Impact of Solar Activity and ENSO on the Early Summer Asian Monsoon During the Last Millennium

The Asian Monsoonal rainfall accounts for the majority of annual regional precipitation in East and South Asia and could be remotely regulated by El Nino-Southern Oscillation (ENSO). Besides, several paleoclimate records and simulations have indicated solar signals in the Asian Monsoon, implying the impact of solar activity on the regional monsoon precipitation. By conducting multi-linear regression analysis to the solar irradiance forced single-forcing experiment in the last millennium, this study presents the comparison of solar and ENSO effects on monsoonal precipitation in South and East Asia during early summer (May-June). Increased total solar irradiance during high solar activity years tends to trigger a favorable environment for developing monsoon onset, leading to more precipitation against ENSO-related patterns over Southeast and South Asia before peak-summer (July-August). The result supports reconstructed terrestrial records and underlines considerable influences of the solar cycle on the variation of the Asian Summer Monsoon. Previous studies have shown the influence of the solar cycle on various climate systems, such as El Nino-Southern Oscillation (ENSO), Atlantic Oscillation, Pacific Decadal Oscillation, etc., despite the relatively small amount of solar irradiance variations to the total amount of solar insolation. In addition, paleo-records indicate the correlation between the long-term solar cycle and the variation of the Asian Monsoon. This study attempts to clarify the role of solar activity in monsoonal precipitation over South and East Asia. We find that during early summer (May-June), high solar activity tends to enhance the meridional temperature gradient over the northern Indian Ocean. An associated meridional circulation is thereby intensified and well-coupled with monsoonal circulations. The coupling contributes to more moisture transport and increased precipitation over the subcontinent, contrasting with the reduced rainfall and suppressed monsoonal circulation over the northern Indian Ocean during warmer ENSO events. This result indicates that the effects of changes in solar irradiation on climate systems should be considered, particularly in the Asian Monsoon region. Furthermore, a physical explanation is provided for the solar signals in the Asian Monsoon in paleo-evidences. The Multi-Linear Regression analysis is used to isolate solar activity and El Nino-Southern Oscillation (ENSO) signals from the CESM-LME project's solar-only experiment Sub-seasonal analyses to the Asian Summer Monsoon are highlighted by varied responses to solar activity in early and peak summer Higher solar activity increases early summer rainfall around Southeast Asia against ENSO-related patterns by enhancing monsoonal circulation