Marine Heat Waves and Twilight Zone | 23 Oct 2024

Source: IE

Why in News? 

Recently, in a study researchers investigated Marine Heatwaves (MHWs) and cold spells within the largely underexplored twilight zone of the ocean. 

  • A cold spell refers to a period of unusually cold weather, often lasting several days or longer.  

Marine Heat Waves (MHWs) 

  • An MHW is an extreme weather event. It occurs when the surface temperature of a particular region of the sea rises to 3 or 4 degree Celsius above the average temperature for at least five days. MHWs can last for weeks, months or even years. 
  • These events are linked to coral bleaching, seagrass destruction, and loss of kelp forests, affecting the fisheries sector adversely. 
  • The most common drivers of marine heatwaves include ocean currents which can build up areas of warm water and air-sea heat flux or warming through the ocean surface from the atmosphere. 
  • Winds can enhance or suppress the warming in a marine heatwave, and climate modes like El Niño can change the likelihood of events occurring in certain regions. 

What are Key Findings Related to MHWs? 

  • Marine heatwaves (MHWs) occurring in the deep ocean are under-reported. 
    • To monitor temperature changes at great depths, specialized buoys suspended in the ocean were deployed globally for extended data collection.  
    • Argo floats, robotic devices that can dive to 2,000 meters and resurface, were used to gather crucial temperature and salinity data. 
  • Global warming is affecting surface ocean temperatures, while the impact on deep ocean waters reveals the deeper, less understood consequences of climate change on oceanic ecosystems. 
  • Unlike surface-level marine heatwaves, atmospheric factors do not influence temperature changes in the deep ocean.  
    • Instead, eddy currents, which are large, swirling loops of water that can extend hundreds of kilometers and reach depths over 1,000 meters, play a crucial role in transporting warm or cold water across vast distances. 
    • The overall ocean warming is leading to stronger eddy currents, amplifying these extreme temperature changes. 
  • MHW impact on Biodiversity: 
    • Extreme temperature shifts in the twilight zone are alarming due to the presence of many fish species and plankton, which are vital to the oceanic food chain and serve as a key food source for small fish. 
    • MHW can reduce oxygen levels and deplete nutrients in the water, endangering marine life and the balance of the ecosystem. 

Twilight Zone in the Ocean 

  • The twilight zone, also known as the mesopelagic or dysphotic zone, is a layer of the ocean that extends from 200 to 1,000 meters below the ocean's surface. 
  • It is a vast ecosystem inhabited by extraordinary creatures, including organisms that live in darkness and those that migrate to and from the surface. 
  • It plays a vital role in the ocean's ability to absorb and store carbon dioxide from the atmosphere. This region also hosts the largest animal migration on Earth. 
  • Organisms in the twilight zone have adapted in several ways, including bioluminescence (used for camouflage) and large mouths (help maximize rare encounters with prey in the dark). 

Eddy Currents 

  • An eddy current in the ocean is a small, circular water current that detaches from the main current and moves independently. 
    • They are formed when segments of a current pinch off, similar to small whirlpools seen in rivers while canoeing. 
  • Eddies can span hundreds of kilometers and extend to depths exceeding 1,000 meters. They can be detected from space using infrared sensors. 
  • They are responsible for several developments. 
    • Cold-core eddies trap nutrient-rich water and transport it along with plankton. 
      • Eddies can create swirling patterns in phytoplankton blooms. 
    • It can displace warm surface water downward or lift cold water upward, leading to rapid temperature changes. 

UPSC Civil Services Examination, Previous Year Question (PYQ) 

Q. With reference to Ocean Mean Temperature (OMT), which of the following statements is/are correct? (2020)

  1. OMT is measured up to a depth of 26ºC isotherm which is 129 meters in the south-western Indian Ocean during January-March.
  2. OMT collected during January-March can be used in assessing whether the amount of rainfall in monsoon will be less or more than a certain long term mean.

Select the correct answer using the code given below:

(a) 1 only  

(b) 2 only 

(c) Both 1 and 2  

(d) Neither 1 nor 2

Ans: (b)