13th May 2023, by Sunita Mishra
In a concerning development, the first cyclonic storm of the season, Cyclone Mocha, has formed over the Southeast Bay of Bengal. Experts predict that atmospheric conditions are highly favorable for the intensification of Cyclone Mocha in the coming days. The India Meteorology Department, the country's nodal agency, has forecasted that the storm is expected to make landfall between Cox’s Bazar, Bangladesh, and Kyaukpyu, Myanmar, as a very severe cyclonic storm by noon on May 14, 2023.
The occurrence of this cyclone in May, the peak month for cyclogenesis in the Indian Ocean, is not surprising. However, the alarming trend of rapid intensification observed in recent cyclonic storms raises concerns among researchers and scientists. They attribute these changes to the increasing global mean temperature, particularly in the Indian Ocean.
According to a study titled 'Changing status of tropical cyclones over the north Indian Ocean,' researchers have noted a decrease in the translation speed of cyclones in the Arabian Sea, indicating slower movement. The intensification of cyclonic activity in the Arabian Sea is closely linked to rising ocean temperatures and increased moisture availability resulting from global warming. The study spanning from 1982 to 2019 shows a significant upward trend in the intensity, frequency, and duration of cyclonic storms and very severe cyclonic storms over the Arabian Sea. While there has been an 8% decrease in cyclone frequency in the Bay of Bengal, there has been a staggering 52% increase in the Arabian Sea during the recent epoch from 2001 to 2019.
Climate scientists have emphasized the impact of warming oceans on cyclone behavior. Dr. Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology and Lead IPCC Author, warns that models often fail to capture the rapid intensification of cyclones due to inadequate inclusion of ocean conditions. Cyclones nowadays can retain their energy for an extended period, as seen in the case of Cyclone Amphan, which caused significant devastation even while traveling over land. Dr. Koll emphasizes that as long as oceans remain warm and winds remain favorable, cyclones will maintain their intensity for a longer duration.
The Bay of Bengal, riding on the wave of global warming in recent decades, has witnessed increased temperatures ranging between 30-32 degrees Celsius. These elevated temperatures play a crucial role in the intensification of cyclonic storms by enhancing convection. Dr. Koll highlights that this rapid intensification has become more frequent in both the Arabian Sea and the Bay of Bengal.
The impacts of climate change on cyclogenesis, especially in the Indian Ocean region, have become a cause for concern due to its high population density along the coastlines. According to the Ministry of Earth Sciences Report, climate models project an increase in tropical cyclone intensity and precipitation intensity in the North Indian Ocean basin. A comparison of pre-1950 and post-1950 periods reveals a rise in severe cyclonic storms by 49% in the Bay of Bengal region and 52% in the Arabian Sea region on an annual scale. Observations further indicate an increase in the frequency of extremely severe cyclonic storms over the Arabian Sea during the post-monsoon seasons of 1998-2018, with medium confidence attributing this rise to human-induced sea surface temperature (SST) warming.
The changing climate, primarily driven by increased greenhouse gases in the atmosphere, has resulted in an energy imbalance in the climate system. Approximately 92% of the energy goes into the ocean, leading to increased Ocean Heat Content (OHC). The IPCC's Special Report on Ocean and Cryosphere in a Changing Climate (SROCC) highlights the rise in global mean surface temperature, sea level, and Ocean Heat Content as primary indicators of climate change. Since the 1950s, the Indian Ocean has experienced the fastest surface warming, with the Ocean Heat Content increasing significantly.
According to the SROCC, future projections indicate a warmer and wetter world over oceans, providing more energy for evaporation and facilitating increased tropical cyclone (TC) activity and rainfall. While there may be fewer cyclones overall, the storms that do form are expected to be more intense, with a higher likelihood of Category 4 or 5 storms. Changes in atmospheric stability and the concentration of energy in a few large storms contribute to this intensification.
Experts highlight the role of the barrier layer, a layer between the top and bottom layers of the ocean, in cyclone behavior. As Ocean Heat Content strengthens, heat penetration to the ocean's bottom layer decreases. Cyclones tend to track where the Ocean Heat Content is higher, and recent observations indicate that cyclones can maintain their strength even when near the coast. This poses a serious threat to the coastal areas of India.
To better understand and predict cyclone behavior, scientists emphasize the importance of incorporating the thermal structure of the upper ocean, rather than relying solely on sea surface temperature (SST), which represents only the skin layer of the ocean. Ocean Heat Content acts as a critical predictor for cyclones, affecting their life cycle, pressure drop, track change, intensity, and storm surge height. Therefore, it is necessary to develop improved parameterization of SST that accounts for Ocean Heat Content, ultimately enhancing cyclone modeling and forecasting accuracy.
In conclusion, the formation and intensification of cyclonic storms like Cyclone Mocha are intrinsically linked to climate change. Rising global temperatures and increasing Ocean Heat Content contribute to the intensification of cyclones in the Indian Ocean region. As climate change continues, the need for robust modeling techniques and accurate forecasting becomes crucial in mitigating the impacts of these severe weather events on vulnerable coastal communities.
Disclaimer: The article is based on information provided by Climate Trends, a research-based consulting and capacity building initiative specializing in environment, climate change, and sustainable development.