Improvement of Regional Climate Simulation over East Asia by Coupled Air-Sea Interaction and Large-Scale Nudging

Abstract

Impacts of coupled air-sea interaction and large-scale nudging are examined in regional climate simulation over East Asia. A slab ocean model (SOM) is coupled with a regional climate model (RCM) to investigate the impact of two-way air-sea interaction, and the spectral nudging is applied to examine the effect of large-scale nudging. The control (CTL) run without coupled air-sea interaction and large-scale nudging has significantly large systematic errors in simulated oceanic precipitation, monsoon circulations, and subtropical high. The errors result from the conditional instability of the second kind (CISK) type physical process caused by uncoupled air-sea interaction. The experiment with a SOM reduces 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 improves 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 are further improved because the spectral nudging decreases the systematic error of large-scale circulations between model solution and large-scale forcing. Simulated results indicate 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 regional climate simulation.