Atmospheric aerosols, which are fine particles suspended in the air, comprise a mixture of mainly sulfates, nitrates, carbonaceous (organic and black carbon) particles, sea salt, and mineral dust. Atmospheric aerosols have a strong impact on the Earth’s radiation budget and climate (Stocker et al., 2014). Aerosols interact with climate system through scattering and absorption of radiation (direct effect) and through modification of the microphysical properties of clouds (indirect effect). Absorbing aerosols, such as black carbon (BC), organic carbon (OC) and dust, can both absorb and reflect sunlight, thus heating the lower-troposphere and cooling the surface. In contrast, non-absorbing aerosols, such as sulfate, generate mainly surface cooling but weak atmospheric heating. In recent decades, the BC emissions are particular large in China and India due to energy combustion and biomass burning and has brought the world’s attention (Ramanathan and Carmichael, 2008; Yang et al., 2022; Wei et al., 2022).
Extensive studies concerning the roles of aerosols in the Earth’s climate did not begin until the 1990s. Aerosols can affect monsoon rainfall, and regional climate change through radiative forcing and microphysical effects (Rosenfeld, 2000; Li, 2004; Nakajima et al., 2007; Li et al., 2007a, 2011a, 2011b; Huang et al., 2014; Guo et al., 2016). Many general circulation model (GCM) studies have investigated the impacts of aerosols on the global and regional changes in precipitation (Menon et al., 2002; Lau et al., 2008; B. Wang et al., 2009; C. Wang et al., 2009; Bollasina et al., 2011, 2013; Cowan and Cai, 2011; Ganguly et al., 2012). The weakening of monsoon circulation and rainfall reduction have been attributed to aerosol effects (Ramanathan et al., 2005; Chung and Ramanathan, 2006; Ramanathan and Carmichael, 2008; Liu et al., 2009; Cowan and Cai, 2011; Bollasina et al., 2011; Salzmann et al., 2014; Krishnan et al., 2016) and equatorial Indian Ocean warming due to increased GHG (Ramanathan et al., 2005; Chung and Ramanathan, 2006; Annamalai et al., 2013; Lee and Wang, 2014; Sabeerali and Ajayamohan 2017).
As an integral component of the Earth’s hydrological cycle, the Indian Summer Monsoon (ISM) is the largest monsoon system critical for the wellbeing of over two thirds of the world’s population. The observational evidence of ISM circulation experienced a significant declining trend from the 1950s together with a weakening local meridional circulation and notable precipitation decreases over north-central India and the west coast that are associated with a reduced meridional temperature gradient (Ramanathan et al., 2001b, 2005; Krishnan et al., 2013; Goyal, 2014; Roxy et al., 2015; Praveen et al., 2020). Although it could potentially be altered by multidecadal variations (Shi et al., 2018) arising from internal modes of climate variability such as the Atlantic multidecadal variation (AMV) and the interdecadal Pacific oscillation (IPO) (Krishnan and Sugi, 2003; Ding et al., 2008, 2009; Zhou et al., 2008; Cheng and Zhou, 2014; Salzmann and Cherian, 2015; Jiang and Zhou, 2019), the high aerosol emissions in South Asia has made the role of aerosol effect a critical issue. The aerosol effects on energy-water cycle and monsoon dynamics are strongly dependent on aerosol distribution and characteristics as well as its spatial and temporal variations. Elevated deep layers of radiation-absorbing aerosols can potentially affect the water cycle by significantly altering the energy balance (Ramanathan et al., 2005; Lau and Kim, 2006; Lau et al., 2006).
The increase of aerosols and associated impact on the ISM has been documented in many modeling studies (e.g., Ramanathan 2005; Lau et al., 2006; Meehl et al., 2008; Ganguly et al., 2012; Bollasina et al., 2013). Ramanathan et al. (2005) used a coupled ocean-atmosphere model to show that absorbing aerosols over India could decrease monsoon precipitation by reducing surface shortwave radiation, which limits the amount of evaporation, as well as increasing atmosphere heating, which stabilizes troposphere over South Asia. The high aerosol regions of the northern portion of the Indian Peninsula and Arabian Sea are cooled relative to the oceans to the south, leading to a reduction of the meridional thermal gradient and a slowing down of the local meridional circulation. The slower circulation reduces surface evaporation and provides a positive feedback, further weakening the monsoon. In a somewhat different modeling approach, Lau et al. (2006) used an atmospheric model together with an observational analysis of Lau and Kim (2006) to emphasize the importance of temporal and regional distributions of both natural and anthropogenic aerosols not only as a forcing agent but also as an integral part of a dynamical feedback mechanism involving clouds, rainfall, and winds that can alter the evolution of ISM system. They proposed the elevated heat pump (EHP) theory which posits that atmospheric heating by deep layers of BC and dust accumulated over the Indo-Gangetic Plain (IGP) and the Tibetan Plato slope (TP) can induce a moisture convergence feedback, leading to increased precipitation in northern India during March to June. Specifically, the accumulation of BC and dust over the northern and southern slopes of TP absorbs shortwave over the longitudes of the Indian Peninsula during pre-monsoon periods and heats the lower and middle troposphere around the TP. The heated air rises via dry convection, creating a positive temperature anomaly in the mid-level to upper troposphere over the southern slope of TP relative to the region to the south. The rising hot air forced by the increased heating in the upper troposphere from North Indian Ocean draws in more warm and moist air over Indian Peninsula, setting the stage for the onset of the ISM.
For our recent study (Chen et al., 2018), the anthropogenic aerosols lead to surface cooling through the decrease in surface short wave flux from October to December. The surface cooling region extends downwind of major emission source areas to western and northern India, as the aerosols are transported by the prevailing winds. The precipitation strongly reduces during October to December over western and northern India, and the reduction is mainly contributed by the vertical convergence term that is associated with changed vertical motion by anthropogenic aerosols. Furthermore, Wang et al. (2013) further demonstrated that absorbing aerosols were particularly important in influencing the circulation and precipitation over the northern India regions in winter.
Similar to radiation effect of BC and OC, the natural dust also absorbs solar radiation that cause an atmospheric heating and surface cooling. Long-range transport of dust aerosols originated in the Middle East is known to be a significant source of aerosol over the Arabian Sea and Indian region during pre-monsoon season. As a result, enhanced aerosol loading in Indian Peninsula during the pre-monsoon season is likely contributed by both anthropogenic and natural aerosols (Satheesh and Srinivasanan, 2002; Badarinath et al., 2010). A recent study by Wei at al. (2022) reveals a decreasing dust transport by altered monsoon flow caused by reduced BC burden over the northern (70°-88°E, 25°-35°N) but increased BC burden over southern (70°-88°E, 15°-25°N) India during the lockdown of COVID-19. Their results indicate that the solar heating in April and May is decreased by BC reduction in northern India and the surface albedo of the TP southern slope is increased due to the reduced BC snow-darkening effect. The northern Indian atmosphere responds to this cooling with a descending motion and enhanced atmospheric stability. Furthermore, the surface and near surface cooling over the southern slope of TP by increased albedo lead to southward cold air advection. The cooling over norther India and warming over southern India from increased crop residue burning BC emissions induce an anomalous southward pressure gradient force. Therefore, an anomalous northward Coriolis force as the emergence of easterly wind anomalies appears to balance the anomalous pressure gradient force. Owing to anomalous easterly wind, the eastward transport of dust from the Middle East and Sahara as well as local dust emissions in the Thar Desert are suppressed.
Despite the large number of studies, there still exists a gap in our knowledge and understanding of anthropogenic aerosols and their climate effect. Aerosol processes are still poorly observed and treated in numerical models. Here, we plan to revisit the aerosol-induced monsoon variability along with aerosol’s temporal transition within the monsoon evolution. In a difference from the prescribed simulates aerosol radiative forcing using simulated transport and simulated aerosol spatial distributions. We use the historical emission inventory which quantified anthropogenic and biomass emission of climate-relevant species for the period 1850–2000 by Lamarque et al. (2010). We focus specially, on the regional anthropogenic aerosols forcing over India to identify the ISM evolution responses. The paper is structured as follows. A brief description of the model and experimental design is given in Section 2. The analyses of ISM characteristics are showed in Section 3, including Indian monsoon characteristics in observation data and model simulation. The changes of ISM onset/withdrawal date and rate of changes are also discussed in this section. Each period during Indian monsoon evolution from pre-monsoon to monsoon withdrawal are discussed in Section 4, 5, 6, including the effect of anthropogenic aerosols on the circulation and precipitation over the Indian Peninsula. The natural dust distribution response to BC climate impacts over monsoon evolution are discussed in Section 7. At the end of the text, a summary and discussions are given in Section 8.