A new study finds rapid freshening in the Southern Indian Ocean due to warming-driven wind shifts moving water from the Indo-Pacific Freshwater Pool. This creates stratification that traps heat, blocks nutrients, harms fisheries, may weaken AMOC, and heighten cyclones and marine heatwaves.
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Picture Courtesy: DOWNTOEARTH
Context
A study in Nature Climate Change highlights a rapid decrease in salinity in the Southern Indian Ocean, a region historically known for its high salt content.
What are the Key Findings of the Study?
Rapid Salinity Decline: High-salinity area in the Southern Indian Ocean has shrunk by 30% in 60 years—the most rapid freshening observed in the Southern Hemisphere.
Massive Freshwater Influx: Equivalent to adding about 90 cubic kilometers of freshwater annually.
Reversal of Natural Processes: Anthropogenic climate change are powerful enough to override the region's natural tendency toward high salinity.
What is the Mechanism Behind this "Freshening"?
The primary cause of this freshening is not increased local rainfall but a shift in ocean dynamics. Global warming has altered wind patterns, which in turn affects ocean currents.
The most direct physical consequence of the freshening is increased ocean stratification—the formation of distinct, non-mixing layers of water. Freshwater is less dense than saltwater, so it forms a stable layer on top, acting like a lid.
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Consequences of Stratification
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How Does this Impact Global Ocean Circulation?
Water from the Southern Indian Ocean region flows towards the Atlantic and reaches the North Atlantic. There, it cools, becomes dense, and sinks, driving the Atlantic Meridional Overturning Circulation (AMOC).
The Risk: If the water arriving in the North Atlantic is too fresh (less dense), it will not sink effectively. This could slow down or even collapse the AMOC.
Global Implication: A slowdown of the AMOC would alter global weather, including the Indian Monsoon.
Implications for India
Cyclone Intensification
The surface heat trapped by stratification acts as fuel for cyclones. The Arabian Sea has seen a 52% rise in cyclone frequency since 2001. (Source: Indian Institute of Tropical Meteorology)
Threat to Blue Economy
Stratification blocks nutrients, leading to a decline in phytoplankton and fish catch. This impacts the livelihoods of millions of people in coastal states.
Monsoon Variability
Changes in ocean temperature and salinity gradients can disrupt the Indian Ocean Dipole (IOD), leading to erratic monsoon patterns that threaten India's agriculture-based economy.
Way Forward For India
Enhanced Monitoring: Strengthen and expand O-SMART scheme to better track salinity, temperature, and mixing in the Indian Ocean.
Global Climate Action: Uphold commitments under the Paris Agreement to limit global warming to 1.5°C, as this is the root cause of the wind pattern shifts.
Sustainable Blue Economy: Implement "climate-smart" fisheries management to adapt to changing nutrient availability and fish migration patterns.
International Collaboration: Utilize platforms like the Quad Climate Working Group for data sharing and collaborative research on Indo-Pacific oceanography.
Conclusion
The freshening of the Southern Indian Ocean signals major climate change, threatening global climate stability, Indian economy, and food security by disrupting ocean systems.
Source: DOWNTOEARTH
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PRACTICE QUESTION Q. Consider the following statements regarding Ocean Stratification: 1. It promotes the vertical mixing of ocean layers. 2. It is caused when less dense freshwater forms a layer over denser saltwater. 3. It prevents nutrient-rich deep water from reaching the surface. Which of the statements given above is/are correct? A) 1 only B) 2 and 3 only C) 1 and 3 only D) 1, 2, and 3 Answer: B Explanation: Statement 1 is incorrect: Stratification restricts or reduces vertical mixing rather than promoting it. It creates a "layered" effect that is resistant to the movement of water between the surface and the deep ocean. Statement 2 is correct: Ocean stratification occurs when water masses with different properties (density, temperature, salinity) form distinct layers. A common cause is when less dense freshwater (from rain, melting ice, or rivers) floats on top of denser, saltier water. Statement 3 is correct: These stable layers act as a physical barrier. Because the denser water stays at the bottom, stratification prevents nutrient-rich deep water from rising to the surface (upwelling), which can limit the growth of phytoplankton. |
Ocean freshening refers to a decrease in the salinity (salt content) of seawater. This usually happens due to melting ice or heavy rain, but in the Southern Indian Ocean, it is currently being caused by the movement of fresh water from the tropics due to shifting winds.
Global warming has altered surface wind patterns. These winds are pushing a massive pool of fresh water from the Indo-Pacific region southward into the Southern Indian Ocean, diluting its historically high salinity.
Stratification is the separation of ocean water into distinct layers that do not mix. A layer of fresh, light water sits on top of salty, dense water. This acts as a barrier, preventing nutrients from coming up to feed marine life and trapping heat at the surface, which worsens global warming impacts.
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