Contribution Of Atmospheric Internal Processes To The Interannual Variability Of The Asian Summer Monsoon

The Asia summer monsoon contains multi-scale systems, In addition to the monsoon mean flow, the low frequency (intraseasonal oscillation) and high frequency (synoptic disturbances) are both active. Moreover, the Asian summer monsoon exists significant interannual variability. Previous studies have found that the interannual variability of Asian summer monsoon is related to the atmospheric external forcing, such as the sea surface temperature, snow cover, and vegetation types. From the atmospheric energetics perspective, this study diagnosed the contribution of atmospheric internal processes, including the scale interaction, heating-circulation feedback, and kinetic advection, to the monsoon variability based on the mean kinetic energy (MKE) budget equation. The major findings are as follows.(1) During the years with strong monsoon circulation over the South Asian monsoon region, the enhancements of low-level MKE are resulted from the conversions of eddy kinetic energy (EKE) and the mean average available potential energy (MAPE). The high-level MKE, however, is mainly maintained by MAPE, but at the same time loses energy to EKE, which helps the growth of high-level eddy perturbations.(2) The<90 days disturbances were further decomposed into high frequency (<20 days) and low frequency (20-90 days) components to investigate the relative contributions of high- and low-frequency eddies interacting with different parts of monsoon mean flow (divergent, vorticity, and vertical wind shear) to the strength of monsoon mean flow. We found that the interaction between low frequency disturbance and monsoon vertical wind shear is the main process for the growth of MKE in low level during strong monsoon years.(3) The relative contribution of the atmospheric external factors (SST) and the internal factors (scale-interaction) to the interannual variability of South Asia summer monsoon was quantitatively compared. We found that the interannual variability of eastern Pacific SST can explain 30%-40% of the interannual variability of South Asia monsoon circulation strength, and scale interaction can explain about 15%-20%. This suggests that, in addition to the underlying surface status, the internal scale-interaction process needs to be considered in the monsoon forecast system.(4) During the years with enhanced convections in the South China Sea (SCS) summer monsoon, the low level MKE is strengthened over the southern SCS (south of 12 N) and Indochina. The increased MKE is associated with the eastward extension of strengthened westerly jet from the Indian summer monsoon region. In the northern SCS (north of 12 N) and western Pacific, an anomalous cyclonic circulation appears which is related to the deepening of monsoon trough.(5) The enhanced MKE in the southern SCS is supported by the interactions between eddy momentum fluxes and mean circulation. The monsoon mean flows lose less kinetic energy to eddies (i.e., mean flows retain more kinetic energy) during strong monsoon years. It is found that the interactions between<20-day eddies and monsoon divergent flows play a major role in inducing positive MKE anomaly. The intraseasonal oscillation-mean flow interaction shows a relatively higher contribution to the MKE change as compared to the eddy-mean flow interaction.(6) The MKE of monsoon trough over the northern SCS is maintained by the interaction between atmospheric heating and ascending motion anomaly. Meanwhile, the enhanced MKE is converted to eddy kinetic energy which is favorable for the growth of eddies. Thus, increased tropical cyclones were observed in the northern SCS. The northward propagating intraseasonal oscillation is also obvious during the years with enhanced monsoon trough.The results of this study may provide physical basis for the development of Asian summer monsoon forecast system. The relatively low predictability of year-to-year monsoon variations in the current atmospheric circulation model may be due to the limited ability of simulating the different components of the monsoon system as well as their interactions. Therefore, improving the model capability in simulating the atmospheric internal processes is one of the key steps directions when developing the monsoon forecast system.