Indian monsoon variability in response to orbital forcing during the late Pliocene

The Asian monsoon is a major component of the global climate system and can be divided into two subsystems, the Indian monsoon and the East Asian monsoon. Insights into monsoon behaviour and dynamics can be gained through studying past warm intervals in Earth's history. One such interval is the Pliocene epoch, specifically the mid-Piacenzian Warm Period (mPWP; 3.264–3.025 Ma). This time is characterised as a period of sustained warmth, with annual mean temperatures 2–3 °C higher than the pre-industrial era. Studies have examined the East Asian monsoon during the mPWP from both a geological data and climate modelling perspective. However, there has been little investigation into the behaviour of the Indian monsoon. Using a coupled atmosphere-ocean global climate model (HadCM3), the Indian summer monsoon response to orbital forcing during the mPWP is studied.

Of the simulated interglacial events (Marine Isotope Stages KM5c, KM3, K1 and G17), MIS KM5c is the only one with a near-modern orbital forcing. This experiment is compared to a pre-industrial simulation to determine the nature of the mPWP Indian summer monsoon in the absence of a different pattern if insolation forcing. The monsoon at MIS KM5c, is simulated to be stronger than pre-industrial, with higher surface air temperatures and precipitation over land due to higher levels of CO2. MIS G17, K1, and KM3 represent interglacial events of similar magnitude with different insolation forcing than MIS KM5c. The Indian summer monsoon is simulated to be significantly stronger for the interglacials K1 and KM3, compared to KM5c. This is due to stronger precession forcing causing an increase in summer surface air temperature and precipitation. When combined with Pliocene geological boundary conditions, these results highlight the significant effect of orbital forcing on the strength of the Indian summer monsoon. The sensitivity of the Indian monsoon to orbital forcing has important implications for any parallels drawn between Pliocene and future monsoon behaviour.