How an IMD discovery in 1920s changed monsoon forecasts, Pg13

Key Highlights:

• In the early 1920s, Sir Gilbert Walker, then head of the India Meteorological Department (IMD), discovered the Southern Oscillation (SO), a seesaw in atmospheric pressure across the Pacific Ocean.
• Walker's discovery was a significant step in understanding external influences on the Indian monsoon, moving beyond purely statistical correlations.
• Decades later, Norwegian-American meteorologist Jacob Bjerknes linked the Southern Oscillation to the periodic warming and cooling of the equatorial Pacific Ocean, known as El Niño, in the late 1960s.
• This connection established the El Niño Southern Oscillation (ENSO) as a coupled ocean-atmosphere phenomenon, profoundly impacting global weather patterns.
• The IMD temporarily ceased providing all-India monsoon forecasts between 1932 and 1988 due to forecasting challenges, resuming them with quantitative elements in 1988.

Detailed Insights:

• El Niño Southern Oscillation (ENSO) is a major driver of year-on-year variations in global climate, influencing weather events worldwide, including the Indian monsoon.
• Sir Gilbert Walker, who served as Director General of Observatories at IMD from 1904 to 1924, meticulously analyzed weather data to identify large-scale atmospheric oscillations.
• Besides the Southern Oscillation, Walker also identified the North Atlantic Oscillation and North Pacific Oscillation, but found the SO most persistent and impactful on the Indian monsoon.
• Jacob Bjerknes demonstrated that El Niño (ocean warming) and the Southern Oscillation (atmospheric pressure changes) form a feedback loop, where one reinforces the other.
• The El Niño phase involves warmer-than-normal equatorial Pacific waters, generally suppressing rainfall over India and Southeast Asia, while La Niña brings cooler waters and opposing impacts.
• Before Walker's work, monsoon forecasts, initiated by IMD in 1877 after the 1876-78 Great Famine, relied heavily on statistical correlations.
• Despite the ENSO link, monsoon forecasting remained complex; IMD's forecasts were limited to northwest India and the peninsular region for a long period, using qualitative terms like "slightly above normal."
• Since 1988, IMD has significantly enhanced its computational abilities, observational networks, and prediction models, leading to improved long-range forecasts, though new challenges like extreme weather events persist.

Scientific/Technical Concepts Involved:

• El Niño Southern Oscillation (ENSO): A large-scale climate phenomenon involving periodic warming (El Niño) and cooling (La Niña) of sea surface temperatures in the equatorial Pacific Ocean, coupled with atmospheric pressure changes.
• Southern Oscillation (SO): The atmospheric component of ENSO, characterized by a seesaw pattern of atmospheric pressure between the eastern and western tropical Pacific Ocean.
• El Niño: The warm phase of ENSO, marked by warmer-than-average sea surface temperatures in the central and eastern equatorial Pacific, impacting global weather.
• La Niña: The cool phase of ENSO, characterized by cooler-than-average sea surface temperatures in the central and eastern equatorial Pacific, generally having opposite effects to El Niño.