• El Niño and La Niña are opposite phases of the El Niño-Southern Oscillation (ENSO) cycle.
• The ENSO is a climatic pattern involving temperature changes in the eastern and central tropical Pacific Ocean, and changes in the patterns of winds, sea level pressure, and rainfall across the Pacific Basin.
• El Niño is the warm phase of ENSO.
• La Niña is the cold phase of ENSO.
• These deviations from normal surface temperatures can have a large-scale impact on global weather conditions and climate.

El Nino

• “El Niño” is named after the Christ Child and was coined by fishermen in Ecuador and Peru.
• It refers to the occasional warming of the central and eastern Pacific Ocean.
• El Niño events occur irregularly, with intervals of 2-7 years, averaging about once every 3-4 years.
• During El Niño, the usual upwelling of cold, nutrient-rich deep ocean water is significantly reduced.
• El Niño typically occurs around Christmas and lasts for a few weeks to a few months.
• Strong El Niño events can sometimes last for much longer time periods, such as in the 1990s when they developed in 1991 and lasted until 1995, and from fall 1997 to spring 1998.

Normal Conditions

• Normal conditions in the Pacific involve a surface low pressure in northern Australia and Indonesia and a high-pressure system over the coast of Peru.
• Trade winds blow strongly from east to west over the Pacific, carrying warm surface waters westward.
• This results in convective storms in Indonesia and coastal Australia.
• Cold nutrient-rich water wells up to the surface along the coast of Peru to replace the warm water pulled to the west.
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Walker circulation (Occurs during Normal Years)

• The Walker circulation is caused by a pressure gradient force between a high-pressure system in the eastern Pacific and a low-pressure system in Indonesia.
• The Walker cell indirectly causes upwelling off the coasts of Peru and Ecuador, bringing nutrient-rich cold water to the surface and increasing fishing stocks.
• The thermocline is a temperature gradient in a body of water, while a halocline is a gradient in salinity. The two can coincide to form a pycnocline, which refers to a rapid change in density with depth.
• Haloclines are common in water-filled limestone caves near the ocean.
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During El Nino year

• In El Niño years, air pressure drops over the central Pacific and the coast of South America.
• The normal low-pressure system is replaced by a weak high in the western Pacific, causing reduced trade winds (Weak Walker Cell) and sometimes even a reversal of the Walker Cell.
• The reduced trade winds allow warm ocean water to accumulate along the coastlines of Peru and Ecuador via the equatorial counter current.
• The accumulation of warm water causes the thermocline to drop in the eastern Pacific, cutting off the upwelling of cold deep ocean water along the coast of Peru.
• El Niño brings drought to the western Pacific, rains to the equatorial coast of South America, and convective storms and hurricanes to the central Pacific.
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Effects of El Nino

• The warmer waters had a devastating effect on marine life existing off the coast of Peru and Ecuador.
• Fish catches off the coast of South America were lower than in the normal year (Because there is no upwelling).
• Severe droughts occur in Australia, Indonesia, India, and southern Africa.
• Heavy rains in California, Ecuador, and the Gulf of Mexico.

How El Nino impacts monsoon rainfall in India

• El Nino and Indian monsoons are inversely related.
• Six of the most prominent droughts in India since 1871 have been El Nino droughts, including the recent ones in 2002 and 2009.
• Not all El Nino years lead to a drought in India, as seen in 1997/98.
• A moderate El Nino in 2002 resulted in one of the worst droughts in India.
• El Nino directly impacts India’s agrarian economy by lowering the production of summer crops such as rice, sugarcane, cotton, and oilseeds.
• This ultimately leads to high inflation and low gross domestic product growth as agriculture contributes around 14 percent of the Indian economy.

El Nino Southern Oscillation [ENSO]

• El Niño is linked with Pacific Ocean circulation pattern known as the Southern Oscillation.
• The Southern Oscillation is a coherent inter-annual fluctuation of atmospheric pressure over the tropical Indo-Pacific region.
• El Niño and Southern Oscillation coincide most of the times, and their combination is called ENSO (El Niño Southern Oscillation).
• Only El Niño results in warm water in the Eastern Pacific and cold water in the Western Pacific.
• Only Southern Oscillation results in low pressure over the Eastern Pacific and high pressure over the Western Pacific.
• ENSO results from the combination of warm water in the Eastern Pacific and low pressure over the Eastern Pacific, as well as cold water in the Western Pacific and high pressure over the Western Pacific.

Southern Oscillation Index and Indian Monsoons

• SO is a see-saw pattern of meteorological changes between Eastern Pacific and Western Pacific.
• The pattern gives rise to vertical circulation along the equator known as Walker Circulation.
• The location of low pressure over the Western Pacific is conducive to good monsoon rainfall in India.
• Shifting eastward from its normal position reduces monsoon rainfall in India.
• El Nino and Southern Oscillation are jointly referred to as ENSO event.
• The Periodicity of SO varies from two to five years.
• Southern Oscillation Index (SOI) is used to measure the intensity of the Southern Oscillation.
• SOI is the difference in pressure between Tahiti in French Polynesia (Central Pacific) and Port Darwin in northern Australia (Eastern Pacific).
• Positive and negative values of SOI indicate good or bad rainfall in India.

Indian Ocean Dipole effect (Not every El Nino year is same in India)

• ENSO was effective in explaining past droughts in India, but in recent decades the ENSO-Monsoon relationship has weakened.
• A similar ocean-atmosphere system to ENSO was discovered in the Indian Ocean in 1999 and named the Indian Ocean Dipole (IOD).
• IOD is defined by the difference in sea surface temperature between two poles in the Indian Ocean.
• IOD develops in the equatorial region of the Indian Ocean from April to May and peaks in October.
• Positive IOD results in the Arabian Sea being warmer and the eastern Indian Ocean becoming colder and dry, with winds blowing from east to west.
• Negative IOD results in Indonesia becoming warmer and rainier, with the opposite wind direction.
• Positive IOD index can negate the effect of ENSO, resulting in increased Monsoon rains in some ENSO years
• The two poles of the IOD, eastern pole (around Indonesia) and western pole (off the African coast) affect the quantity of rains for the Monsoon in India
• Equatorial Indian Ocean Oscillation (EQUINOO) is the atmospheric component of IOD and oscillates between the Bay of Bengal and the Arabian Sea.
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Impact on IOD on Cyclonogeneis in Northern Indian Ocean

• Positive IOD (Arabian Sea warmer than the Bay of Bengal) results in more cyclones than usual in the Arabian Sea.
• Negative IOD results in stronger than usual cyclogenesis (Formation of Tropical Cyclones) in the Bay of Bengal. Cyclogenesis in the Arabian Sea is suppressed.

The El Niño Modoki

• El Niño Modoki is a coupled ocean-atmosphere phenomenon in the tropical Pacific.
• It is different from conventional El Niño, which is characterized by strong anomalous warming in the eastern equatorial Pacific.
• El Niño Modoki is associated with strong anomalous warming in the central tropical Pacific and cooling in the eastern and western tropical Pacific.
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El Niño Modoki Impacts

• The El Niño Modoki phenomenon is characterized by the anomalously warm central equatorial Pacific flanked by anomalously cool regions in both west and east.
• Such zonal gradients result in anomalous two-cell Walker Circulation over the tropical Pacific, with a wet region in the central Pacific.

La Nina

• El Niño event is followed by a return to normal weather conditions.
• Strong trade winds can cause abnormal accumulation of cold water in the central and eastern Pacific, leading to La Niña event.
• La Niña occurred in 1988 and was responsible for summer drought in central North America.
• La Niña was followed by very active hurricane seasons in 1998 and 1999 in the Atlantic Ocean.
• Hurricane Mitch, which developed during this period, was the strongest October hurricane in about 100 years of record keeping.
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Effects of La Nina

• La Niña causes lower-than-normal air pressure over the western Pacific, leading to increased rainfall.
• La Niña results in abnormally heavy monsoons in India and Southeast Asia, cool and wet winter weather in southeastern Africa, and wet weather in eastern Australia.
• La Niña causes a cold winter in western Canada and the northwestern United States, and winter drought in the southern United States.
• La Niña enhances the rainfall associated with the Southwest monsoon but has a negative impact on rainfall associated with the Northeast monsoon.
• Rainfall associated with the summer monsoon in Southeast Asia tends to be greater than normal, especially in northwest India and Bangladesh.
• Strong La Niña events are associated with catastrophic floods in northern Australia.
• La Niña events are also associated with rainier-than-normal conditions in southeastern Africa and northern Brazil.
• Drier-than-normal conditions are observed along the west coast of tropical South America, the Gulf Coast of the United States, and the pampas region of southern South America.
• La Niña usually has a positive impact on the fishing industry of western South America due to upwelling of cold, nutrient-rich waters to the surface.

Madden-Julian Oscillation (MJO)

• Madden-Julian Oscillation (MJO) is an oceanic-atmospheric phenomenon.
• It is the largest element of the intra-seasonal variability in the tropical atmosphere.
• It affects weather activities across the globe.
• MJO brings major fluctuation in tropical weather on weekly to monthly timescales.
• It can be characterized as an eastward moving ‘pulse’ of cloud and rainfall near the equator.
• It typically recurs every 30 to 60 days.
• MJO is a traversing phenomenon.
• It is most prominent over the Indian and Pacific Oceans.

Phases of Madden-Julian Oscillation

• The MJO has two phases: the enhanced convective phase and the suppressed convective phase.
• Strong MJO activity often splits the planet into two halves, one in each phase.
• During the enhanced convective phase, winds converge at the surface and push air up throughout the atmosphere, leading to increased rainfall and condensation.
• During the suppressed convective phase, winds converge at the top of the atmosphere, forcing air to sink and leading to warmer, drier conditions.
• The dipole structure of the MJO moves from west to east in the tropics and causes more cloudiness, rainfall, and storminess in the enhanced convective phase and more sunshine and dryness in the suppressed convective phase.
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How Does MJO Affect Indian Monsoon?

• IOD, El Nino, and MJO are oceanic and atmospheric phenomena that affect large-scale weather.
• IOD only pertains to the Indian Ocean, while El Nino and MJO affect weather on a global scale.
• MJO is a traversing phenomenon that goes through eight phases.
• It brings good rainfall over the Indian subcontinent when it is over the Indian Ocean during the monsoon season.
• MJO brings bad news for the Indian monsoon when it stays over the Pacific Ocean for a longer cycle.
• MJO is linked with enhanced and suppressed rainfall activity in the tropics, making it important for the Indian monsoonal rainfall.
• MJO has a periodicity of nearly 30 days, and if it is above 40 days, it could lead to a dry monsoon.
• Shorter MJO cycles are better for the Indian monsoon as it visits the Indian Ocean more often.
• The presence of MJO over the Pacific Ocean with El Nino is detrimental for monsoon rains.