New insights about the Late Eocene monsoon-like atmospheric circulation using the IPSL Earth System Model: what is certain, less certain and very uncertain !
Résumé
New late Eocene simulations have been done with the fully coupled IPSL-CM5A2 model using a pCO2 of 1120 ppm and an Eocene-like land-sea mask. The model simulates very-well strong seasonal precipitations over some regions in India, Myanmar or eastern China. In the same regions, several Asian fossils dated from the late Eocene have been used to suggest the presence of a monsoon as soon as 40 Ma. However, major features of the modern monsoon, usually hardly recognizable in the data, such as the presence of a winter Siberian high-pressure cell, the existence of a strong summer Somali Jet, or the winter/summer wind inversion, are lacking in our simulation.
The large spatial extent of the Tethys Sea at that time induces the set-up of a strong anticyclonic circulation westward of the Himalaya - Tibetan Plateau and produces intense southward winds in the eastern part of the anticyclone. Those winds seem to block the summer northward flow of equatorial moist airmasses which are diverted eastward over India at lower latitudes than today. Another striking feature of our late Eocene simulation is the strong westerly wind that prevails all year round at tropical latitudes. This large zonal wind prevents the onset of deep moist convection as it tends to induce downward subduction of dry and cold air masses on its way over the southern Tibetan Plateau.
As the reconstruction of the paleogeography in late Eocene Asia is not consensual, we have confronted our new findings to a range of potential boundary conditions. Specifically, several important features such as the height and position of the Tibetan Plateau or the extent of the Greater India are still debated. Moreover, coeval with these changes, the Eocene cooling and the several phases of the Paratethys sea’s marine regression and transgression might also have affected significantly the resulting climate. We show here that our general climatic features remain stable under the several forcings tested. Strong westerly winds associated to the Tethysian anticyclone result in a large arid band over the region surrounding the Tibetan Plateau and prevent a migration of the ITCZ northward of 10°N in northern India. The eastern part of the Asian continent is more sensitive to the seasonal shift of the ITCZ and appears more humid, with a southern limit of the arid zone located around 18 to 24°N.