Discussion on linear long-term trends in aerosol and cloud properties over India and its surrounding waters

• Insignificant AOD trends over northern India along Indo-Gangetic plains.
• Upward AOD trends over oceanic regions surrounding Indian subcontinent.
• Aerosol effects on cloud trends are noticeable over few parts of the study region.

Spatial and seasonal variations in the linear long-term trend estimates of aerosol and cloud properties over Indian subcontinent and the surrounding oceanic regions of Bay of Bengal (BoB) and Arabian Sea (AS) are studied and discussed utilizing 12 complete years (2003–2014) of Moderate Resolution Imaging Spectroradiometer (MODIS) derived Aerosol and cloud products. Annual Aerosol Optical Depth (AOD) trends (in terms of AOD/year) are found to be positive (upward) over most of the study region with a spatial mean (median) value of ∼0.0065 (0.0064) and exhibited significant spatial and seasonal heterogeneity. Over Indian landmass AOD trends and their statistical significance decreased towards north along the Indo-Gangetic plains (IGP), for which the probable causes are discussed. Same kind of pattern in AOD trends has been observed as we move deeper into the oceanic regions of BoB and AS, away from Indian subcontinent. Observed trend patterns are discussed in light of the possible increase in emissions (over Indian landmass) and transported aerosol component, co-variation with trends in meteorological parameters and their possible feedbacks. Trend maps in seasonal AOD are shown to understand the aerosol build up over the study region under varying meteorological conditions. Seasonal AOD trend patterns resembled the synoptic scale wind circulation over the study region revealing that the upward trend in aerosol abundance over the adjoining oceanic regions of India is a result of effective transport of increasing emissions over India on to them. No significant trends in cloud properties (over the whole study region) are depicted in concert with that of aerosols, except over few pockets. The study also highlighted the role of large scale atmospheric processes in modulating the shape of the AOD time series over the regions with significant abundance of natural aerosol component (dust).