Stratospheric aerosols and the Indian monsoon

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Abstract
This paper reports on a research study of the association between stratospheric aerosols and Indian monsoon precipitation. It was found that low‐latitude aerosols precede below average precipitation and high‐latitude aerosols precede above average precipitation. A chi‐squared analysis shows that the association is above the 99% level of significance. The second part of the research concerns the transformation of low‐latitude aerosols into high‐latitude aerosols as they move poleward in the second year of their existence. The model predicts that below average monsoon precipitation of the first year after a low‐latitude eruption would be followed by above average monsoon precipitation in the second year. The findings support the prediction. Table 7 shows that below average monsoon years are twice as likely to be followed by above average monsoon years as by below average monsoon years. Because aerosol research suffers from inadequate reporting of the events (see appendices A, B, and C), the third section of this research study uses El Niño events as a proxy for the presence of low‐latitude stratospheric aerosols. Previous research of Handler (1984, 1986) and Parker (1986) has shown that there is a high level of statistical association (>98%) between the presence of low‐latitude stratospheric aerosols and El Niño events. The 22 moderate and strong El Niño events (Quinn et al., 1978) are used as a proxy for the presence of low‐latitude aerosols. Six of the 22 events are not useful for the analysis because of the presence of interfering aerosols. Of the 16 possible El Niño events with no interfering aerosols present, 13 show the proper sequence of below average and then above average monsoon precipitation. Since recent aerosols data are generally more complete, a separate analysis of the 1942–1984 period is presented. In that 43 year period, the Indian monsoon fits the stratospheric forcing model very closely. Low‐latitude aerosols are associated with below average monsoon precipitation and high‐latitude aerosols are associated with above average monsoon precipitation. Six years are indeterminate because of the interfering effects of high‐ and low‐latitude aerosols. The data are consistent with the global climate models of Kutzbach and Guetter (1986). The forcing of a low‐latitude aerosol is assumed equivalent to a decrease in solar radiation in that both reduce the equator‐to‐pole temperature gradient. The Kutzbach and Guetter study has shown that a small change of solar radiation can produce a large change in monsoon precipitation. Finally, the research suggests that the long term, secular behavior of Indian monsoon precipitation can be related to the frequency of low‐latitude volcanic eruptions. During the early part of the 20th century, low‐latitude eruptions were frequent and the average monsoon precipitation was low. During the period 1942–1962 the frequency of low‐latitude eruptions was low and the average monsoon precipitation was high.