Dipole pattern in aerosol-induced atmospheric warming trends over the Indian subcontinent in the last two decades

Understanding the patterns of aerosol-induced perturbation in radiation budget and its drivers is crucial in climate science. Here, we examined spatio-temporal trends in aerosol-induced atmospheric warming and the top-of-the-atmosphere (TOA) and surface cooling over the Indian Subcontinent under clear-sky and all-sky conditions using clouds and the earth's radiant energy system data for the period 2000-2021. Overall, the regional mean TOA and surface cooling were found to increase by 0.06 W m-2 yr-1 and 0.09 W m-2 yr-1, respectively. Over the last two decades, the aerosol-induced atmospheric warming in all-sky conditions increased over the subcontinent landmass and outflow regions over the ocean while it declined over dust-dominated arid regions. This dipole pattern was driven by a combination of an overall increase in aerosol optical depth, a gradual increase in the fraction of scattering aerosols over the Indian landmass dominated by anthropogenic sources, a decline in dust loading over the arid sources. As a result, atmospheric warming efficiency declined in most parts of the Indian subcontinent. A comparative meta-analysis revealed that aerosol-induced atmospheric warming was over-estimated by the existing studies where aerosol direct radiative forcings were estimated by 1-D radiative transfer model utilizing modeled optical properties based on incomplete information about in-situ physico-chemical properties derived from ground-based measurements. Our analysis showed that TOA and surface cooling by aerosols were higher in clear-sky conditions relative to the actual all-sky condition by up to 11 W m-2 and 16 W m-2, respectively; therefore, atmospheric warming reported for clear-sky conditions would be biased high over the subcontinent. As India embarked on a clean air mission, changes in aerosol loading and its composition are expected to alter the dipole pattern further in the future, impacting the regional climate via dynamic feedback.