Impact of precession on monsoon characteristics from coupled ocean atmosphere experiments: changes in Indian monsoon and Indian ocean climatology

Using a fully coupled ocean–atmosphere model, we have investigated the response of the Indian and southeast monsoons to changes in precession, with special emphasis on the Indian Ocean. We designed our experiments in order to explore the differences between periods where the monsoon activity is strong compared to the period of reference (115 ka BP), and the seasonal timing of the insolation forcing is different. The two extreme cases are such that the maximum insolation occurs between the vernal equinox and the summer solstice in one case and between the summer solstice and the autumnal equinox in the other. Our results show that, even though the changes in the land–sea contrast that drive the monsoon flow follow quite well the differences in the insolation forcing, the regional distribution of monsoon characteristics (monsoon flow, precipitation etc.) from Africa to Southeast Asia and over the ocean substantially varies from one simulation to the other. Large differences are found in the simulated surface temperature and salinity in the Indian Ocean. They are mainly due to changes in the hydrological cycle over the basin (precipitation, evaporation and river runoff). Our results strongly emphasise that the timing of the seasonal cycle must be taken into account in analyses of past monsoons, and that it may explain some of the leads and lags reported between insolation and various monsoon proxies reported in the literature