Aerosol and rainfall variability over the Indian monsoon region: distributions, trends and coupling

Abstract

Abstract. Aerosol solar absorption over the Indian monsoon region has a potential role of modulating the monsoon circulation and rainfall distribution as suggested by recent studies based on model simulations. Prior to the onset of the monsoon, northern India is influenced by significant dust transport that constitutes the bulk of the regional aerosol loading over the Gangetic-Himalayan region. In this paper, a multi-sensor characterization of the increasing premonsoon aerosol loading over northern India, in terms of their spatial, temporal and vertical distribution is presented. Aerosol transport from the northwestern arid regions into the Indo-Gangetic Plains and over the foothills of the Himalayas is found to be vertically extended to elevated altitudes (up to 5 km) as observed from the space-borne lidar measurements (CALIPSO). In relation with the enhanced pre-monsoon aerosol loading and the associated solar absorption effects on tropospheric temperature anomalies, this paper investigates the monsoon rainfall variability over India in recent past decades from an observational viewpoint. It is found that the early summer monsoon rainfall over India is on the rise since 1950s, as indicated by historical rainfall data, with over 20% increase for the period 1950-2004. This large sustained increase in the early summer rainfall is led by the observed strengthening of the pre-monsoon tropospheric land-sea thermal gradient over the Indian monsoon region as indicated by microwave satellite measurements (MSU) of tropospheric temperatures from 1979-2007. Combined analysis of changes in tropospheric temperatures and summer monsoon rainfall in the past three decades, suggest a future possibility of an emerging rainfall pattern of a wetter monsoon over South Asia in early summer followed by a drier period.

Effect of the summer monsoon on aerosols at two measurement stations in Northern India – Part 1: PM and BC concentrations

“…At the end of the dry season, before the monsoon, the concentrations were close to the annual maxima, especially in Mukteshwar. This can be attributed dust events which occur when air masses arrive from the desert areas close to the Arabian Sea, such as the Thar Desert (Gautam et al, 2009). The effect of the summer monsoon is very clear in this figure.…”

“…As already reported in the literature (Dey and Tripathi, 2008;Gautam et al, 2009Gautam et al, , 2011Ram et al, 2008Ram et al, , 2010, mineral dust events are frequent in the Indo Gangetic Plains and occur in the Himalayan foothills, too, during the premonsoon season. The dominance of mineral dust in the high concentration events observed here is backed by information that most of the events had a PM 2.5 /PM 10 ratio of about 0.1-0.2 indicating a dominant coarse mode from primary particles (Fig.…”

“…Pre-monsoon and early monsoon is the dust season in India (e.g. Dey and Tripathi, 2008;Gautam et al, 2009Gautam et al, , 2011Ram et al, 2008Ram et al, , 2010. The prospect of the high concentrations during monsoon being mineral dust is discussed in Sect.…”

Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models

“…However, other authors have noted overall negative trends. Ramanathan et al (2005) found negative trends particularly in July up to 2000, while with data up to 2004 Gautam et al (2009) found significant declining trends from July to September over India. Meanwhile, a recent comparison of trends in four different gridded rainfall data sets for India noted area-averaged rainfall decreases since the 1950s (Bollasina et al, 2011), although with considerable spatial differences (especially in north-eastern peninsular India) and relatively few gridpoints yielding statistically significant trends.…”

Contrasting aerosol characteristics and radiative forcing over Hyderabad, India due to seasonal mesoscale and synoptic‐scale processes

“…In contrast, in the late monsoon (August-September) it is high, reaching 470 mm in August 2008, further reducing afterwards. Note also the great variability in the rainfall amounts between the two years, able to cause significant variations in aerosol load and properties between the two contrasting monsoon years (Gautam et al, 2009). The flow chart of the wind speed (WS) and direction is shown in Figure 3 on a seasonal basis.…”