From El Nino, LPA, to IOD: Monsoon terminology everyone should know

IMD predicted normal monsoon for India in this season, unlike the forecast by Skymet

India Meteorological Department (IMD), the nation’s official weather forecaster, on Tuesday forecast that India will get a 'normal' monsoon this year, unlike Skymet Weather, which predicted "below-normal" monsoon rains in the country.

"India to see normal rainfall during the southwest monsoon season (from June to September). It is likely to be 96 per cent (with an error margin of 5 per cent) of the long-period average (LPA) of roughly 87 cm," M Ravichandran, secretary, Ministry of Earth Sciences, said in a press conference.

Here are a few terms to know related to monsoon:

La Nina

La Nina is abnormal cooling of the central and eastern Pacific Ocean waters off the coasts of Ecuador and Peru.

Such cooling (Sea surface temperature (SST) falling 0.5 degrees Celsius or more below a 30-year average for at least five successive three-month periods) is caused by strong trade winds flowing west along the Equator, taking warm water from South America towards Asia.

The warming of the western equatorial Pacific causes enhanced evaporation and concentrated cloud-formation activity around that region, with consequences for India as well.

The latest La Nina event lasted from July-September 2020 to December-February 2022–23, making it one of the longest ever. It brought copious rains to India—just as two previous "strong" La Ninas in 2007–08 and 2010–11, followed by one "moderate" episode in 2011–12, had done.

El Nino

During El Nino, the trade winds diminish or even change direction, blowing from east (South America) to west (Indonesia). Warm water masses migrate into the central and eastern equatorial Pacific Ocean as the winds blow from west to east. As a result of the rise in SSTs, rainfall increases in western Latin America, the Caribbean, and the US Gulf Coast, whereas convective currents are cut off in Southeast Asia, Australia, and India.

According to the most recent update from the US National Oceanic and Atmospheric Administration, ENSO-neutral conditions are projected to "persist through the Northern Hemisphere's early summer [of] 2023." That is, at least until June. However, "a transition to El Nino is expected by July-September 2023," with its odds "increasing throughout the fall (September-November)."

The Australian Bureau of Meteorology has also predicted "a 50 per cent chance of an El Nino developing later in 2023."

Long-Period Average (LPA)

IMD uses the long-period average (LPA) to determine if the rainfall is normal, below normal, or above normal.

The LPA, according to IMD's website, "is the rainfall recorded over a particular region for a given interval (like a month or season) averaged over a long period like 30 years, 50 years, etc."

Usually, in India, a 50-year LPA covers large variations on either side caused by years of unusually high or low rainfall due to El Nino or La Nina.

India defines average, or normal rainfall as between 96 per cent and 104 per cent of a 50-year average with 88 centimetres (35 inches) of rainfall for the four-month season beginning June.

IMD maintains LPAs for the entire country on a national and local level.

Indian Ocean Dipole (IOD)

It is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia. 

The IOD affects the climate of Australia and other countries that surround the Indian Ocean basin, and is a significant contributor to rainfall variability in this region.

(With agency input)