Reduction of systematic errors in regional climate simulations of the summer monsoon over East Asia and the western North Pacific by applying the spectral nudging technique

In this study, the systematic errors in regional climate simulation of 28-year summer monsoon over East Asia and the western North Pacific (WNP) and the impact of the spectral nudging technique (SNT) on the reduction of the systematic errors are investigated. The experiment in which the SNT is not applied (the CLT run) has large systematic errors in seasonal mean climatology such as overestimated precipitation, weakened subtropical high, and enhanced low-level southwesterly over the subtropical WNP, while in the experiment using the SNT (the SP run) considerably smaller systematic errors are resulted. In the CTL run, the systematic error of simulated precipitation over the ocean increases significantly after mid-June, since the CTL run cannot reproduce the principal intraseasonal variation of summer monsoon precipitation. The SP run can appropriately capture the spatial distribution as well as temporal variation of the principal empirical orthogonal function mode, and therefore, the systematic error over the ocean does not increase after mid-June. The systematic error of simulated precipitation over the subtropical WNP in the CTL run results from the unreasonable positive feedback between precipitation and surface latent heat flux induced by the warm sea surface temperature anomaly. Since the SNT plays a role in decreasing the positive feedback by improving monsoon circulations, the SP run can considerably reduce the systematic errors of simulated precipitation as well as atmospheric fields over the subtropical WNP region.