Sahana Ghosh, Mongabay India
In April-May 2019, extreme severe cyclone Fani, with its 11-day lifespan on ocean and land, became the longest-lived cyclone to spawn in the Bay of Bengal. It rapidly intensified from a cyclonic storm to an extremely severe storm (category 4) in 24 hours. A year later, in the same season, Amphan barrelled into India’s eastern coast with a ferocity that made it the costliest tropical cyclone on record in the north Indian Ocean (the Bay of Bengal and the Arabian Sea).
Amphan, the first super cyclone in 21 years since the 1999 Odisha super cyclone, also rapidly intensified into a super cyclone (category 5) in 24 hours, but it brewed for a relatively shorter period over the ocean compared to Fani, which also travelled along the same region in the Bay of Bengal.
“The fact that despite remaining over the ocean for a longer time (seven days), Fani did not intensify into a super cyclone as Amphan did in a very short period of time prompted us to study Amphan’s rapid intensification,” explains physical oceanographer and climate scientist Saurabh Rathore, lead author of a recent paper that documented the presence of a marine heatwave before and during super cyclone Amphan and the interactions between the two extreme events.
Extreme events in the ocean and atmosphere can meld into compound extreme events. “Extremely high sea surface temperatures before cyclone Amphan created conditions conducive for a marine heatwave. This strong marine heatwave coincided with the cyclone track and intensified the cyclone. Later on, the cyclone dissipated the marine heatwave,” Rathore at LOCEAN-IPSL Sorbonne University, Paris, told Mongabay-India.
“Marine heatwaves and the tropical cyclone are very random in nature, and they can form independently of each other. But in this case, the cyclone needed the high sea surface temperature to form. The marine heatwave also needed a high temperature to cross the threshold to be categorised as a marine heatwave. So the high ocean temperature helped align both the events,” said Rathore.
Subsurface warming also contributed to the interplay between the marine heatwave and the super cyclone. “Various other factors helped sustain the cyclone and marine heatwave. We need to have better predictive capacities for marine heatwaves because when they coincide with other extremes such as cyclones, they can turn out to be big disasters,” added Rathore.
Recent research showed that the Arabian Sea along the west coast of India is experiencing a rapid increase in marine heatwave days in the last decade, sometimes stretching out through a season. In the last four decades, between 1982 and 2019, every decade has seen roughly 20 more marine heatwave days.
The increasing trend of marine heatwave (MHW) days became more noticeable after the year 2000 and is clearly visible after 2015. The accelerated trend of the heatwave days is found to be driven by the rapid rise in the mean sea surface temperature (SST) of the Arabian Sea in the recent decade. More research is needed to flesh out the links between cyclones and marine heatwaves, experts say.
The Arabian Sea has witnessed a rise in cyclone activity linked to the rising ocean temperatures and increased availability of moisture under global warming in the last two decades, according to research by IITM-Pune. Scientists documented a 52% increase in the number of cyclones in the Arabian Sea, while very severe cyclones increased by 150% between 2001-2019.
They also noted an 80% increase in the total duration of cyclones in the Arabian Sea during the last two decades. The duration of very severe cyclones has increased by 260%. In contrast, there’s a slight drop in cyclone frequency in the Bay of Bengal, which traditionally has been a hotbed of cyclone-associated devastation.
Scientists deciphering tropical cyclone characteristics in a changing climate say unpacking features and quantifying behaviour of tropical cyclones are crucial to predict cyclone landfall with sufficient lead time and for coastal protection.
A recent analysis by IIT-Bhubaneswar also finds that the frequency and intensity of tropical cyclones over the Arabian Sea have shot up in the last 30 years (1990-2019) during pre-and post-monsoon seasons.
Overall, the percentage of storms that can hold themselves in a severe cyclonic storm (SCS) and above category for more than four days has increased over both the Arabian Sea (83%) and Bay of Bengal (76%) for both the pre-and post-monsoon in 30 years.
“Tropical cyclones are rapidly intensifying, becoming more severe as well as maintaining their strength for a longer duration in the northern Indian Ocean; they are also bringing in more rainfall,” says climate scientist Sandeep Pattnaik at IIT-Bhubaneswar, co-author of the recent analysis on the northern Indian Ocean cyclones that builds on previous work deciphering cyclone characteristics in a changing climate.
In 2021, when cyclone Tauktae tore across the Gujarat coast in west India along the Arabian Sea, remnants of the cyclone also deluged Delhi, India’s capital, with record rainfall rendering that May the rainiest in 13 years. Along with Tauktae, cyclone Yaas which rammed into India’s eastern coast on the Bay of Bengal, brought above-normal rain to Nepal in the premonsoon season, ending months of drought.
Marked as a very severe cyclonic storm, Yaas was a relatively weaker storm than 2019’s Fani, an extremely severe cyclonic storm, but it unleashed more rainfall. Other examples of challenging cyclones in recent years are cyclones Jawad, Asani in the Bay of Bengal and Nisarga in the Arabian Sea.
Pattnaik’s analysis used 60-year India Meteorological Department (IMD) and IBTrACS (International Best Track Archive for Climate Stewardship) dataset for the Bay of Bengal and the Arabian Sea cyclones across pre and post-monsoon seasons. The team finds that tropical cyclones in the Arabian Sea have a longer lifespan (6-7 days) compared to those that swirl over the Bay of Bengal (5-6 days).
“It is also interesting to note that tropical cyclones in Bay of Bengal lasting more than 4-5 days in severe cyclonic storm or above stage made frequent landfall over India due to the absence of high-pressure region. In addition, key atmospheric-ocean mechanisms favour more violent land-falling TCs formed over the Bay of Bengal compared to the Arabian Sea.
Forecasting agencies can tap into the findings to augment their understanding for better prediction of the track, duration of the storm, intensity, rainfall, and steering currents which will, in turn, lead to better preparedness to minimise losses, Pattnaik said.
He added that multiple factors such as an increase in air and ocean surface temperature boost the duration (longevity) and strength of cyclones. Effective atmosphere-ocean interactions are also contributing factors.
Ocean science expert Prasad Bhaskaran says storms with more longevity and slower translation speed (average moving speed of tropical cyclones making landfall) can directly impact the storm surge in coastal and near-shore environments.
“Storms that move slowly over an area can result in more damage than faster-moving cyclones,” added Bhaskaran at IIT-Kharagpur, referring to a co-authored paper published in 2020 that looked into track lengths (the distance in kilometers from the depression stage to landfall) of 185 tropical cyclones over the northern Indian Ocean between 1982 and 2018 using IMD datasets.
Their investigation showed that for all tropical cyclone stages starting from Deep Depression to Super Cyclonic Storm, the track length decreased by 13%, and the time that the cyclone spends over the ocean (residence time) increased by 7.3%.
But high-intensity tropical cyclones (SCS) showed an increase in both track length and residence time in the recent decade, 2009 to 2018, which can increase the chances for rapid intensification.
“We also found that the translation speed of cyclones making landfall in the Bay of Bengal dropped by 2.5 kilometers per hour in the past three decades. When a tropical cyclone moves slowly, it spends a long time passing over the region and the region is exposed to the cyclone’s effects for a longer time. So the drop in translation speed, together with the increase in the longevity of cyclones, is a concern that we have flagged in our research,” Bhaskaran told Mongabay-India.
Bhaskaran adds unravelling the reasons for the rapid intensification of cyclones is a research gap. “We need to work on this because the models are not able to capture the rapid intensification. The IMD prediction failed for Tauktae when it rapidly intensified and made landfall in Gujarat,” added Bhaskaran.
“One of our analyses also shows that relatively weaker tropical cyclones like Yaas can still produce heavy rain and devastation,” says Pattnaik, adding that storms such as Yaas have a better precipitation efficiency than others–they have a better ability to convert available moisture and hydrometeors (any form of water, liquid or solid, present in the atmosphere, excluding clouds) to rainfall.
“Yaas will be remembered purely because of its extreme amount of rainfall in the West Bengal-Odisha border region, and Fani, a compact storm, will be remembered for the wind impact.”
The timing and location of the origin of the pre-monsoon cyclone shape rainfall. “When the cyclone is forming just before the monsoon, as was the case with Yaas, a lot of moisture will be brought into the cyclone system because they will suck a lot of moisture from the south-westerly flow. So you have more moisture which is likely leading to better rainfall efficiency, which is likely translating into more rainfall,” explains Pattnaik. “If you know a particular storm’s precipitation efficiency, that can offer a clue to operational agencies to predict if a storm will give more or less rainfall.”
Former IMD Director KJ Ramesh recalls that forecasting Fani also enabled the agency to use all tools at its disposal and work in tandem with Indian National Center for Ocean Information Services for tapping into ocean data.
“Earlier, we factored in atmospheric interactions in cyclone forecasts, but cyclone Okchi prompted the use of the dynamic atmosphere-ocean coupled model, which proved extremely useful in Fani. Going ahead, we will need to consider land surface interactions as well because what we have observed in recent years is that cyclones are not weakening after landfall- they pick up moisture over land and get another lease of life,” Ramesh told Mongabay-India.
This article was first published in Mongabay India.
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