Simulation of East Asian Summer Monsoon (EASM) in SP-CCSM4

Abstract:

The East Asian Summer Monsoon (EASM) simulated by the Super-Parameterized Community Climate System Model version 4 (SP-CCSM4) is evaluated against observations, and the mechanisms explaining the onset of the Mei-yu season are revisited. The May-June-July-August (MJJA) seasonal mean state of EASM is generally reproduced well in the model, including the precipitation, monsoon trough in the South China Sea (SCS), cross-equatorial airflow and the large-scale monsoon airflow. The northward propagation of the monsoonal precipitation is also well simulated in the model. The development of the 30-60-day oscillation related to EASM is investigated using lagged regression and composite analysis. The model captures the mechanisms associated with the Northward Propagation of the Intraseasonal Oscillation (NPISO). A warm sea surface temperature (SST) anomaly, cyclonic vorticity and the moisture convergence lead the convection, favoring the northward propagation of the convection. Additionally the easterly windshear contributes to the NPISO. Compared to the observation, in the model the onset of the Mei-yu season takes place 5 days earlier and the duration of the Mei-yu’s rainy episode is shorter. The onset of the Mei-yu sea son is influenced by the phase relationship of convective activity between the tropical West Pacific Warm Pool (WPWP) and SCS. The results indicate that dry conditions over the tropical WPWP concurrent with maximum convection over SCS favor a faster northward propagation of the ISO, which yields to the early onset of the Mei-yu season. The onset of the Mei-yu season is also influenced by the seasonal mean conditions. During events with an earlier onset, the MJJA mean precipitation is enhanced. The increased precipitation is caused by a stronger moisture transport and the westward extension of the western North Pacific Subtropical High (WNPSH).