Abstract
In this paper, we evaluate the capability of the Beijing Climate Center Climate System Model (BCC-CSM) in simulating and forecasting the boreal summer intraseasonal oscillation (BSISO), using its simulation and sub-seasonal to seasonal (S2S) hindcast results. Results show that the model can generally simulate the spatial structure of the BSISO, but give relatively weaker strength, shorter period, and faster transition of BSISO phases when compared with the observations. This partially limits the model’s capability in forecasting the BSISO, with a useful skill of only 9 days. Two sets of hindcast experiments with improved atmospheric and atmosphere/ocean initial conditions (referred to as EXP1 and EXP2, respectively) are conducted to improve the BSISO forecast. The BSISO forecast skill is increased by 2 days with the optimization of atmospheric initial conditions only (EXP1), and is further increased by 1 day with the optimization of both atmospheric and oceanic initial conditions (EXP2). These changes lead to a final skill of 12 days, which is comparable to the skills of most models participated in the S2S Prediction Project. In EXP1 and EXP2, the BSISO forecast skills are improved for most initial phases, especially phases 1 and 2, denoting a better description for BSISO propagation from the tropical Indian Ocean to the western North Pacific. However, the skill is considerably low and insensitive to initial conditions for initial phase 6 and target phase 3, corresponding to the BSISO convection’s active-to-break transition over the western North Pacific and BSISO convection’s break-to-active transition over the tropical Indian Ocean and Maritime Continent. This prediction barrier also exists in many forecast models of the S2S Prediction Project. Our hindcast experiments with different initial conditions indicate that the remarkable model errors over the Maritime Continent and subtropical western North Pacific may largely account for the prediction barrier.
Highlights
The boreal summer intraseasonal oscillation (BSISO) and Madden-Julian oscillation (MJO) are two major modes of atmospheric intraseasonal variability over the tropics
From EXP1 to EXP2, the BSISO forecast skill remains almost unchanged in the first week but is improved in the second week
We evaluated the capability of the Beijing Climate Center (BCC) model in simulating and forecasting the BSISO
Summary
The boreal summer intraseasonal oscillation (BSISO) and Madden-Julian oscillation (MJO) are two major modes of atmospheric intraseasonal variability over the tropics. Tangible progresses have been made, the intraseasonal forecasting by dynamic models is still challenging due to its remarkable sensitivity to various model settings, such as model physics, initial conditions, and air-sea coupling (Seo et al 2005; Fu et al 2008, 2011; Ling et al 2014; Jie et al 2017). Results have partially shown that the Beijing Climate Center (BCC) S2S forecast model, which participates in the S2S Prediction Project, exhibits relatively low skill in MJO and BSISO forecasts. To reduce the deficiency in S2S forecasts, Liu et al (2017) improved model initial conditions and enhanced MJO forecast skill by using the BCC S2S model.
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