Numerical Simulation And Mechanism Study On The Impacts Of Tibetan Plateau Thermodynamic Forcing Over South Asian Summer Monsoon

The activities of Asian summer monsoon are very complicated under the interactions of sea-ocean-atmosphere. The scientists always consider the mechanical and thermal forcing of Tibetan Plateau (TP) as an important factor in regularing Asian monsoon. But, in boreal summer, there still exist academic difference on the physical mechanism of TP effect over the formation and maintenance of South Asian Summer Monsoon (SASM). Most of the studies indicate that the surface sensible heat over TP and the latern heating in upper troposphere could have an impaction on the surrounding circulations, and it is the mainly contributor to the buildup of the South Asian High (SAH) together with the formation and maintenance of SASM. On the contrary, some scholars have raised a different viewpoint, the insulation of Himalaya make warm entropy accumulated on the south foothill and the cold entropy on the north. The high entropy caused the moist convections and linked directly with the vertical profile of virtual temperature, so the temperature in high troposphere becomes warmer and drives the SASM circulation. Therefore, it is necessary to have a new acquaintance on the mechanical and thermal effect of TP and Himalaya.Based on various observations and numerical simulations, the mechanical and thermal effects of TP and Iran Plateau (IP) are studied on the topic of formation and maintenance of SASM. A further discussion has been taken to find out the heating effect over south slope of TP and thermal forcing of Himalaya. Besides, based on the thermal wind balance and geostrophic wind balance, we discuss the physical mechanism of diabatic heating near the SAH ridge line could warm the troposphere temperature on its west, and test this hypothesis by numerical simulations. The main conclusions are derived as follows:(1) The topographical sensitive tests by FGOALS-s2atmospheric component indicate that, the thermal contrast of large scale land sea distribution is the dominant control of the SASM south branch, the mechanical and thermal impact of TP and IP is the main cause of the precipitation and circulation over India continent and south slope of TP. While on the condition of mechanical forcing of TP and IP without sensible heating, the air flow move along isentropic surface around the plateau, the precipitation cannot shift to the north of India continent, and the north branch of SASM cannot buildup. While the surface sensible heat exists over TP, the circulation must adapt to the diabatic heating, the air flow near TP can penetrate the isentropic surface and tend to cross the plateau, bring the water vapor at low level to the high level of troposphere. The large scale diabatic heating cause the cyclonic low level circulation, in the thermal experiment of TP solo, the cyclonic circulation anomaly cause the precipitation over west of TP to decrease and south of TP with East Asia to increase; while in the thermal experiment of IP solo, the cyclonic anomaly cause the precipitation over west of TP to increase. The linear composition of the results of these two experiments are almost in accordance with the results of whole thermal experiment, the precipitation pattern is also very similar to the land-sea contrast experiment, it indicates that the heating effect of TP and IP is the dominant control of the formation and maintenance of SASM north branch, while the insulation effect is not important.(2) The results of slope, platform and north India thermal experiments show that the slope heating is the main cause of the Asia continental cyclonic circulation formation at low level, while the thermal effects of the platform and north India on monsoon precipitation and circulation are quite weak and can be neglected. The diabatic heating over the TP slope bend the isentropic surface on each level and cause the vertical motion at the slope surface, form the precipitation of SASM north branch. The heating on the platform cause the upper level temperature change greatly, but due to the little water vapor above4000m, the precipitation of SASM is not affected much by platform heating. In addition, the thermal forcing over India north continent has a limited effect on the local precipitation, but it is not the main driven force of SASM.(3) The sensitive test of Himalaya orographic uplift shows that, as the orography increasing, the sensible heating of Himalaya south slope is also intensified, it cause the SASM north branch precipitation to increase. Moreover, with the relationship of thermodynamic balance and Sverdrup balance, we come to the exponential relationship between the meridional wind change with the distribution of diabatic heating and potential temperature. The calculated wind response is well in coordinated with the increase of diabatic heating, vertical velocity, precipitation and the simulated wind, moreover, the circulation pattern is determined by the spatial pattern of potential temperature and vertical heating scale.(4) According to the thermal wind balance, we diagnose that the upper-level temperature maximum over west TP is a kind of quasi-geostropic adaption. It is not the result of moist convection heating or the surface high entropy, but is a response of temperature to the circulations. On the condition of thermal wind balance, the temperature must achieve its maximum on the SAH ridge line, and the diabatic heating in the mid-troposphere over East of TP and East Asia is the driven force of this warm center, the change of the temperature in the west is in proportion to the zonal gradient of diabatic heating.(5) According to the relationship we found, a new physical mechanism has been proposed on how the heating over SAH ridge line increasing the temperature on its west. Because in the subtropical region, the atmospheric motion should obey quasi-geostropic relationship, the Coriolis force must equal to pressure gradient force. While the diabatic heating generate on the east of TP, the vertical inhomogeneous diabatic heating cause the meridional wind to change on the upper level by Sverdrup balance, so the Coriolis force enhanced and the balance of the two force break up, the air particle must move to the west to intensify the pressure gradient, result in the air column to be thick over west of TP, the SAH intensified, and the thickness field increasing, cause the temperature to increase; meanwhile, because of the generation of the downward vertical velocity, the convections over west of TP are weaken, the precipitation tend to decrease. The observed decadal change of circulation and precipitation in Asian boreal summer support the possibility of the secondary circulation’s existence. We further test the rationality of this hypothesis by numerical simulations. By adding a fixed heating source on the ridge line over east of TP, the temperature maximum response over west is intensifed obviously, the zonal cross-section of the circulation indicate that there exist a east wind anomaly on the upper troposphere, and a descend motion anomaly under the temperature maximum; in addition, a quasi60days cycle heating source test has been preformed, and the results show that, the heating period can be propagate from east of TP to its west, the time serials of temperature maximum and vertical velocity on west of TP are in the same phase with the heating source over east. Then we come to the conclusion that the formation of the upper-level temperature maximum over west of TP is not driven from the moist convection caused by insulate effect of Himalaya, but is the response to the circulation by the large scale diabatic heating on east of TP.All the evidence shows that, the formation of SASM is dominant controlled by land sea thermal contrast and Asia continental heating effect.