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Source: Hindu

Context

A new study calls into question the notion that tropical rain always causes the ocean surface to become more buoyant (float), despite the fact that freshwater is less dense than seawater.  It demonstrates that rain can occasionally boost surface stability rather than promote mixing.

About Ocean Surface Buoyancy

• Traditionally, it has been believed that rainfall over oceans introduces freshwater to the surface, which, being less dense than saltwater, causes the upper ocean layer to become lighter. 
• This process results in positive buoyancy flux, enhancing vertical mixing. 
• Such mixing is crucial for ocean-atmosphere energy exchange and upwelling of nutrients, especially in the tropics, supporting marine biodiversity and influencing weather patterns like monsoons.
• However, a new study challenges this conventional notion. It reveals that in certain conditions, rainfall may stabilize the ocean surface rather than mix it.

What is Buoyancy?

Buoyancy is the upward force exerted by a fluid, such as water, on an object that is submerged in it. This force determines whether the object will float, sink, or remain suspended within the fluid.

Buoyancy in Oceans:

Buoyancy in oceans refers to the tendency of water masses to rise or sink depending on their density. This density is affected by temperature and salinity. Warmer or fresher water is less dense and tends to stay near the surface, while colder or saltier water is denser and tends to sink.

The vertical movement of these water layers creates stability or instability in the ocean:

• When lighter (less dense) water lies above heavier water, the water column is unstable, leading to vertical mixing.
  
  
• When heavier (more dense) water lies above lighter water, the water column is stable, and mixing is suppressed.
  

This concept is crucial for understanding ocean circulation, climate regulation, and marine ecosystems.

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Source: Woods Hole 

Buoyancy Flux:

Buoyancy flux is a measure of the rate at which buoyancy (the tendency of a fluid parcel to rise or sink due to density differences) is added to or removed from a fluid, typically at the ocean or atmosphere's surface.

In the context of the ocean, buoyancy flux helps describe how external forces like heat exchange or freshwater input (from rainfall or melting ice) affect the density of surface water and therefore influence vertical mixing.

• A positive buoyancy flux occurs when lighter water is added to the surface (e.g., due to rainfall or heating), making the water column less dense and more unstable, which promotes mixing.
  
  
• A negative buoyancy flux happens when the surface water becomes denser (e.g., due to cooling or evaporation), which stabilizes the water column and inhibits mixing.
  

In simple terms, buoyancy flux indicates how strongly and in what direction density-driven mixing is encouraged or suppressed in oceans or the atmosphere. It is crucial in understanding climate dynamics, ocean circulation, and weather patterns.

Heavy Rain and the Role of Cold Pools 

• According to one study, after heavy rainfall, the ocean surface becomes heavier and more stable due to cold pools (cold, dry air accompanying rain clouds), which cool the surface by obscuring sunlight and transferring heat from the ocean to the atmosphere.
• This method reduces ocean water mixing, resulting in more stable circumstances.

Key Findings of the New Study

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Source: Nature

Data & Methodology

• Researchers used data from 22 buoys located across equatorial oceans.
  
  
• Parameters recorded: rainfall, sea surface temperature (SST), wind speed, and heat flux.
  
  
• Analysis covered over 31,000 hours of rainfall events.
  
  
• Key metric studied: Buoyancy Flux – combines effects of heat exchange and freshwater input.
  
  

Key Findings

• Light Rain (0.2–4 mm/hr):
  
  
• Positive buoyancy flux observed.
  
  
• Result: Surface destabilization, promoting vertical mixing.
  
  
• Heavy Rain:
  
  
• Negative buoyancy flux prevalent.
  
  
• Reason: Strong cold pools and greater heat loss from ocean.
  
  
• Result: Surface stabilization, suppressing mixing.
  
  
• Diurnal Variation:
  
  
• Rainfall causes more instability at night than during the day.
  
  
• Geographic Zones Identified:
  
  
• Cold Rain Zones: Western Pacific & Indian Ocean – significant heat loss.
  
  
• Hot Rain Zones: Central Pacific – less heat loss.
  
  
• Role of Atmospheric Downdrafts:
  
  
• Downdrafts affect SST and buoyancy changes, influencing mixing behavior.
  

Significance of the Study

• Scientific Implications: Disputes the common idea that rain always increases buoyancy.
• Rain can either stabilize or destabilize the ocean surface, depending on the conditions.
• Climate Relevance: Ocean mixing facilitates heat, carbon, and nitrogen cycle.
• Misinterpreting rainfall's function can distort climate and weather models.
• Practical impact: Improves predicting accuracy in oceanography and climate research.
• This helps to better comprehend the climate-ocean feedback cycle.

Source: The Hindu