Global Overturning Circulation 

Context: A team of researchers from the Goa-based National Centre for Polar and Ocean Research and the School of Earth, Ocean, and Atmospheric Sciences in Goa University reconstructed the past deep-water circulations of the Indian Ocean. 

What is Global overturning circulation? 

  • Global overturning circulation — the equatorward transport of cold, deep waters and the poleward transport of warm, near-surface waters — controls ocean heat distribution and atmospheric carbon dioxide levels, thus playing a critical role in global climate.   

About the Research

  • A new study (Nature Communications) by a team of researchers from the Goa-based National Centre for Polar and Ocean Research and the School of Earth, Ocean and Atmospheric Sciences in Goa University has now sort out the issue of deep-water circulation of Indian Ocean. 
  • Studies have indicated that tectonically driven changes in the ocean gateways such as the closure of the Central American Seaway, a body of water that once separated North America from South America, since the late Miocene period, had a dramatic impact on the circulation. 

Indian Ocean Deep water circulation 

  • The Indian Ocean does not have any major deep-water formations of its own. It acts only as a host for NCW and AABW.  
  • The northern parts of the Indian Ocean are located at one of the terminals ends of the GOC, far away from the deep-water formation regions and oceanic seaways.  
  • Few studies have been carried out in the Indian Ocean to reconstruct past deep water circulations based on iron-manganese crust records and authigenic neodymium isotope composition of sediment cores.  
  • But iron-manganese crusts are situated at deeper depths and are bathed only by AABW, making it suitable only for the reconstruction of the history of AABW, and authigenic neodymium isotope records are available only from the Bay of Bengal region. But they too cannot help as the Himalayan rivers that empty into the Bay also bring in substantial amounts of Neodymium particulates. 

Findings of the study 

  • Recent study by the scientists have generated an authigenic neodymium isotope record from the Arabian Sea and reconstructed the DWC record of the Indian Ocean for the period from 11.3 million years ago (Miocene era) to 1.98 million years ago (Pleistocene era).   
  • The record shows a clear shift from the Pacific water dominated deep circulation system before about nine million years ago, to the onset of a modern-like deep water circulation system in the Indian Ocean comprising of Antarctic bottom water and northern component water during the Miocene-Pliocene transition (about six million years ago).  
  • This finding suggests a widespread impact of the late Miocene Central American Seaway closure on the evolution of ocean deep water circulation and validates the so-called Panama Closure Hypothesis. 

What is Panama Closure Hypothesis? 

  • The “Panama Hypothesis” states that the gradual closure of the Panama Seaway, between 13million years ago (13Ma) and 2.6Ma, led to decreased mixing of Atlantic and Pacific water Masses, the formation of North Atlantic Deep water and strengthening of the Atlantic thermohaline circulation, increased temperatures and evaporation in the North Atlantic, increased precipitation in Northern Hemisphere (NH) high latitudes, culminating in the intensification of Northern Hemisphere Glaciation (NHG) during the Pliocene, 3.2–2.7Ma. 
  • Previously it was thought that tectonic changes might have led to the formation of two separate water bodies — northern component water in the North Atlantic and Antarctic Bottom Water (AABW) in the Southern Ocean.  
  • Consequently, it is also hypothesised that there would have been large-scale changes in the Deep-Water Circulation (DWC) in the oceans across the world, thus impacting global climate through ocean-atmosphere carbon dioxide and heat exchanges. 
  • But these formulations have remained untested due to lack of adequate data.  
  • Some records that are available are from near the deep-water formation regions mostly from the Pacific and the Atlantic Oceans.  
  • Hence, they might not necessarily reflect the impact and change in deep water circulation. 
  • But the present study is highly significant since it provides unequivocal evidence in support of the theory that the closing of the gap between North and South America had led to the evolution of the modern form of GOC. 

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