Context: Recently, the Indian Meteorological Department (IMD) has announced that it expects monsoon rainfall to be normal this year.

Key Points

• In its first stage Long Range Forecast (LRF) for monsoons, the IMD provided that Southwest monsoon seasonal (June to September) rainfall over the country as a whole is likely to be normal (96-104%).
• The monsoon seasonal (June to September) rainfall is likely to be 100% of the Long Period Average (LPA) with a model error of 5%.
• It is estimated that the deficient rainfall will be 9 percent.
• The IMD provided that Neutral El Nino Southern Oscillation (ENSO) conditions are prevailing over the Pacific Ocean and Neutral Indian Ocean Dipole (IOD) conditions are prevailing over the Indian Ocean.
• La Nina, or cooler-than-usual sea surface temperatures in the east-central Pacific Ocean, is typically associated with better monsoon rains and colder winters in India while El Nino is associated with below-normal rainfall in India.
• The southwest monsoon season, that replenishes the country's farm-dependent economy, first hits the southern tip of Kerala usually in the first week of June and retreats from Rajasthan by September.
• Since 2012, IMD is also using the dynamical global climate forecasting system (CFS) model developed under the Monsoon Mission to generate experimental forecasts.

About Models for Forecasting

• Dynamical Model: It is also called the Monsoon Mission Coupled Forecast System. It relies on the supercomputers, mathematically simulating the physics of the ocean and the atmosphere.
• This model is better at forecasting the state of the weather a week or two in advance and is not yet considered reliable by meteorologists in forecasting the monsoon.
• Statistical Model: It takes into consideration the global weather models pointing to negligible chances of El Nino, a warming of the central equatorial Pacific which is associated with the drying up of monsoon rain. The IMD relies on this model.
• In any given year, there is a 33% chance of a normal monsoon that’s why there is high confidence that the monsoon in 2020 would be normal.

About Monsoon

• It is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea.
• Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase.
• The term is sometimes incorrectly used for locally heavy but short-term rains.
• India’s geography and geology are climatically pivotal: the Thar Desert in the northwest and the Himalayas in the north work in tandem to create a culturally and economically important monsoonal regime.
• As in much of the tropics, monsoonal and other weather patterns in India can be wildly unstable: epochal droughts, floods, cyclones, and other natural disasters are sporadic, but have displaced or ended millions of human lives.

About El Nino

• It is a climate cycle in the Pacific Ocean with a global impact on weather patterns.
• The cycle begins when warm water in the western tropical Pacific Ocean shifts eastward along the equator toward the coast of South America.
• Normally, this warm water pools near Indonesia and the Philippines. During an El Nino, the Pacific’s warmest surface waters sit offshore of north-western South America.
• During an El Nino, the trade winds weaken in the central and western Pacific. Surface water temperatures off South America warm up, because there is less upwelling of the cold water from below to cool the surface.
• The clouds and rainstorms associated with warm ocean waters also shift toward the east.
• The warm waters release so much energy into the atmosphere that weather changes all over the planet.

About La Nina

• La Nina means ‘little girl’ in Spanish and is also known as El Viejo or ‘cold event’. 
• Here, the water temperature in the Eastern Pacific gets colder than normal. 
• As a result of this, there is a strong high pressure over the eastern equatorial Pacific. Now, there is low pressure in the Western Pacific and off Asia. 
• La Nina causes drought in Peru and Ecuador, heavy floods in Australia, high temperatures in Western Pacific, Indian Ocean, off the Somalian coast and good monsoon rains in India. 
• A La Nina is actually beneficial for the Indian monsoon.
• Generally, El Nino and La Nina occur every 4 – 5 years. El Nino is more frequent than La Nina. Typically, the episodes last for nine to twelve months.

About Indian Ocean Dipole

• It also known as the Indian Nino, is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern part of the ocean.
• The IOD involves an aperiodic oscillation of sea-surface temperatures (SST), between “positive”, “neutral” and “negative” phases.
• A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, with a corresponding cooling of waters in the eastern Indian Ocean—which tends to cause droughts in adjacent land areas of Indonesia and Australia.
• The negative phase of the IOD brings about the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.
• The IOD also affects the strength of monsoons over the Indian subcontinent.

About Indian Meteorological Department (IMD)

• It is the National Meteorological Service of the country which was established in 1875.
• It is under Ministry of Earth Sciences.
• It is the principal government agency in all matters relating to meteorology, seismology and allied subjects.