds that should have moved towards the Indian coast now move towards the Peruvian coast.
The warm water causes lots of clouds getting formed in that area, causing heavy rains in Peruvian desert during El Niño years.
This robs the Indian subcontinent of its share in the Monsoon rains. The greater the temperature and pressure difference, the greater would be the shortage in the rainfall in India.
--------------------------------------------------------------------------------
# La-Nina :
La Niña, “anti-El Niño” or simply “a cold event” is the cooling of water in the Eastern Pacific Ocean.
The following happens in La-Nina:
The water in Eastern Pacific, which is otherwise cool; gets colder than normal. There is no reversal of the trade winds but it causes strong high pressure over the eastern equatorial Pacific.
On the other hand, low pressure is caused over Western Pacific and Off Asia.
This has so far caused the following major effects: Drought in Ecuador and Peru. Low temperature, High Pressure in Eastern Pacific.
Heavy floods in Australia; High Temperature in Western Pacific, Indian Ocean, Off coast Somalia and good rains in India. Drought in East Africa.
For India, an El Niño is often a cause for concern because of its adverse impact on the south-west monsoon; this happened in 2009. A La Niña, on the other hand, is often beneficial for the monsoon, especially in the latter half. The La Niña that appeared in the Pacific in 2010 probably helped 2010’s south-west monsoon end on a favorable note. However, it also contributed to the deluge in Australia, which resulted in one of that country’s worst natural disasters with large parts of Queensland either under water from floods of unusual proportions or being battered by tropical cyclones.
---------------------------------------------------------------------------------
# Periodicity :
This distortion is pressure and temperature recurs every 4-5 years. But it may not happen exactly after 4-5 years or it may not happen at all. It periodicity is thus quite uncertain.
El Nino and La Nina episodes typically last nine to 12 months, but some prolonged events may last for years. They often begin to form between June and August, reach peak strength between December and April, and then decay between May and July of the following year. While their periodicity can be quite irregular, El Nino and La Nina events occur about every three to five years. Typically, El Nino occurs more frequently than La Nina.
-----------------------------------------------------------------------------
# Impact of El-Nino :
Normal or High rainfall in Eastern/Central Pacific
Drought or scant rainfall in western pacific/Asia
This leads to a lot of undesirable circumstances.
“When the rainfall for the monsoon season of June to September for the country as a whole is within 10% of its long period average, it is categorised as a normal monsoon. When the monsoon rainfall deficiency exceeds 10%, it is categorised as an all-India drought year.” – IMD
