The growing intensity of Cyclone Mocha, expected to make landfall on Sunday, is another reminder of how cyclones nowadays can retain their energy for many days consistently, which is a result of warming oceans, experts say
The formation of Cyclone Mocha over the Southeast Bay of Bengal has marked the arrival of the cyclone season. Atmospheric conditions are very conducive for cyclone Mocha to intensify further. According to the India Meteorology Department, Mocha is likely to cross southeast Bangladesh and north Myanmar coasts between Cox’s Bazar, Bangladesh and Kyaukpyu, Myanmar, by noon of May 14, 2023 as a very severe cyclonic storm.
The formation of a tropical cyclone is very timely as May is the peak month for cyclogenesis in the Indian Ocean. However, the rapid intensification of these storms is concerning. Researchers and scientists are pointing to the increasing global mean temperature behind the changes in the cyclogenesis, particularly over the Indian Ocean.
Fertile ground for rapid intensification
According to a study ‘Changing status of tropical cyclones over the north Indian Ocean’, the translation speed of cyclones (the speed at which cyclones move) has decreased in the Arabian Sea. This means that cyclones are now moving slowly. The increase in cyclonic activity in the Arabian Sea is tightly linked to the rising ocean temperatures and increased availability of moisture under global warming. During the study period (1982–2019), a significant increasing trend in the intensity, frequency, and duration of cyclonic storms (CS) and very severe CS (VSCS) was observed over the Arabian Sea. There was a 52% increase in the frequency of CS during the recent epoch (2001–2019) in the Arabian Sea, and a decrease of 8% in the Bay of Bengal.
“Weather conditions in the ocean are very supportive for rapid intensification of the system. Though the Bay of Bengal might have seen a decline in cyclogenesis but intensity of the cyclones has increased manifold. Models are unable to pick up this rapid intensification of cyclones because they do not include ocean conditions properly and that is their limitation. Cyclones nowadays can retain their energy for quite a long number of days. One example of this trend was Cyclone Amphan, which continued to travel over land as a strong cyclone and resulted in massive devastation. As long as oceans are warm and winds are favourable, cyclones will retain their intensity for a longer period,” said Roxy Mathew Koll, climate scientist at the Indian Institute of Tropical Meteorology and lead IPCC author.
Koll further added, “The Bay of Bengal has been riding on the wave of global warming during the past few decades. Temperatures have been between 30-32°C in the Bay of Bengal. These high temperatures play a very important role in the intensification of CS as they infuse more convection. This kind of rapid intensification has become frequent recently both in the Arabian Sea as well as in the Bay of Bengal.”
Impacts of climate change on cyclogenesis: an Indian Ocean perspective
Tropical cyclones (TCs) are one of the most devastating natural disasters. More than 75% of total TCs over the globe causing the human deaths of 5,000 or more have occurred over the North Indian Ocean (NIO) during the past 300 years.
The threats from increasing cyclonic activity due to human-induced climate change have been well established. The Indian Ocean region, including the Arabian Sea (AS) and the Bay of Bengal is of particular concern because of the high population density along its coastlines.
According to the Ministry of Earth Sciences Report ‘Assessment of Climate Change over the Indian Region’, climate model simulations project a rise in TC intensity (medium confidence) and TC precipitation intensity (medium-to-high confidence) in the North Indian Ocean basin.
A comparison between pre-1950 and the post-1950 (commonly referred to as the warming era) periods indicated that there is a rise in number of severe cyclonic storm (SCS) from 94 to 140 (i.e. a 49% increase) in the Bay of Bengal region, and from 29 to 44 (i.e., 52% increase) in the Arabian Sea region on an annual scale.
Frequency of extremely severe cyclonic storms (ESCS) over the Arabian Sea has increased during the post-monsoon seasons of 1998–2018 (high confidence). However, there is medium confidence in attributing this observed increase to human-induced SST warming.
“Sea surface temperatures (SSTs) are increasing. The mechanism behind the formation of cyclones does not change but weather conditions are changing. Cyclones have been intensifying at a faster pace in the recent past. The reason behind this is not just an increase in the SSTs but also rising ocean heat content (OHC). Earlier the system used to take 2-3 days before forming into a tropical storm but nowadays it changes from a depression into a CS in just a day. Atmosphere not only interacts with SSTs but also with the entire ocean. It is quite evident from research that OHC is changing because of climate change,” said M M Ali, meteorologist and oceanologist, Andhra Pradesh State Disaster Management Authority and emeritus scientist G and group director, Atmosphere, Indian Space Research Organisation (ISRO).
According to the IPCC’s Special Report on Ocean and Cryosphere in a Changing Climate (SROCC), the climate is changing primarily because of increased long-lived greenhouse gases in the atmosphere that have increased radiative forcing of the climate system.
A consequence is an energy imbalance at the top of the atmosphere of which about 92% goes into the ocean, increasing the ocean heat content (OHC). Primary indicators of a changing climate include an increase in the global mean surface temperature, sea level, and OHC.
According to the IPCC, at the ocean surface, temperature has, on average, increased by 0.88°C (0.68 -1.01) between 1850–1900 and 2011–2020, with 0.60°C (0.44 – 0.74) of this warming having occurred since 1980. Since the 1950s, the fastest surface warming has occurred in the Indian Ocean.
The ocean surface temperature is projected to increase between 1995 – 2014 and 2081 to 2100 on average by 0.86 [0.43 to 1.47, likely range] °C in SSP1-2.6 scenario and by 2.89 [2.01 to 4.07, likely range] °C in SSP5-8.5 scenario.
The ocean heat content has increased from 1971- 2018 by 0.396 [0.329 to 0.463, likely range] yotta joules and will likely increase until 2100 by two to four times that amount under SSP1-2.6 and four to eight times that amount under SSP5-8.5 scenario.
According to IPCC’s Special Report on Ocean and Cryosphere in a Changing Climate, both SST and OHC are projected to increase in the future. There will be a warmer and wetter world over oceans and more energy available for evaporation, facilitating more TC activity and more rainfall.
There may be fewer but more intense storms (i.e, relatively higher chances of Category 4 or 5 storms), in part because of changes in atmospheric stability, and in part because a few bigger storms can replace many smaller storms in terms of their impact on the ocean.
“Barrier layer is a layer between the top and bottom layers of the ocean. This layer is getting stronger with increasing OHC and hence heat is not penetrating to the bottom layer of the ocean. Cyclones follow OHC and track where it is more. That is what we have seen in the Arabian Sea and Bay of Bengal, where usually cyclones tend to weaken as they track near the coast due to increasing wind shear and less moisture. However, these days cyclones tend to hold on to their strength even when they are near the coast. This is a serious threat to the Indian coasts on either side,” added Ali.
The rate of intensification and final intensity of the tropical cyclones has been found to be more sensitive to the initial spatial distribution of the mixed layer than to the SST alone. More than 50% of the cyclones intensified in the north Indian Ocean have a significant (at 95% level) negative correlation with SST. Thus, it is the deep surface layer, not simply the surface itself that is important to cyclone intensification.
Importance of ocean heat content for cyclone studies
There’s a need to reconsider the input parameters that are used into the models as a source of energy for the cyclones. The energy in the ocean for cyclones is available in the upper layers of the oceans rather than in the skin layer which is represented by SST. The thermal structure of the upper ocean is a more critical and sensitive predictor for cyclones compared to SST, which need to be incorporated in numerical models for better and accurate forecasting.
Analysis has indicated that OHC acts as a more important predictor than SST to estimate the life cycle of a cyclone through pressure drop, track change, intensity and to predict storm surge height. The parameter also explained the increasing number of intensifying cyclones.
Since the atmosphere interacts with the ocean through SST, a better parameterisation of SST that takes into account the OHC is required to be developed. One such option is to get the ocean mean temperature of the upper layer that interacts with the atmosphere. Since this parameter has the same units as that of SST, assimilating it in the cyclone models is simple as it is well recognised that SST plays a dominant role in the formation and intensification of tropical cyclones.