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Abstract

South Asian summer monsoon (SASM) delivers substantial rains to the Indian subcontinent and drives strong upwelling in the Arabian Sea, making marine upwelling records and terrestrial rainfall records two primary proxies for reconstructing past SASM variability. However, on orbital timescales, these two sets of records vary largely in opposite directions: the upwelling records are in-phase with Southern Hemisphere (SH) summer insolation, whereas the rainfall records are in-phase with Northern Hemisphere (NH) summer insolation. This leaves a long-standing debate on whether SASM is driven by NH or SH insolation. Here, combining paleoclimate records with transient climate simulations that explicitly separate the effects of the NH and SH insolation forcing, we show that the SASM rainfall is dominated by the NH insolation, whereas the Arabian Sea upwelling is forced predominantly by the SH insolation. When boreal summer occurs at perihelion, insolation is strongly enhanced not only in the NH but also in the tropical-subtropical SH. The former enhances the SASM rainfall through Eurasian warming, while the latter weakens the Arabian Sea upwelling by inducing South African warming and subsequent atmospheric teleconnections over the Indian Ocean. Our study reconciles the long-standing debate, and more broadly, reveals that warming in South Africa could exert a significant and previously overlooked remote forcing on the SASM system in past and future climate changes.