Marine heatwave fuelled super cyclone Amphan

Subject :Geography

Section: Climatology

Context:

A 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.

Marine heatwaves

The high sea surface temperatures are more susceptible to generating extreme temperature conditions that persist over days to months and are termed as Marine Heat- waves (MHWs).
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.

Relation between Marine heatwaves and tropical cyclones:

The anthropogenic warming of the oceans and atmosphere facilitates the generation and intensification of extreme events such as MHWs and tropical cyclones.
The 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.
Both marine heat waves and tropical cyclones are the extreme events of the ocean-atmosphere coupled system.
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.

Sea surface temperature in Bay of Bengal:

The Bay of Bengal exhibits high sea surface temperatures (about 28°C) through-out the year and is more prone to tropical cyclones.
The Bay of Bengal is home to about 5-7% of the total number of tropical cyclones occurring globally each year and this makes the North Indian Ocean vulnerable to the highest number of fatalities globally.
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.

Factors responsible of Super Cyclone Amphan

It was found 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 that coincided with the cyclone track and facilitated its rapid intensification in a short period.
The study has compared the super cyclone Amphan to a previous extremely severe cyclone Fani in May 2019 with a near similar trajectory.
The total life span of Amphan over the ocean was five days as com- pared 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.
The study also inferred that 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.
Apart from the surface warming, the study also shows that ocean stratification and warming below the surface also play a crucial role during this phenomenon of compound extreme events.