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Simulation of East Asian Summer Monsoon (EASM) in SP‐CCSM4: Part I—Seasonal mean state and intraseasonal variability

The mean state and intraseasonal variability of the East Asian Summer Monsoon (EASM) simulated by the Super‐Parameterized Community Climate System Model version 4 (SP‐CCSM4) and the conventionally parameterized CCSM4 are evaluated against observations. The SP‐CCSM4 model has a better simulation of the May‐June‐July‐August seasonal mean state of EASM than CCSM4, although it produces a dry bias over the EASM area compared to observations. The dry bias in SP‐CCSM4 is associated with the erroneous northward displacement of the western North Pacific subtropical high. The SP‐CCSM4 model simulates the reasonable monsoon onset and northward propagation of the monsoonal precipitation, yet the rainband marches faster and reaches to a higher latitude than in observations. The mechanisms associated with the northward propagation of the intraseasonal oscillation (ISO) of EASM are also captured by SP‐CCSM4. The cyclonic vorticity and the moisture convergence lead the convective activity, favoring the northward propagation of convection. The easterly wind shear and air‐sea interaction mechanisms in the model are realistic and show contributions to the northward propagation of the ISO of the model. The SP‐CCSM4 model captures many facets of the stepwise northward propagation of the precipitation belt in the EASM region, including the Mei‐yu season. However, compared to the observations, 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 CCSM4 model has large deficiencies in simulating the intraseasonal variability of EASM.