Contribution of atmospheric internal processes to the interannual variability of the South Asian summer monsoon

Apart from the external forcings, the interannual variability of the South Asian summer monsoon can arise from atmospheric internal dynamics, although its mechanism has not been fully understood. Based on a three-dimensional mean kinetic energy (MKE) budget equation, the contributions of internally generated processes associated with mean flow-transient eddy interaction and circulation-heating feedback to the interannual variability of the South Asian summer monsoon were diagnosed using two reanalysis data sets. In years with enhanced convections over South Asia, a strengthened low (high)-level MKE related to southwesterly (easterly) jet is observed over the Arabian Sea. The enhancement of MKE at the low level is maintained by both the anomalous barotropic energy conversion between eddy kinetic energy (EKE) and MKE and the baroclinic energy conversion from mean available potential energy (MAPE) to MKE. Among the three-dimensional barotropic energy conversion processes, the vertical eddy momentum flux interacting with the monsoon vertical wind shear anomaly plays a leading role in supporting the MKE anomaly. Moreover, the contribution of eddy activities related to the intraseasonal oscillation (ISO) of 20-90 days is larger than that of high-frequency (<20days) disturbances. This mean flow-ISO interaction dominates the atmospheric energy conversion processes and external boundary factors that may exert the interannual variations of monsoonal MKE in the lower troposphere. Different from the low-level dynamics, scale interaction between eddies and the mean flow shows an insignificant contribution to the strengthening of the easterly jet in the higher troposphere. The Nino3.4 sea surface temperature anomaly appears to be a key factor modulating the interannual variations of the high-level monsoon easterly.