Asynchronous responses of East Asian and Indian summer monsoons to mountain uplift shown by regional climate modelling experiments

Abstract

It has been demonstrated in climate models that both the Indian and East Asian summer monsoons (ISM and EASM) are strengthened by the uplift of the entire Asian orography or Tibetan Plateau (TP) (i.e. bulk mountain uplift). Such an effect is widely perceived as the major mechanism contributing to the evolution of Asian summer monsoons in the Neogene. However, geological evidence suggests more diachronous growth of the Asian orography (i.e. regional mountain uplift) than bulk mountain uplift. This demands a re-evaluation of the relation between mountain uplift and the Asian monsoon in the geological periods. In this study, sensitivity experiments considering the diachronous growth of different parts of the Asian orography are performed using the regional climate model COSMO-CLM to investigate their effects on the Asian summer monsoons. The results show that, different from the bulk mountain uplift, the regional mountain uplift can lead to an asynchronous development of the ISM and EASM. While the ISM is primarily intensified by the thermal insulation (mechanical blocking) effect of the southern TP (Zagros Mountains), the EASM is mainly enhanced by the surface sensible heating of the central, northern and eastern TP. Such elevated surface heating can induce a low-level cyclonic anomaly around the TP that reduces the ISM by suppressing the lower tropospheric monsoon vorticity, but promotes the EASM by strengthening the warm advection from the south of the TP that sustains the monsoon convection. Our findings provide new insights to the evolution of the Asian summer monsoons and their interaction with the tectonic changes in the Neogene.

Influence of the Tibetan Plateau uplift on the Asian monsoon-arid environment evolution

“…Wu et al [77], through numerical experiments by only considering the dynamic and thermal effects of the Iranian Plateau (the TP excluded), found that the monsoon circulation in the South Asian region south of 20°N was mainly regulated by the land-sea thermal contrast, while to the north it was controlled by the thermal forcing of the Iranian Plateau, showing considerable effects of the Iranian Plateau in maintaining the South Asian monsoon. The sub-regional climatic simulation by Tang et al [79] also indicated that the Iranian Plateau would promote the development of the South Asian summer monsoon and constrain the East Asian summer monsoon.…”

“…[79] Only the Himalayas and adjacent narrow terrain were retained, and the surface sensible heat flux from the Himalayas or from the plain of south of the plateau was suppressed.…”

“…Based on the geological evidence of the tectonic uplift of the northern TP since the Pliocene, Zhang and Liu [76] designed numerical experiments with topographic height reduced in a limited range (average topography decreased by about 800 m) also found that the uplift of northern TP led to intensified northern East Asian summer monsoon, thus increased precipitation in the northern East Asian monsoon region north of the Yangtze River. Most recently, Tang et al [79] conducted a series of sensitivity experiments on the uplift effect of different sub-regions of the TP on the South Asian and East Asian summer monsoons by using a regional climate model with a higher resolution. Their results also indicated some differences in responses of the Indian summer monsoon and East Asian summer monsoon to the TP uplift: the former was mainly controlled by the mechanical barrier effect of the topography in the southern plateau, while the latter was strengthened due to the thermal forcing in the central and northern plateau.…”

The resolution sensitivity of the South Asian monsoon and Indo-Pacific in a global 0.35° AGCM

“…Several adjustments to the deep convection scheme have been introduced since Gregory and Rowntree (1990), such as the addition of down-drafts (Gregory and Allen 1991), convective momentum transport using a flux gradient approach (Stratton et al 2009) and a convective available potential energy (CAPE) closure scheme, based on Fritsch and Chappell (1980a, b), which uses a dilute CAPE calculation. In the default MetUM GA3 configuration, the time scale for the CAPE closure is set at 90 min unless large-scale vertical velocities over 0.3 m s −1 are detected in the column, in which case the time scale is reduced to ensure numerical stability.…”

The link between Tibetan Plateau monsoon and Indian summer precipitation: a linear diagnostic perspective

“…Numerical experiments confirmed that the thermal forcing of TP plays an important role on the establishment and development of the Asian summer monsoon (Broccoli and Manabe 1992;Liu et al 2001Liu et al , 2004Schneider and Bordoni 2008;Wu et al 2012b;Tang et al 2013;Chen et al 2014). In the preceding spring season, the surface thermal (ISMI).…”