The ozone layer is commonly understood as a protective layer in the Earth’s atmosphere that absorbs the harmful ultraviolet (UV) radiation of the Sun, shielding the living organisms on Earth from excessive UV exposure. Scientists have detected an unusual enhancement of ozone over the North Bay of Bengal during winter, at an altitude lower than where ozone normally reaches its highest concentration. The unusual ozone enhancement was observed at around 21–23 kilometres in the lower stratosphere, whereas the ozone layer normally reaches its maximum concentration at about 25–30 kilometres.
The amount of ozone in the detected layer was significantly higher than normally observed over eastern India. The Ozone partial pressure levels were approximately 50 nanobars (nbar) higher than the climatological mean for that latitude level. The unusual structure persisted for more than a day and was observed during both daytime and nighttime.
The discovery provides quantitative experimental evidence of the redistribution of stratospheric ozone over the Indian subtropical region during winter. The findings were published in Earth and Space Science by the American Geophysical Union.
Nationwide NetRAD-ASMA Campaign
The discovery was made during Phase-1 of the Network of ST/MST Radars and Balloon-borne Measurement Campaigns of the Asian Summer Monsoon Anticyclone (NetRAD-ASMA) campaign, a nationwide scientific programme that brought together research institutions across India to understand complex processes taking place in the atmosphere.
Scientists used a countrywide network of atmospheric radars, weather balloons, satellites, and advanced atmospheric models to obtain a detailed picture of ozone distribution and atmospheric conditions. The Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute under the Department of Science and Technology, Government of India, played an important role in the studies.
A key contribution came from the indigenously developed 206.5 MHz Stratosphere–Troposphere Radar at ARIES, Nainital. Data from the radar were collected alongside observations from similar radars at Haringhata, Gadanki, and Kochi. Weather balloons carrying sophisticated instruments were launched to measure atmospheric parameters and ozone concentrations.
Observations from the Aura Microwave Limb Sounder (MLS) and INSAT-3DR satellites further enabled scientists to examine the unusual ozone structure from space and compare it with balloon- and ground-based measurements. Scientists, including Dr Manish Naja and Dr Samaresh Bhattacharjee of ARIES, participated in the study.
Evidence behind the Ozone Enhancement
The enhanced ozone layer being detected during both daytime and nighttime suggested that sunlight-driven photochemical processes were unlikely to have been the primary cause.
Satellite observations also detected the unusual ozone structure, supporting the measurements obtained from weather balloons.
Researchers examined atmospheric convection, wind shear, horizontal transport, and vertical air movements to identify the processes responsible for the unusual event. Their observations revealed persistent downward air movement in the lower stratosphere above the region.
By combining evidence from radars, balloons, satellites, and atmospheric models, scientists associated the unusual ozone enhancement with the transport of ozone-rich air from the mid-latitudes, along with downward atmospheric movement and compression of the air mass. The downward air motion compressed ozone-rich air and helped form the layer, while the transport of ozone-rich air into the area resulted in further enhancement. Together, these atmospheric processes produced a persistent anomaly that remained through day and night.
Significance for Atmospheric Research
The ozone layer protects life on Earth by absorbing harmful ultraviolet radiation from the Sun. Understanding how its concentration and distribution can vary unexpectedly helps researchers better understand atmospheric circulation and dynamical processes that influence ozone distribution in the stratosphere.
The findings strengthen understanding of how atmospheric currents transport and redistribute ozone over the Indian subtropical region. They also provide new insights into the complex processes occurring in the Earth’s atmosphere and how atmospheric circulation and dynamics influence stratospheric ozone.
The study demonstrates the value of coordinated observations from atmospheric research facilities across India. It also highlights the contribution of indigenously developed technology, particularly the ARIES Stratosphere Troposphere Radar (ASTRAD) and balloon-borne measurements, to national atmospheric research programmes.
The unusual ozone layer over the North Bay of Bengal, therefore, represents important quantitative experimental evidence of ozone redistribution in the Indian subtropics. By combining radar, balloon, satellite and atmospheric model observations, scientists have been able to capture a rare atmospheric phenomenon and improve understanding of the atmospheric processes underlying ozone redistribution in the stratosphere.
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