Cyclone Asna: An Unusual Formation Driven by Anomalous Monsoon winds
Cyclone Asna, a rare and scientifically intriguing tropical storm, formed under unusual circumstances in August 2024, showcasing the complex interplay of monsoon dynamics and environmental factors. Originating as a depression over land, the storm intensified into a cyclone over the Arabian Sea, making it the first such occurrence in the North Indian Ocean (NIO) region since 1981 and the first in the Arabian Sea since 1976. The cyclone's formation, driven by an atypical monsoon wind system, has sparked significant interest within the meteorological community due to its rarity and the unique conditions that led to its development.
The Formation of Cyclone Asna
On August 29, 2024, a deep depression formed over Gujarat, India, following a period of excessive rainfall in the region. The depression, initially located 60 km northwest of Bhuj and 80 km northeast of Naliya, intensified as it moved west-southwestwards towards the Arabian Sea. By August 30, the depression had evolved into a tropical cyclone, named Asna, as it emerged over the northeast Arabian Sea off the coasts of Kutch, Gujarat, and Pakistan.
Historically, cyclones in the Arabian Sea during August are rare, with only four recorded between 1891 and 2023. Cyclone Asna's formation is particularly notable as it is the first land-originating cyclone in the Arabian Sea during August in nearly five decades. This event marks only the fourth such occurrence in August since 1891, with previous instances recorded in 1976, 1964, and 1944.
The Unusual Monsoon Wind System
The primary factor behind Cyclone Asna's formation and intensification lies in the unusual monsoon wind system observed during the 2024 season. Typically, monsoon depressions originate over the Bay of Bengal and move westward across the Indian subcontinent. However, in the case of Cyclone Asna, the depression originated over land, a rare occurrence, and later transitioned into a cyclone over the sea.
Kieran Hunt, a research fellow in Tropical and Himalayan Meteorology at the University of Reading, UK, highlights that the current monsoon circulation has been atypical. "For a long time, we thought a necessary prerequisite for the formation of such land-based systems was lots of soil moisture, so that the land could provide adequate moisture to disturbances, allowing them to intensify as they would if they were over the ocean," Hunt explained. However, he noted that recent observations suggest the configuration of large-scale winds plays a more critical role in such formations.
During the 2024 monsoon season, the monsoon trough, which typically curves at its eastern end, shifted slightly westward. This shift facilitated the formation of the initial depression over land rather than the Bay of Bengal. Additionally, the monsoon exhibited a break-like circulation, characterized by strong easterly winds over southern India. These winds, cyclonic in nature, contributed to the development of low-pressure systems (LPS) and depressions along their northern edge.
The Boreal Summer Intra-seasonal Oscillation (BSISO), a significant monsoonal event, further amplified these conditions. The BSISO, which involves the transfer of heat from the Indian Ocean to the Western Pacific Ocean, created an environment conducive to the formation of the LPS. This LPS eventually intensified into the deep depression that led to Cyclone Asna, causing heavy rainfall and flooding in Gujarat.
Scientific Significance and Rare Occurrence
The rarity of Cyclone Asna's formation, especially during August in the Arabian Sea, makes it a subject of significant scientific interest. Typically, cyclones in the Arabian Sea form over warm ocean waters, not over land. The last recorded instance of a land-originating cyclone that transitioned into a sea-based storm occurred in 1976.
Ashok Kumar Das, a scientist and head of the IMD Met Centre in Ahmedabad, emphasized the rarity of this event, stating, "This is a rare occurrence. The last time something similar happened was in 1976 when a depression formed on land, transitioned into a cyclone, and then moved out to sea. Usually, it’s the other way around."
The current sea surface temperatures in the Arabian Sea, ranging from 27-28°C, just above the 26.5°C threshold necessary for cyclone formation, further contributed to the intensification of Cyclone Asna. As the cyclone moved towards Oman, it encountered even warmer waters, exceeding 32°C, which likely influenced its trajectory and strength.
Climate Change and Increased Cyclone Activity
Recent studies suggest that climate change is contributing to the increasing frequency and intensity of cyclones on India's western coast. The warming of the Arabian Sea, driven by global climate change, has made regions like Gujarat, Maharashtra, and Rajasthan more susceptible to intense storms.
M Rajeevan, former secretary of the Ministry of Earth Sciences and a climate scientist, remarked on the unusual nature of Cyclone Asna, noting that depressions typically do not intensify into cyclones during the monsoon season in the Arabian Sea due to high wind shear. However, the unusually warm temperatures in the Indian Ocean may have facilitated the formation and intensification of Cyclone Asna.
A study by the Council on Energy, Environment, and Water (CEEW) identified that over 41% of districts in India's western zone are now considered cyclone hot spots. The study underscores the need for enhanced safety measures in cyclone-prone regions, including the construction of robust structures and the prioritization of early response through well-prepared shelters to mitigate potential loss of life and property.
Impact and Response
The formation of Cyclone Asna followed a period of severe weather in Gujarat, which had already experienced significant rainfall and flooding. Between August 26 and 29, the state received 48% excess rainfall, with the dry regions of Saurashtra and Kutch particularly affected. Rain-related incidents claimed 26 lives, displaced more than 18,000 people, and prompted extensive rescue operations by the Indian Coast Guard and Indian Army.
The Indian Coast Guard conducted multiple helicopter operations, rescuing stranded individuals from flood-stricken areas. The Army mobilized resources to provide critical assistance and disaster relief in affected districts, including Vadodara, Jamnagar, Dwarka, Rajkot, Morbi, and Bhuj.
Conclusion
Cyclone Asna's formation over land and subsequent intensification into a cyclone over the Arabian Sea is a rare and scientifically intriguing event that highlights the complex dynamics of monsoon systems and their interaction with environmental factors. As climate change continues to alter global weather patterns, the frequency and intensity of such unusual events may increase, necessitating further research and preparedness to mitigate their impacts. The case of Cyclone Asna underscores the need for continued monitoring and analysis of monsoon dynamics, particularly in the context of a warming planet.
References
https://www.hindustantimes.com/india-news/originated-over-land-cyclone-over-sea-a-rare-storm-gains-strength-off-gujarat-101725043334662.html
https://currentaffairs.adda247.com/cyclone-asna-forms-over-kutch/
https://www.downtoearth.org.in/climate-change/cyclone-asna-formation-due-to-unusual-monsoon-wind-system-expert
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