IMPACT OF PRECESSION ON MONSOON CHARACTERISTICS FROM COUPLED OCEAN ATMOSPHERE EXPERIMENTS

Precession cycle modulates the seasonal distribution of the incoming solar radiation at the top of the atmosphere with a periodicity of about 23 kyr. Summer insolation is the largest for periods during which the Earth is near the perihelion of its orbit during summer. The associated continental warming favours the deepening of the summer thermal low over the Northern Hemisphere continents and the inland advection of moist air from the tropical oceans, strengthening the monsoon activity. Different orbital configurations (precession) can lead to large June-July-August (summer) insolation forcing. Amongst these, the maximum insolation can occur between the vernal equinox and the summer solstice or between the summer solstice and the autumnal equinox. Using a fully coupled ocean-atmosphere model we investigate the response of the Indian and southeast monsoons to changes in precession and we explore the differences between periods where the monsoon activity is strong compared to the period of reference but the seasonal timing of the insolation forcing is different. Our aim is to determine if extreme phases in the seasonal forcing can lead to different signatures in the monsoon response. We focus on the Asian monsoon and on the atmospheric and oceanic circulation in the Indian Ocean. Our results show that, even though the changes in the land-sea contrast that drives the monsoon flow follows quite well the differences in the insolation forcing, the regional distribution over the continental regions affected by the monsoon and 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 related to various feedbacks, where the changes in the hydrological cycle over the basin through precipitation, evaporation and river runoff play and important role. Our results strongly emphasize that the timing of the seasonal cycle need to be considered in analyses of monsoon changes in the past, and that it may explain some of the lead and lags reported in the literature between insolation and various monsoon proxies.