Geological controls on the evolution of Asian climate with specific reference to topography, ice sheets and CO2

The hydrological regime in East and South Asia is dominated by the monsoons, whilst central
Asia is characterized as arid. Defining the timing of the onset of aridity and the intensification
of the monsoons in Asia, has generated significant debate over the years. The uplift of the
Tibetan Plateau, the retreat of the Paratethys Sea, the atmospheric carbon dioxide (CO
2
decrease, and the associated global cooling after the Eocene/Oligocene transition are all
considered to be major drivers of Asian aridity and monsoonal intensification. Here, a series
of sensitivity simulations of a fully coupled ocean–atmosphere climate model (HadCM3) are
carried out to investigate the effect of the atmospheric CO
2
variability, the uplifts of the
Tibetan Plateau, Mongolian Plateau and Tian Shan orogen, and the Greenland and Antarctic
ice-sheets formation, on Central Asian aridity, the East Asian Monsoon circulation and
monsoon driven precipitation. Results show that increasing of the CO
2
conditions for the East Asian and South Asian Monsoons and a steeper transition from non-
monsoonal to monsoonal conditions in South Asia, while Arid East Central Asia becomes
wetter during the non-monsoonal months. Results also show that even though ice-sheet
coverage and CO
2
causes wetter
changes play a role on the Asian climate and its sub-systems, the latitudinal
position and elevation of the Tibetan Plateau are the primary drivers for the Asian climate
evolution. Absence of high-elevation in Asia shows increased surface temperatures, and
decreased moisture availability and precipitation over Asia. Additionally, with the Tibetan
Plateau at lower elevations, the westerlies flow zonally limiting the precipitation over East
Asia to the summer months. Above an elevation threshold of 3000 m, simulations show a
shift to a modern-like East Asian Monsoon circulation and Arid Central Asia conditions
suggesting that the shift towards modern-like conditions over Asia is controlled by the high-
elevation Central Asia.