Continental drift and plateau uplift control origination and evolution of Asian and Australian monsoons

Evolutions of Asian and Australian monsoons have important significance for understanding the past global change but are still a controversial subject. Here, we explore systematically the effects of plate movement and plateau uplift on the formation and evolution of the Asian and Australian monsoons by numerical simulations based on land-sea distributions and topographic conditions for five typical geological periods during the Cenozoic. Our results suggest that the timings and causes of formation of the monsoons in South Asia, East Asia and northern Australia are different. The Indian Subcontinent, which was located in the tropical Southern Hemisphere in the Paleocene, was influenced by the austral monsoon system simulated at that time. Once it moved to the tropical Northern Hemisphere in the Eocene, the South Asian monsoon established and remained persistently thereafter. However, the monsoons of East Asia and northern Australia did not appear until the Miocene. The establishment of the simulated low-latitude South Asian (northern Australian) monsoon appeared to have strongly depended on the location of mainland India (Australia), associated with northward plate motion, without much relation to the plateau uplift. On the contrary, the establishment of the mid-latitude East Asian monsoon was mainly controlled by the uplift of Tibetan plateau.

3 of 25Ma was estimated in a similar way as H25 = H45 + (H15 -H45)*20/30. Finally, we modified major topographic and bathymetric features according to a large amount of geological evidence and overlaid the resulted paleo-topography/bathymetry on the corresponding reconstructed land-sea distributions using ArcGIS. For example, we modified the paleo-topography of the Rocky Mountains by first calculating the paleo-tomodern elevation ratios of different geological periods obtained from geological evidence (Table S1) and then applied the mean ratios of a given geological period to the modern topography to get the paleo-topography. The same approach was employed to estimate paleo-topography for the Andes Mountains (Table S2), Greenland and Antarctica (Table   S3), and the Pamirs, Tienshan Mountains, Iran Plateau, and Mongolian Plateau (Table S4).
For the paleo-topography of the Tibetan Plateau (TP), we first divided the TP into 5 terranes (Table S4). For each terrane block, we calculated the paleo-to-modern elevation ratios for each geological period and then applied the ratios to the modern TP topography to get the paleo-topography. The paleo-elevation data used in the reconstructions of the TP paleotopography are listed in Table S5. Another consideration is the paleo-latitudinal positions of the TP and the Pamirs as determined by paleomagnetism and other geological evidence. For example, paleomagnetism evidence indicated that Dingri County of Tibet (29°N, 87°E at the present) was located at 11°N, 17°N, and 23°N during 40Ma, 25Ma, and 10Ma 51 , respectively, while its longitudinal position did not change much over time. Other evidence suggested that the southern rim of the TP was located at 10-15°N during the Eocene 52 .
Accordingly, we adjusted the latitudinal positions of the TP by moving it as a whole from south to north to its locations during LE, LO, and LM. Similarly, we modified the paleolatitudes of the Pamirs, also representing a south-to-north movement over time 53 (Table S6): Exps. 1-5 corresponding to the 5 geological periods during the Cenozoic: Present-day (PD), late Miocene (LM), late Oligocene (LO), late Eocene (LE) and mid-Paleocene (MP), using the reconstructed land-sea configurations and topography/bathymetry at 0Ma, ~10Ma, ~25Ma, ~40Ma, and ~60Ma, respectively, regridded onto the FAMOUS grids and smoothed appropriately. To simplify the physical processes and allow us to focus on the effects of land-sea configuration and topography, we used the pre-industrial CO 2 level of 280 ppmv for all but the LE period (Exp. 4), during which the CO 2 level was set as 4×pre-industrial level in reference to existing geological records 54 and previous simulation work 27 . Taking into account the fact that large-scale tectonic uplifts of TP mainly occurred in the Eocene or later (Table S5) The second set of numeric simulations included 6 comparative experiments (Table S7).
Exps. 1a-4a used the same land-sea configurations as in Exps. 1-4, but the global topography was set to zero or the sea level. Additionally, there were two extra experiments 4b and 4c for the LE period. The only difference between Exp. 4b and Exp. 4 was the CO 2 level, which was set as 280 ppmv in Exp. 4b as compared to 4×280 ppmv in Exp. 4, while Exp. 4c presents a low CO 2 condition (280 ppmv) without global topography. All experiments with topography are abbreviated as "OROG", while all zero-topography experiments as "FLAT" in the following.

Changes in the East Asian summer monsoon circulation and the causes. An important
consideration when defining the East Asian monsoon is the alternating seasonal winds originated from the tropical ocean and interior of the land mass. The climate model used in this study showed good performance in simulating distributions of the East Asian regional circulation and precipitation (Fig. S3). Since the northernmost limit of the summer 850 hPa winds originating from the tropical ocean marks the northern boundary of the typical summer monsoon circulation, it can be used to illustrate the northward expansion of the monsoon region in East Asia during the Cenozoic. Figure S4 shows the progressively northward migration of the northern boundary of the summer monsoon circulation from the southern coast of China (~26°N) in MP (~60Ma), to roughly the Yangtze River valley (~32°N) in LE (~40Ma) and LO (~25Ma), and then further to North China in LM (~10Ma) and finally reaching north of 42°N in PD (0Ma). Prior to 25 Ma (Figs S4a-c), the majority of the East Asian land mass, especially North China, was dominated by the westerly circulation in summer. During LE (40 Ma), there was a strong low pressure system centered at ~110°E, 55°N and the southwesterly winds in the southern part of this system produced increased summer rainfall over the Korean Peninsula and Japan Islands (Fig. 1b). However, this circulation pattern does not qualify as the typical summer monsoon circulation for East Asia as the monsoon winds originating from the tropical ocean did not reach the region beyond the Yangtze River valley at this time (Fig. S4b). In other words, a simple increase in summer precipitation does not necessarily indicate the presence of a typical monsoon climate. Only after 10 Ma, meridional (southerly) winds progressed northward beyond 35°N ( Fig. S4d) and eventually reached their modern position of ~42°N (Fig. S4e).

Forcings of the changes in summer rainfall in East Asia. Topography played an
important role in shaping the modern East Asian monsoon circulation. Figure S5 shows the results of the experiments for the same land-sea configuration as in Fig. S4, but with no global topography. In these cases, there was barely any area over the mainland East Asian that could be defined as being controlled by a typical summer monsoon circulation. This means that for East Asia, as the land-sea configuration remained relatively stable during the Cenozoic, the uplift of the TP (and global topography) was critical for the establishment of the East Asian monsoon. Without the topography, summer monsoon circulation would not have existed even under the present-day land-sea configuration (Fig. S5d). Changes in the summer circulation pattern caused by removal of topography are reflected in distributions of rainfall in East Asia (Figs S6a-d), as the differences in summer rainfall between the basic experiments (Exp. 1-4, Table S6) and the corresponding comparative experiments (Exp. 1a-4a, Table S7)