Relevance: GS-1: Important Geophysical phenomena such as earthquakes, tsunamis, volcanic activity, cyclones etc.
Key Phrases: Super Cyclonic Storm Amphan, Tropical Cyclone, Ocean Warming, Intergovernmental Panel on Climate Change (IPCC), Marine Heatwave (MHW), Global Warming, World Meteorological Organisation, Cyclone Fani, Super Cyclone.
Context:
- Super Cyclonic Storm Amphan, a powerful and catastrophic tropical cyclone caused widespread damage in Eastern India, specifically in West Bengal and Odisha, and in Bangladesh, in May 2020.
- It was the strongest tropical cyclone to strike the Ganges Delta since cyclone Sidr of 2007 and the first super cyclonic storm to have formed in the Bay of Bengal since the 1999 Odisha cyclone.
Tropical Cyclone:
- A tropical cyclone is a rapidly rotating storm originating over tropical oceans from where it draws the energy to develop.
- They are irregular wind movements involving the closed circulation of air around a low-pressure centre.
- This closed air circulation (whirling motion) results from the rapid upward movement of the hot air subjected to Coriolis force. The low pressure at the centre is responsible for the wind speeds.
- It has clouds spiralling towards the eyewall surrounding the "eye", the central part of the system where the weather is normally calm and free of clouds.
- Its diameter is typically around 200 to 500 km but can reach 1000 km.
- A tropical cyclone brings very violent winds, torrential rain, high waves and, in some cases, very destructive storm surges and coastal flooding.
- The winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Conditions Favourable for Tropical Cyclone Formation
- Large sea surface with a temperature higher than 27° C.
- Presence of the Coriolis force enough to create a cyclonic vortex
- Small variations in the vertical wind speed
- A pre-existing weak low-pressure area or low-level-cyclonic circulation
- Upper divergence above the sea level system
What is a super cyclone?
- Super cyclone refers to a tropical storm, where wind speeds cross 220kmph (137mph).
What causes Ocean Warming?
- Rising greenhouse gas emission is the primary factor for anthropogenic (human-induced) climate change.
- The increase in carbon dioxide concentration can trap the radiation into the atmosphere and not let it go into space. This increases the average surface air temperature resulting in what we know as global warming.
- As the capacity of the atmosphere to absorb the heat is very less, more than 90% of the extra heat that has been trapped in the climate system has been absorbed by the oceans since 1970, according to IPCC AR5, and IPCC AR6 reports.
Consequences of ocean warming:
- Due to this, oceans are warming globally from the surface to
deeper depths, with severe consequences such as:
- increasing intensity and frequency of extreme events,
- rising sea levels, melting glaciers, and
- changing the weather pattern across the globe.
Vulnerability of Bay of Bengal:
- The Bay of Bengal, home to about 5-7% of the total number of tropical cyclones occurring globally each year, exhibits high sea surface temperatures (about 28°C) throughout the year and is more prone to tropical cyclones, thus making the North Indian Ocean vulnerable to the highest number of fatalities globally.
Marine Heatwave (MHW)
- A marine heatwave (MHW) is a period of abnormally high temperatures relative to the average seasonal temperature in a particular region of a sea or ocean.
- Marine heatwaves are caused by a variety of factors, including shorter-term weather phenomena such as fronts, intra-seasonal, annual, or decadal modes like El Niño events, and longer-term changes like climate change.
- The Intergovernmental Panel on Climate Change (IPCC) Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) finds that it is "virtually certain" that the global ocean has absorbed more than 90% of the excess heat in our climate systems, the rate of ocean warming has doubled, and MHW events have doubled in frequency since 1982.
- This intense warming of the ocean due to MHW has severe socio-economic consequences such as fish mortality, and coral bleaching, and also has the potential to interact and modify other extreme events such as tropical cyclones.
Co-occurrence of Marine Heatwaves (MHWs) and Tropical Cyclone:
- Previous studies have shown that due to global warming, the tropical Indian Ocean, at the surface, is warming at a faster rate as compared to the rest of the global ocean.
- The high sea surface temperatures are more susceptible to generating extreme temperature conditions that persist over days to months and are termed Marine Heatwaves (MHWs).
- The anthropogenic warming of the oceans and atmosphere
facilitates the generation and intensification of extreme events such
as:
- Marine Heatwaves (MHWs) and
- Tropical cyclones.
- Both marine heat waves and tropical cyclones are the extreme events of the ocean-atmosphere coupled system.
- The study, published in the Frontiers in Climate, is the first study conducted in the Indian Ocean that investigates the interaction between a marine heatwave and super cyclone Amphan in the Bay of Bengal in May 2020.
- The co-occurrence of multiple extreme events (e.g., in our case the co-occurring marine heatwave and tropical cyclone) are termed compound extreme events.
Do you know?
- Amphan was the first super cyclone in the Bay of Bengal in the last 21 years and intensified from category 1 (cyclonic storm) to category 5 (super cyclone) in less than 24 hours.
- Amphan was also the costliest tropical cyclone on record in the North Indian Ocean, with reported economic losses of approximately $14 billion in India, according to the World Meteorological Organisation, and 129 casualties across India and Bangladesh.
- According to the latest IPCC report (AR6), Amphan was the largest source of displacement in 2020, with 2.4 million displacements in India alone of which around 8,00,000 were pre-emptively evacuated by the authorities.
Reasons for intensification of cyclone Amphan into super cyclonic:
- The presence of a strong MHW beneath the track of the cyclone with an extremely high anomalous sea surface temperature of more than 2.5°C coincided with the cyclone track and facilitated its rapid intensification in a short period.
- Apart from the surface warming, ocean stratification and warming below the surface also played a crucial role during this phenomenon of compound extreme events.
Comparison of the super cyclone Amphan to cyclone Fani (May 2019):
- The total life span of Amphan over the ocean was five days as compared to Fani which was for seven days but Fani did not turn into a super cyclone as Amphan did.
- The main difference between these two cyclones was the presence of MHW in the case of Amphan, which was not there in the case of Fani.
- Despite short duration and unfavourable atmospheric conditions relative to Fani, Amphan turned into a super cyclone, primarily fuelled by a strong MHW on its way.
Conclusion:
- The recent study along with previous studies also discusses that such compound or individual extreme events are going to increase in the future due to global warming and the Indian Ocean will witness the increased intensity and frequency of such climate extremes.
- Hence, the study provides new perspectives on the interactions between different extreme events that could aid in improving the current understanding of compound extreme events with severe socio-economic consequences in affected countries.
- The study will be very helpful for the political and scientific authorities to make better disaster management and mitigation planning for our vulnerable coastal communities from such extreme events.
Source: The Hindu
Mains Question:
Q. What is a marine heatwave (MHW) and how does it help fuel a tropical cyclone into a super cyclone?
