Aerosol-cloud interactions constrain climatic trends in rainfall and temperature of India

Aerosol-cloud interactions (ACI) is a key uncertainty in our ability to forecast future climate. Robust evidences of aerosol-induced modifications to the structure and lifetime of both, rain bearing and non-rain bearing clouds has emerged from satellite observations across the globe in last two decades. These observations were also substantiated by many process-level simulation studies using weather models at cloud resolving scales in last decade. Thus, the significance of ACI at process scale on short-term meteorological perturbations is well agreed. However, the role of aerosol-cloud interactions on trends at climate scale is not evident yet. For example, if cloud occurrence is increasing over India, it is not clear if there is any substantial role of ACI in comparison to other governing factors. Here, we will present our analysis on the association of ACI with the recent trends in clouds, temperature and rainfall over India using satellite observations and global climate model simulations.

First, we will discuss data analysis of simulations from CMIP5 models, to quantify the importance of ACI on extreme climate indices over Indian monsoon region. The climate models were grouped based on whether the models represent only aerosol-radiation interactions (REM_ADE) or the full suite of aerosol-radiation-cloud interactions (REM_ALL). Compared to REM_ADE, including all aerosol effects significantly improves the model skills in simulating the observed historical trends of all three climate indices over India. Specifically, AIE enhances dry days and reduces wet days in India in the historical period, consistent with the observed changes. However, by the middle and end of the 21st century, there is a relative decrease in dry days and an increase in wet days and precipitation intensity. Further, we will also illustrate unprecedented satellite evidences of aerosol induced positive trends in marine cloud occurrences and surface temperature during pre-monsoon over the Bay of Bengal (BOB) region. In last 15 years, increased aerosol emissions over North India have led to an increase in aerosol loading till 3 km over the BOB outflow region in monsoon onset period. The elevated aerosol loading stabilizes the lower troposphere over the region in recent years and leads the low-level cloud occurrences (below 3 km) to increase in recent years by ~20%. Incidentally, the sea surface over entire BOB is steadily warming under climate change except the pollution outflow region, suggesting potential contributing to the observed non-intuitive cooling trends in sea surface temperatures.

Our findings underscore the crucial role of ACI in trends and future projections of the Indian hydroclimate and emphasizes the crucial need for improved aerosol representations in coupled models for accurate predictions of regional climate change over South Asia.