The Slowdown in Overturning Circulation

Why in News?

Recently, Deep Ocean currents in Antarctica are slowing down earlier than predicted, potentially disrupting the crucial overturning circulation.

• The slowdown in circulation and declining oxygen levels in the deep ocean have been observed, indicating significant changes in the Antarctic deep ocean over the past three decades.
• The consequences of this phenomenon are further underscored by the impacts of melting Antarctic ice on the overturning circulation.

What is Overturning Circulation?

• About:
  • Overturning circulation refers to the global network of ocean currents that redistribute heat, carbon, and nutrients across the world's oceans.
  • In Antarctica, it involves the sinking of dense, oxygen-rich water from the surface, its spread along the sea floor, and slow rise in distant regions.
• Process:
  • In polar regions, the surface water cools due to low temperatures and exposure to frigid air masses.
  • The cooling leads to the formation of sea ice, which extracts freshwater from the surrounding seawater. This process increases the salinity and density of the remaining water.
  • The high salinity and density cause the surface water to become denser, making it more likely to sink.
    • The dense water sinks to deeper layers, forming what is known as bottom water.
  • The sinking of dense water drives the deeper limb of the overturning circulation. It flows towards the equator, while at the same time, warmer surface water from lower latitudes moves towards the poles.
  • As the deep-water travels, it gradually mixes with surrounding water masses, exchanging heat, carbon, and nutrients. Eventually, the upwelling of this modified water occurs in other regions, completing the overturning circulation.
• Importance:
  • Overturning circulation plays a crucial role in maintaining climate stability on Earth.
  • It facilitates the transport of heat, carbon, and nutrients, influencing the planet's climate system.
  • Additionally, it ensures the supply of oxygen to the deep ocean, supporting marine life and its ecosystems.
• Impact of Slowdown in Overturning Circulation:
  • The observed slowdown of deep ocean currents in Antarctica, occurring earlier than anticipated, raises concerns about climate stability.
  • A reduced flow of bottom water results in a decline in oxygen supply to the deep ocean, impacting oxygen-dependent organisms.
  • Lower oxygen levels may lead to behavioral changes, migrations, and disruptions in the marine food chain.
    • Moreover, the slowdown amplifies global warming as the ocean's capacity to store carbon dioxide and heat diminishes, intensifying the greenhouse effect.
• Melting Antarctic Ice and its Contribution:
  • Melting Antarctic ice disrupts the formation of Antarctic bottom water, making surface waters fresher and less dense, impeding their sinking.
    • This disruption weakens the overturning circulation, further diminishing the oxygen supply to the deep ocean.
  • The replacement of bottom water with warmer, oxygen-depleted waters exacerbates the decline in oxygen levels.
  • Additionally, melting ice contributes to rising sea levels through thermal expansion as warmer water occupies more space.

Source: TH