No abstract

The Indian summer monsoon (ISM) and associated rainfall over India (ISMR) exhibit variability ranging from interannual to centennial periods which can be attributed mainly to the interactions among the atmosphere, the land and surrounding warm oceans. ISM also shows internal variability within a season termed the intraseasonal variability (ISV) manifested as fluctuations between the active spells with good rain and the break spells with scanty rainfall. These rainfall fluctuations are associated with corresponding changes in circulation. This active-break cycle is most pronounced over the seasonal trough zone (Blanford, 1886) over the Indo-Gangetic plain of central India during the peak monsoon months of July and August when the entire country comes under the sway of ISM. This is part of the planetary scale inter-tropical convergence zone (ITCZ) which gets established over the monsoon core zone (MCZ, Rajeevan et al., 2010) on the culmination of the onset phase of ISM (delineated in Figure 1). Frequent occurrences of intense active/break spells in a season will have a decisive impact on the seasonal monsoon rainfall over the country. For example, a prolonged break spell during the monsoon season of 2002 resulted in drought conditions (Sajani et al., 2007). The evolution of the monsoon in a season or its subseasonal behavior is as important as the seasonal rainfall because the spatio-temporal distribution of rains during the season is inextricably linked to India's agriculture and shapes several other aspects of life in the country.The Indian subcontinent comprising varied geographic features, dense population, industrial zones and surrounding oceans, is a source region for aerosols. The regional aerosols over India influence the monsoon by altering radiation and functioning as nuclei of ice and liquid clouds. Conversely, the ISM circulation and precipitation affect aerosol emissions, transport, dry deposition and wet scavenging. There are several observational (e.g.,