As a prominent and independent component of the grand Asian summer monsoon system, EASM features a huge monsoon low over the East Asian continent and a seasonally northward march of intense southerly flows. The huge monsoonal low has been attributed to the seasonal variation of solar radiation and the associated thermal contrast between the Asia Land and the Pacific Ocean. The influence of the dynamical and thermal forcing by the Tibetan Plateau is another mechanism for the EASM formation. However, these explanations are still incomplete in understanding the physical processes of the EASM formation. Recent evidence has recognized that diabatic heating, especially the atmospheric condensational heating associated with moisture processes, could contribute to the formation and maintenance of EASM. In our study, there is a reasonable hypothesis that the feedback of synoptic eddy activities also plays an indispensable role in the complete EASM regime.
A revisit on the three–dimension circulation structure of EASM suggests that upon the grand low–level monsoonal low with strong humid southerly flows extending from the tropics to Northeast Asia, there exists a distinct meridional difference in the vertical structure of EASM with a boundary of around 35.5oN. In the southern domain, EASM features a meridional overturning cell and a baroclinic structure with an upper–level high versus a lower–level low in geopotential height. While in the northern domain, the EASM exhibits an equivalent barotropic structure with an upper–level low versus a lower–level low, and the ascending accompanied with the northward motion and the descending with southward motion dominate to the east and west of 120oE over northeastern Asia, respectively. Such differences in the three–dimensional structures imply some different dynamics over the two domains.
What are the physical processes responsible for the formation of circulation systems with different vertical structures of EASM? Here, we answer it by focusing on the feedbacks of diabatic heating and synoptic eddy activities by diagnosing the QGPV equation, a powerful tool for identifying the seasonal–mean atmospheric response to the forcing. Regardless of friction, the diabatic heating as well as the transient eddy heat and vorticity transport are all important sources of forcing terms on the time–mean flow. The diabatic heating and the transient eddy heating forcing are both the thermal effect on the mean flow and could induce baroclinic geopotential tendencies. Differently, the geopotential tendencies induced by the transient eddy vorticity forcing present the equivalent–barotropic vertical structure.
Consistent with previous results, the baroclinic monsoonal circulation in the southern EASM domain is dynamically determined by diabatic heating, especially condensational heating. Specifically, the mid–tropospheric diabatic heating would induce the baroclinic circulation response characterized by the positive PV and negative geopotential tendency at the low level versus the negative PV and positive geopotential tendency at the upper level. By the advection effect of meridional wind on the ambient vorticity, the circulation response to the diabatic heating tends to shift westward, and then, in the final state of equilibrium, the southerly wind prevails at the lower level and the northerly wind prevails at the upper level. Thereby, a local meridional cell dominates over the southern EASM region and becomes a part of the tropical monsoonal circulation south of 20oN.
However, in the northern domain of EASM, if only considering the effect of diabatic heating, a regional high should prevail above the monsoonal low and the northern low is only confined to the lower troposphere, while this is clearly inconsistent with the observational fact, so we try to emphasize the role of the transient eddy forcing in the formation of the observed equivalent–barotropic low pressure here. In summer, the northern EASM region experiences strong heat and vorticity transport by synoptic transient eddies, which in turn feedback on the time–mean flow. The transient eddy heat flux diverges over the south of 40oN, yields the cooling effect on the middle troposphere, and consequently induces a distinct baroclinic structure with an upper–level low and a lower–level high tendency in the northern EASM. The structure of atmospheric response induced by the transient eddy heating forcing is exactly opposite that induced by the diabatic heating forcing, suggesting that the mid–tropospheric cooling effect by the transient eddy heat forcing might partly offset the diabatic heating forcing. However, the transient eddy vorticity forcing could produce a negative geopotential tendency centered at the upper layers with a barotropic vertical structure. Although the center of the initial atmospheric response induced by the transient eddy vorticity forcing lies northwest to the observed barotropic low over northeastern Asia, due to the adjustment of the relative vorticity and stretching vorticity advected by the globally zonal mean zonal flow, the initial atmospheric geopotential tendency induced by the transient eddy vorticity forcing could deviate from the initially–formed area and shift southeastward, and eventually, coincide with the observed regional barotropic low. Combined with the effect of the diabatic heating on the low–level flow, there presents a distinct vertical structure of regional low centered roughly at 42.5oN with deep southerlies and upward motion in the east and deep northerlies and downward motion in the west in the northern domain.
In summary, we conclude that in the complete EASM regime, the feedbacks of diabatic heating and transient eddy forcing are both important and nonnegligible mechanisms responsible for the formation of EASM. The vertical circulation structure over the southern EASM region is mainly determined by the impact of the diabatic heating that mainly comes from the local latent heating. However, the steady vertical structure of the northern EASM is attributed to the combined effect of the feedbacks of diabatic heating and synoptic eddy activities. Especially, in the northern EASM domain, the formation of the equivalent–barotropic circulation system at the mid–upper troposphere is induced by the synoptic transient eddy vorticity forcing.
Indeed, the role of transient eddy forcing on the formation of the summertime low-level low pressure in the northern EASM has also been noticed by Lin et al. (2021). Although their result shows that the effect of transient eddy forcing is negligible for the formation of Northern East Asia Low based on a linear baroclinic model with simplified physical processes and omitted non–linear interactions, Lin et al. (2021) has already realized some limits on their study and recognized the underestimation of the effect of transient eddy forcing. This underestimation is because the coarse–resolution reanalysis cannot resolve synoptic eddies sufficiently (Sang et al. 2021). Therefore, even though the feedback of transient eddy activities is somewhat weaker than that of diabatic heating in quantity, we still believe that the transient eddy, especially the transient eddy dynamical feedback works on the formation of the EASM structure in the northern domain.
The role of the midlatitude transient eddy activities in shaping the EASM structure gives us new insights into understanding the formation and variation of EASM from the perspective of wave–mean flow interactions. This study only focused on the transient eddy feedback on the summertime mean state of the East Asian monsoon system. How the transient eddy feedback affects the seasonal evolution, interannual and other timescale variabilities of EASM are still open questions, which need further deep exploration to improve the research on variation and prediction of EASM.