Marine Heatwaves (UPSC Geography Notes)

Scientists discovered that sea surface temperatures (SSTs) in the Pacific Ocean off the western coast of the United States had been unusually high. This 6.5 million square kilometre marine heatwave (MHW) has the potential to destroy marine life and induce droughts in the surrounding areas. This has an influence on the Indian Subcontinent as well. In this article, you will learn about what Marine Heatwaves are, their causes, impacts, how to measure these Marine Heatwaves, and some details concerning research done on the Indian Ocean.

Marine Heatwaves is found under GS Paper I Geography syllabus of the UPSC IAS Exam. So read in detail to excel in the exams.

What are marine heatwaves ?

• A marine heat wave is often characterised as a coherent region of extremely warm sea surface temperature that lasts for days to months. Marine heat waves occur when sea temperatures are higher than usual for a lengthy period of time.
• Over the last decade, MHWs have been recorded in all major ocean basins. These occurrences are connected to coral bleaching, seagrass damage, and kelp forest loss, all of which have a negative impact on the fishing industry.
• According to a study, during the May 2020 maritime heatwave, 85 percent of the corals in the Gulf of Mannar on the Tamil Nadu coast bleached.
• Example of major marine heatwave occurrences are:
  • Great Barrier Reef in 2002
  • Mediterranean in 2003.
  • Northwest Atlantic, 2012.
  • The Northeast Pacific hurricanes of 2013-2016 had a significant and long-term influence on the oceanographic and biological conditions in those locations.

Also read about Mitigating Risk of Heat Waves in the linked article.

Learn about Coral Reefs by clicking here.

What are the causes of marine heatwaves?

1. Summer/ Winters – Heatwaves can occur in both the summer and the winter, when they are referred to as “winter warm-spells.” These winter occurrences can have serious repercussions, such as in Australia’s southeast, where the spiny sea urchin can only colonise farther south when winter temperatures top 12 °C.
2. Ocean Currents – The most common source of marine heat waves is ocean currents, which may build up patches of warm water and air-sea heat flux, or warming via the ocean surface from the atmosphere.
3. Winds – Sunlight normally penetrates the atmosphere and heats the ocean’s surface. When there are light winds, the warm water does not mix with the cooler water below. It floats on the surface of the water and continues to heat up, resulting in marine heat waves.
4. Climate Change – Large-scale climate influences such as the El Nino Southern Oscillation (ENSO) can too cause marine heatwaves.

Also read about La-Nina in the linked article.

What are the impacts of marine heatwaves?

• Habitat Destruction – Marine heatwaves can alter the habitat ranges of some species, such as the spiny sea urchin off the coast of southeastern Australia, which has been moving southward towards Tasmania at the expense of the kelp forests on which it feeds.
• Loss of Biodiversity – Marine heatwaves can have a significant impact on biodiversity. Marine heatwaves in northern Australia caused widespread bleaching of the Great Barrier Reef in 2016.
• Deoxygenation and Acidification – Marine heatwaves frequently occur in conjunction with other factors such ocean acidification, deoxygenation, and overfishing. In such instances, Marine Heatwaves not only wreak more havoc on ecosystems, but they also raise the danger of deoxygenation and acidification.
• Economic Loss – Marine heatwaves can result in economic losses due to their effects on fisheries and aquaculture. The abalone harvest in Western Australia’s north was damaged by a maritime heatwave in 2011.
• Alter the Ecosystem Structure – Marine heat waves influence ecosystem structure by promoting certain species and inhibiting others. It has been linked to mass death of marine invertebrates and may drive animals to shift their behaviour, putting them at greater danger of injury.

How to measure marine heatwaves?

• A marine heatwave occurs when seawater temperatures reach a seasonally variable threshold (often the 90th percentile) for at least 5 days in a row. Heatwaves that occur within two days of each other are considered to be part of the same event.

The following properties of maritime heatwaves are also used to predict their occurrence:

• Duration: the number of days it takes from the start date to the finish date.
• Annual frequency: the number of occurrences that occur in a given year.
• Intensity: the temperature anomaly above the climatology, in degrees Celsius, indicating how much warmer than usual for that time of year it is, and this is either a maximum number or an average for the whole event.
• Cumulative intensity: the integral of intensity throughout the course of the event, comparable to degree heating days or degree heating weeks.
• Annual marine heatwave days: the number of days in a year that were affected by a heatwave.

A consequence of a heatwave is the Urban Island Heat effect. To know how both go hand-in-hand with each other, visit the linked article.

What the study says on Marine Heatwaves in the Indian Ocean?

This is the first study to demonstrate a direct link between marine heatwaves and atmospheric circulation and rainfall. The prevalence and consequences of heatwaves have been observed throughout the world’s waters, but their impact in the tropical Indian Ocean is little known. The following are some of the study’s findings:

• The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) predicts that if global warming hits 1.5°C to 2°C, sea surface temperature throughout the Indian Ocean would rise by 1° to 2 °C.
• Maritime heatwaves in the Western Indian Ocean and the Bay of Bengal aggravated the arid conditions across the central Indian subcontinent.
• The Western Indian Ocean region had the largest increase in marine heatwaves, with around 1.5 events per decade, followed by the north Bay of Bengal, with 0.5 events per decade.
• Rainfall has surged dramatically over south peninsular India in response to the heatwaves in the north Bay of Bengal.
• Between 1982 and 2018, the Western Indian Ocean witnessed 66 events, while the Bay of Bengal saw 94. These changes are the effect of heatwaves altering the monsoon winds.

Conclusion

The frequency, severity, and regions under the influence of the marine heatwaves are increasing year after year, it becomes extremely important to improve ocean observational arrays to reliably monitor these phenomena, as well as the updating of our weather models to successfully predict the challenges posed by a warming world. Many parties, including researchers, legislators, environmentalists, the business sector (aquaculture, fisheries, ecotourism), and civil society, must work together to find effective solutions to these marine heatwaves. Local management authorities should raise awareness among all stakeholders and develop forecasting tools in order to achieve a coordinated response. National and subnational governments should create and implement policies to protect communities and increase regional ocean resilience.

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