Improvement of regional climate simulation of East Asian summer monsoon by coupled air-sea interaction and large-scale nudging

Abstract

Impacts of coupled air-sea interaction and large-scale nudging were examined through regional climate simulation of the East Asian summer monsoon (EASM). A slab ocean model (SOM) was coupled with a regional climate model (RCM) to investigate the impact of two-way air-sea interaction, and the spectral nudging was applied to examine the effect of large-scale nudging. The control run without coupled air-sea interaction and large-scale nudging had significantly large systematic errors in simulated oceanic precipitation, monsoon circulations, and subtropical high. The errors resulted from an erroneous physical process caused by uncoupled air-sea interaction. The experiment with a SOM reduced the unreasonable physical process because simulated sea surface temperature (SST) decreases with enhanced ocean surface mixing from intensified low-level wind. Therefore, coupled air-sea interaction in the SOM run improved the simulations of seasonal mean precipitation, monsoon circulations, and subtropical high. In the experiment with a SOM and large-scale nudging, simulated precipitation and synoptic fields were further improved because the spectral nudging decreased the systematic error of large-scale circulations between model solution and large-scale forcing. Simulated results indicated that adjusting simulated SST to atmospheric conditions by coupling an ocean model with a RCM and improving boundary conditions by implementing large-scale nudging are necessary for advanced simulation of the EASM using RCMs.