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 pre-monsoon 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.
Highlights
A growing body of evidence indicates that tropospheric aerosols, a major component of the global climate system, significantly influence the Earth’s radiation budget and climate forcing
With the growing attention on the potential effects of aerosol radiative forcing on the Indian monsoon rainfall and circulation in recent years (Menon et al, 2002; Ramanathan et al, 2005; Lau et al, 2006; Meehl et al, 2008), here we analyze the pre-monsoon aerosol loading and the monsoon rainfall variability from long-term observations over India
We showed that the Aerosol Index (AI) variability over northern India during the pre-monsoon season is strongly associated with the inter-annual variability of tropospheric temperatures, with a large sustained widespread warming trend centered over the Himalayan-Gangetic region (Gautam et al, 2009b)
Summary
A growing body of evidence indicates that tropospheric aerosols, a major component of the global climate system, significantly influence the Earth’s radiation budget and climate forcing. With the growing attention on the potential effects of aerosol radiative forcing on the Indian monsoon rainfall and circulation in recent years (Menon et al, 2002; Ramanathan et al, 2005; Lau et al, 2006; Meehl et al, 2008), here we analyze the pre-monsoon aerosol loading and the monsoon rainfall variability from long-term observations over India. Long-term rainfall data over India are obtained from the Indian regional/sub divisional Monthly Rainfall data set generated by the Indian Meteorological Department This dataset is available from 1871 to 2004 from a network of rain-gauges which consist of 306 almost uniformly distributed stations for which rainfall data are available from 1871 (Parthasarathy et al, 1995). These data were obtained from http://ingrid.ldeo.columbia. edu/SOURCES/.NASA/.GPCP/.V2/
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