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Earlier Onset of the Indian Monsoon in the late 20th century: the Role of Anthropogenic Aerosols

Key Findings

  • Aerosols are likely responsible for the observed earlier onset, resulting in enhanced June precipitation over most of India.
  • Increased atmospheric stability, which inhibits the monsoon migration in May, leads to thermodynamical changes over the northwestern continental region, including increased surface temperature and near-surface moist static energy.
  • These support a stronger June flow and, facilitated by a relative warming of the Indian Ocean, a vigorous northwestward precipitation shift.

Massimo A. Bollasina, Yi Ming, and V. Ramaswamy. Journal: Geophysical Research Letters. DOI: 10.1002/grl.50719

Summary

This study investigates the impact of the late 20th century increase of anthropogenic aerosols on the onset of the Indian summer monsoon. Aerosols are likely responsible for the observed earlier onset, resulting in enhanced June precipitation over most of India. This shift is preceded by strong aerosol forcing over the Bay of Bengal and Indochina, mostly attributable to the direct effect, resulting in increased atmospheric stability that inhibits the monsoon migration in May.

While most studies have focused on variations of the summer-mean monsoon climate, subseasonal changes have received much less attention. Possible changes at the time of the onset could be of equal importance for the agriculture-based monsoon region, as the onset heralds the beginning of the rainy season over the dry Indian sub-continent.

These results highlight the importance of aerosol-precipitation-circulation interactions, an issue which has not been fully addressed in the past but is emerging as a major uncertainty in the understanding of the aerosol impact, especially at regional scale. Additionally, this study underscores the role of regional land-surface processes in mediating the aerosol impact.

The nature of changes in the monsoon onset is still an underexplored topic, especially in the context of recent observed long-term monsoon variations. This study takes into account various anthropogenic and natural forcings, and offers a broader perspective on aerosols -- beyond black carbon. The authors provide new insights into the physical pathway for the aerosol-monsoon interaction, which is still uncertain and largely debated in the scientific community.

Figure 1: Top panel shows precipitation trend over India from 1950-1999, based on observations. Bottom panel shows historical simulations forced only by anthropogenic aerosols, using GFDL’s CM3 climate model. The black dots mark the grid points for which the trend exceeds the 95% confidence level.

Figure 2: From observations: The 1951-1999 trend in monsoon onset expressed as number of 5-day intervals (pentads) is shown in shaded areas, and the pentad of monsoon onset, 1951-1970 average, is shown in black contours,. Positive values denote later onset and negative values denote earlier onset. The green dots mark the grid points for which the trend exceeds the 95% confidence level.