Evaluation Of The Simulation And Prediction Of The Central Indian Ocean Mode In S2S Models

The Indian summer monsoon is a typical monsoon system on the Asian continent.It directly affects the production and life of billions of people.The interseasonal variability(ISV)is one of the most important variabilities during the Indian summer monsoon(ISM).Simulation of ISV during the ISM is a big challenge in models,which affects the simulation and forecasting skill of the ISM.Recently,some studies found that central Indian Ocean(CIO)mode over the Indian Ocean has a close relation with ISVs during the ISM.The CIO mode can be used as a new diagnostic criterion for the simulation of the ISM.In this study,the simulations and predictions of the CIO mode in subseasonal-to-seasonal(S2S)models are examined.The significant positive correlation between the CIO mode and the ISM precipitation is verified.In addition,the possible way to improve the model are proposed by comparing the well-simulated and poorly-simulated cases.The specific conclusions are as follows:Results confirm that a better simulation and forecast of the CIO mode in S2S air-sea coupled models is associated with a better simulation of northward-propagating ISVs and leads to stronger variance of intraseasonal rainfall during the ISM.It highlights that the CIO mode can be used as a new diagnostic criterion for the simulation of the ISM in the diagnostic climate model.Intercomparsion in multi-models indicates that the best model displays significant skill in predicting the CIO mode around one month,and the worst performer is not well simulated on initial conditions.Most of the CIO mode cases can be well simulated on initial conditions,but the number decreases with time prolonging.At the lead time of 5 days,the numbers of simulated good and bad cases are closest to the average value,leading to statistical significance of the comparsion.To analyze the reasons for poorly-simulation of the CIO mode cases,the positive CIO mode cases are classified into the well-simulated and the poorly-simulated groups.By comparing the two groups,it is shown that the barotropic energy conversion from the meridional shear of background zonal winds(^?)enhances the kinetic energy on intraseasonal timescales during the CIO mode in the well-simulated group.As a result,the enhanced meridional wind anomalies transport moisture from the tropics to the subtropics and enhance mid-tropospheric moisture loading in the subtropics which nourish the northward-propagating intraseasonal variabilities.In constract,meridional shear of mean zonal wind is weak in poorly-simulated group,which leads to the deviation in the simulation of the CIO mode.Further diagnosis shows that the seasonal variation of(^?)during the ISM is mainly controlled by the horizontal eddy flux.The simulation of horizontal eddy flux in the model is weak,which causes the low simulation of barotropic instability during the ISM,leading to the poorly-simulated CIO mode.Based on the above conclusions,this paper points out the way to improve models,which is not only reducing the error in wind field,but also improving the simulation of wind field shear.Additionally,the improvement of horizontal eddy flux parameterization is an important method to capture the CIO mode in models.These results are of great scientific significance and practical value for the simulation and prediction of the ISM.