Asian summer monsoon anomalies induced by aerosol direct forcing: the role of the Tibetan Plateau

Abstract

In this paper we present results of a numerical study using the NASA finite-volume GCM to elucidate a plausible mechanism for aerosol impact on the Asian summer monsoon involving interaction with physical processes over the Tibetan Plateau. .During the pre-monsoon season of March-April, dusts from the deserts of western China, Afghanistan/Pakistan, and the Middle East are transported into and stacked up against the northern and southern slopes of the Tibetan Plateau. The absorption of solar radiation by dust heats up the elevated surface air over the slopes. On the southern slopes, the atmospheric heating is reinforced by black carbon from local emission.The heated air rises via dry convection, creating a positive temperature anomaly in the mid-to-upper troposphere over the T ibetan Plateau relative to the region to the south. In May through early June in a manner akin to an "elevated heat pump", the rising hot air forced by the increasing heating in the upper troposphere, draws in warm and moist air over the Indian subcontinent, setting the stage for the onset of the South Asia summer monsoon. Our results suggest that increased dust loading coupled with black carbon emission from local sources in northern India during late spring may lead to an advance of the rainy periods and subsequent ly an intensification of the Indian summer monsoon.The enhanced rainfall over India is associated with the development of an aerosolinduced large -scale sea level pressure anomaly pattern, which causes the East Asia (Mei-yu) rain belt to shift northwestward, suppressing rainfall over East Asia and the adjacent oceanic regions .2

Absorbing Aerosols and Summer Monsoon Evolution over South Asia: An Observational Portrayal

“…However, the impact of aerosols on monsoon rainfall in a coupled model was found to be different from that in uncoupled models with specified SSTs (Chung et al 2002;Menon et al 2002;Lau et al 2006) as a result of the response of SSTs themselves to aerosol forcing. Ramanathan et al (2005), using a coupled ocean-atmosphere model with aerosols over South Asia prescribed according to measurements, found that, while aerosol absorption of solar radiation and consequent heating of the atmosphere leads to enhanced upward motion over India during winter, it also leads to a weakening of the monsoon circulation and a reduction of rainfall over India during summer.…”

“…Absorbing aerosols have been shown to be influential in the context of long-term changes (or trends) in summer monsoon rainfall (e.g., Ramanathan et al 2005;Chung and Ramanathan 2006;Lau et al 2006;Meehl et al 2008) and its interannual variability (LK06). The summer season is particularly challenging because extensive cloudiness at this time brings the aerosol indirect effects into play in addition to the land and ocean surface-related feedbacks.…”

“…The contribution of absorbing aerosols to the long-term changes of rainfall over India was investigated by Chung et al (2002), Menon et al (2002), Ramanathan et al (2005), Lau et al (2006), Chung and Ramanathan (2006), and recently by Meehl et al (2008). Chung et al (2002) used an atmospheric model with sea surface temperatures (SSTs) fixed to their climatological cycle and aerosol forcing imposed in terms of a radiative forcing perturbation (from October to May) over the Indian Ocean, derived from INDOEX measurements.…”

“…The authors attributed this variation to the heating of the air and its effects on temperature profile, convection strength, and induced large-scale ascending motion. Lau et al (2006) have also ascertained the effects of absorbing aerosols on the summer monsoon with an atmospheric model forced by specified SSTs and proposed an "elevated heat pump" hypothesis: anomalous accumulation of absorbing aerosols (transported dust from the nearby deserts and BC from regional sources) against the southern slopes of the Himalayas induces a large-scale upper-level heating anomaly over the Tibetan Plateau in April and May that reinforces the meridional temperature gradient and intensifies the monsoon over India in June and July.…”

Modeling all-sky global solar radiation using MODIS atmospheric products: A case study in Qinghai-Tibet Plateau

“…The complex terrain and the amount and distribution of solar energy on the Qinghai-Tibet Plateau have significant influences on the Asian monsoon and the global climate changes (Bai and Xu, 2004;Lau et al, 2006). However, the atrocious geographical and natural conditions make it quite difficult for the scientists to carry out more and frequent solar radiation observations.…”