Explained: How rocks in Meghalaya cave connect Northeast monsoon to El Niño

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Mains Paper 1: Geography | Salient features of World’s Physical Geography

From UPSC perspective, the following things are important:

Prelims level: El Nino, Karst Topography

Mains level: Impact of El-Nino


News

A new study led by researchers from a US university, on the rock formations in a cave near Cherrapunji in Meghalaya, has found new evidence to suggest that India’s winter rainfall are influenced by the state of the ocean waters in the faraway Pacific.

El Niño & monsoons

  • India’s summer monsoon which brings in about 70% of annual rainfall in the country, is already known to be heavily influenced by the variability in sea-surface temperatures of Pacific Ocean.
  • This is a condition referred to as El Niño Southern Oscillation (ENSO).
  • A warmer than usual Pacific Ocean, off the coast of South America, is known to suppress the monsoon rainfall in India.

Connection with retreating Monsoon

  • This relationship is not so strongly established with the winter monsoon, also called as the northeastern monsoon, which occurs during the months of October, November and December.
  • It is vital for several regions in the Northeast and India’s eastern coast.
  • More than 50% of the annual rains in coastal Andhra Pradesh, Rayalaseema, Tamil Nadu, south interior Karnataka, and Kerala comes during these winter months.
  • ENSO is known to have an impact on the winter monsoon as well but is weaker and opposite.
  • The warming of sea-surface waters, for example, is seen to help winter rainfall rather than suppressing it.
  • The impact varies in time and space. The influence is weaker in October and stronger in November and December.

Deducing monsoon variations from stalagmites

  • The latest study claims to have found new evidence to suggest that the state of Pacific Ocean do indeed impact the winter rains.
  • Their findings are based on more than three years of research on stalagmites (mineral deposits, mainly limestone, in caves) of the Mawmluh Cave, near Cherrapunji, in the East Khasi Hills district.
  • These solid stalagmite structures, or mineral deposits, are the result of slow but steady water dripping in the caves, and contain several thin layers of different kinds of minerals that get picked up while the water is flowing.
  • From a careful study of the composition of these stalagmites, scientists can deduce the amount of rainfall that could have happened over the caves in the past, or even whether the water was a result of local rainfall, or had flown in from a different place.
  • Using such techniques, the researchers in this case were able to estimate local variations in rainfall in the past, and then correlate it with old ocean records of the Pacific Ocean.

Stalagmites revives drought history

  • The stalagmites indicate the recurrence of intense, multi-year droughts in India over the last several thousand years.
  • Stalagmite records from monsoon regions, including India, are vital to understanding past variability in the global climate system and the underlying reasons for this variability.

Back2Basics

Karst Topography

  • Karst is a landscape which is underlain by limestone which has been eroded by dissolution, producing towers, fissures, sinkholes, etc.
  • Karst topography is a landscape formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum.
  • It is characterized by underground drainage systems with sinkholes, caves etc.

Stalactite and Stalagmite

  • The water containing limestone in solution, seeps through the roof in the form of a continuous chain of drops.
  • A portion of the roof hangs on the roof and on evaporation of water, a small deposit of limestone is left behind contributing to the formation of a stalactite, growing downwards from the roof.
  • The remaining portion of the drop falls to the floor. This also evaporates, leaving behind a small deposit of limestone aiding the formation of a stalagmite, thicker and flatter, rising upwards from the floor.
  • Sometimes, stalactite and stalagmite join together to form a complete pillar known as the column.
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