Orbit-induced rainfall dipole pattern in South Asia over the past 425 ka

On decadal, centennial, and millennial time scales, the South Asian summer monsoon (SASM) precipitation often exhibits distinct east-west or north-south dipole precipitation patterns, presenting large spatial variations in precipitation across the region. However, it remains unclear whether these spatial differences in monsoon precipitation persist over longer orbital timescales. To address this issue, we analyzed a series of equilibrium simulations using the NorESM-L model, spanning the past 425 ka, and investigated the sensitivity of precipitation to potential forcings. Our investigations, in agreement with previous studies, demonstrate that the strength of the SASM is primarily influenced by northern hemisphere summer insolation. The northward shift of the ascending core of the Hadley circulation further modulates the spatial distribution of precipitation in South Asia on the orbital timescale. In general, during periods of strong (weak) SASM associated with high (low) northern hemisphere summer insolation, the simulated precipitation in the northern part of South Asia is higher (lower) compared to the southern region. These findings align to some extent with available geological records, supporting the simulated north-south dipole precipitation pattern in South Asia. This temporal and spatial variability in precipitation within the South Asian monsoon domain highlights the complex hydrological conditions in the monsoon tropic, indicating that higher precipitation is not always accompanied by higher temperatures on the orbital timescale. These findings depart from modern understanding of synchronized precipitation and temperature patterns in the South Asian monsoon region.