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 Indian Ocean circulation.

What is Global overturning circulation (GOC)?

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

Concept: Panama Closure Hypothesis

• This news essentially talks about the Panama Closure Hypothesis.
• Panama Hypothesis states that the gradual closure of the Panama Seaway, between 13 million years ago (13 Ma) and 2.6 Ma, led to decreased mixing of Atlantic and Pacific water Masses.
• This led to the formation of North Atlantic Deep water circulation.
• It strengthened the Atlantic thermohaline circulation, increased temperatures and evaporation in the North Atlantic, increased precipitation in Northern Hemisphere high latitudes.

Impact of Panama closure

• It is 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.

Impact on Indian Ocean gyre

• The Indian Ocean does not have any major deep-water formations of its own.
• It acts only as a host for NCW and AABW.
• Further, the northern parts of the Indian Ocean are located at one of the terminal ends of the GOC, far away from the deep-water formation regions and oceanic seaways.

What has the new research found?

• 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.
• It comprises of Antarctic bottom water and northern component water during the Miocene-Pliocene transition (about six million years ago).
• This 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.

Back2Basics: Indian Ocean Circulation

• The Indian Ocean circulation/gyre, located in the Indian Ocean, is one of the five major oceanic gyres, large systems of rotating ocean currents, which together form the backbone of the global conveyor belt.
• The Indian Ocean gyre is composed of two major currents: the South Equatorial Current, and the West Australian Current.
• Normally moving counter-clockwise, in the winter the Indian Ocean gyre reverses direction due to the seasonal winds of the South Asian Monsoon.

How does it function?

• In the summer, the land is warmer than the ocean, so surface winds blow from the ocean to the land.
• However, during the winter, these temperatures reverse, making the winds blow from the land to the ocean.
• Because most of the air pressure gradient is retained behind the Tibetan plateau, air pressure gradients over the Indian Ocean and the gyre are small.
• This results in winds of moderate strength, due to the protection from the full-force winds blowing off the Mongolian high-pressure region.
• Because of these moderate, dry winds, the Winter Monsoon season in the Indian Ocean region is the dry season for most of Southern Asia.
• Due to this seasonal wind cycle, the currents of the Indian Ocean, which make up the Indian Ocean gyre, are directly affected, causing reversal.

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