Evaluation Of Forecast Skill By BCC S2S Model For Boreal Summer Intraseasonal Oscillation Over The Asian Summer Monsoon Regions

This study evaluated the simulate and forecast skill of boreal summer intraseasonal oscillation(BSISO)in the air-sea coupled model of Beijing Climate Center(BCC)which participated the subseasonl to seasonal prediction(S2S)project.The sources of model bias were diagnosed and discussed.The first mode of BSISO(BSISO1)that propagates northeast/northward with a period of 30-90 days and the second mode of BSISO(BSIS02)that propagates northwest/northward with a period of 10-30 days can be obtained from the first two modes and third-fourth modes of multivariate empirical orthogonal function(MV-EOF),respectively.Simulating results shows that the forecast skill of BSISO in the BCC S2S is decreased by the forecast lead days,the BSISO1 has about 11 days of forecasting skill,and the BSIS02 is 9 days.Possible reasons of forecasting skills are as follows:(1)The poor simulation of BSISO1 with lead time of 11 days or longer is likely due to insignificant increases in low-level cyclonic vorticity,moistening and warm temperature anomalies to the north of the convection,which were induced by the interaction between background mean flows and BSISO-related anomalous fields.The BCC S2S model can predict the background monsoon circulations,such as low-level southerly and the northerly and easterly vertical shears,but has limited capability in forecasting the distributions of circulation and moisture anomalies.The loss of prediction skill is related to the model adjusting toward its intrinsic BSISO mode as the forecast lead time increases.(2)The predictions after lead 9d show significant biases in the period,propagation characteristics and strength of BSIS02.BCC S2S reveals a higher forecast skill of BSIS02 structure and propagation over the western North Pacific,while it significantly underestimated convection signal of BSIS02 over the tropical Indian Ocean.It is suggested that the unrealistic Indian Ocean BSIS02 is related to the biases of model mean state.Also,the model underestimates the abundant moisture and vigorous convection over the eastern Indian Ocean and Bay of Bengal.BCC S2S underestimating of BSIS02 convection might be attributable to the relatively weak vorticity to the northwest of BSIS02 convection.By diagnosing the vorticity equation,we found that although the model well simulates the positive contributions of geostrophic vorticity advection and convergence effect to the northwest of convection,their values are smaller than re-analyzed.Moreover,the simulated relative vorticity advection shows an opposite effect to re-analysis in the prediction of lead 9d,weakening the favorable environment of BSIS02 development associated with positive vorticity to the northwestern part of convection.