A modeling study of the response of Asian summertime climate to the largest geologic forcings of the past 50 Ma

Abstract

The modern Asian summer monsoon is known to be affected by the modern continental geometry, orography, atmospheric composition, interglacial climate state, and orbital configuration. All of these factors, however, have undergone substantial changes since the Indian and Asian continents collided 50 million years ago. Within the framework of one general circulation model we evaluate the relative importance of each of these factors for the spatial patterns of summertime climate fields and precipitation-weighted 18 O. We find that the continental-scale structure of the monsoon circulation remains robust, but there are important impacts on local environmental conditions. The largest differences in climate are associated with changes in orbital configuration and topography because of how those factors influence local gradients in the summer climate variables. Finally, model calculations are consistent with the modern understanding of monsoon dynamics in which the strength of the circulation and the distribution of rainfall are tightly coupled to the spatial patterns of low-level moist static energy and upper tropospheric temperature.

The neglected Indo‐Gangetic Plains low‐level jet and its importance for moisture transport and precipitation during the peak summer monsoon

“…This unblocked mode simulated by the 2°C AM4 and 5 models has serious implications for studies that suggest the IAM is triggered, driven or maintained by orographic heating [Huber and Goldner, 2012;Wu et al, 2012;Chen et al, 2014;Xu et al, 2014]. If so, then perhaps theories regarding orographic surface heating should be revisited using higher-resolution models [Ma et al, 2014;Roe et al, 2016] and the diagnostics applied here.…”

Competing Topographic Mechanisms for the Summer Indo‐Asian Monsoon

“…Along the way, we evaluate the three main hypotheses whether topographic thermal forcings He et al, 2018;Wu et al, 2007; or the insulation mechanism (Biasutti et al, 2018;Boos, 2015;Boos & Hurley, 2013;Ma et al, 2014), or mechanical and orographic forcing are necessary or sufficient conditions for generating a modern-like IAM. the models' biases and brings the regional circulation in better agreement with observations is necessary for understanding monsoon-topography interaction (Acosta & Huber, 2017;Johnson et al, 2015;Meehl et al, 2006;Roe et al, 2016;Shields et al, 2016). Additionally, our high-resolution modeling approach enables us to also look at how smaller terrains such as the Western Ghats and Mizoram Mountains impact the regional circulation.…”

“…See the supporting information for further details and comparison between our simulation and various reanalysis products. As previously shown, low‐resolution models tend to misrepresent crucial topography‐monsoon interactions; therefore, utilizing a very high‐resolution configuration that reduces the models' biases and brings the regional circulation in better agreement with observations is necessary for understanding monsoon‐topography interaction (Acosta & Huber, ; Johnson et al, ; Meehl et al, ; Roe et al, ; Shields et al, ). Additionally, our high‐resolution modeling approach enables us to also look at how smaller terrains such as the Western Ghats and Mizoram Mountains impact the regional circulation.…”

Paleogene evolution and demise of the proto‐Paratethys Sea in Central Asia (Tarim and Tajik basins): Role of intensified tectonic activity at ca. 41 Ma

“…A third view suggests that the westward retreat of the proto-Paratethys Sea was the controlling factor for both the aridification by removing a moisture source, and the intensification of the Asian monsoons by increasing the land-sea contrast (Ramstein et al, 1997;Zhang et al, 2007). However, recent proxy data and climate models show that the sea was too shallow to affect the thermal contrast generating monsoons (Bougeois, 2014;Bougeois et al, 2018;Licht et al, 2016Licht et al, , 2014Roe et al, 2016;Zhang et al, 2012). Recent indications from proxy and model data rather show that Asian aridification since the early Eocene is linked to a decrease in the westerly moisture flux (Caves et al, 2015) and that the proto-Paratethys Sea provided moisture via the westerlies which precipitated in Central Asia during wintertime (Bougeois, 2014;Bougeois et al, 2018).…”