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Interdecadal variation of biases in a regional climate model simulation of summer climate of East Asia

In this study, a 55‐year climate simulation over East Asia is conducted by using the regional climate model RegCM4.6 and the variations of biases in different climate backgrounds are investigated. The results show that the biases of rainfall and western Pacific subtropical high (WPSH) simulated by RegCM4.6 are not stable, instead they exhibit an obvious feature of interdecadal variation that is similar to the interdecadal variation of rainfall and WPSH themselves. The interdecadal change of the simulation bias occurs in the late 1970s, when abrupt interdecadal changes took place in global atmosphere and ocean as well as in East Asia climate. The interdecadal variation of the bias of the simulated WPSH is closely related with the interdecadal variation of the simulated rainfall bias. After 1978, the simulated WPSH is displaced further south than the observed WPSH, leading to excessive rainfall in the simulation and larger positive bias in south China. These overestimated rainfall and convective clouds in south China during 1979–2009 consume a lot of water vapour in the atmosphere and weaken the downwards solar radiation, resulting in a cold and dry atmosphere and large negative biases of virtual temperature and geopotential height at 500 hPa (H500) during 1979–2009 compared to that during 1955–1978. Corresponding to the above circulation anomalies, the simulated WPSH further weakens and shifts southwards after the late 1970s. A positive feedback loop is formed due to the positive feedback between the WPSH and cumulus convective process.