ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 31, SEPTEMBER 2014, 1167–1180 The Boreal Summer Intraseasonal Oscillation Simulated by Four Chinese AGCMs Participating in the CMIP5 Project ZHAO Chongbo ∗1 , ZHOU Tianjun 2 , SONG Lianchun 3 , and REN Hongli 1 1 Laboratory for Climate Studies, National Climate Center, China Meteorological Administration, Beijing 100081 2 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029 3 National Climate Center, China Meteorological Administration, Beijing 100081 (Received 25 October 2013; revised 30 December 2013; accepted 10 February 2014) ABSTRACT The performances of four Chinese AGCMs participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) in the simulation of the boreal summer intraseasonal oscillation (BSISO) are assessed. The authors focus on the major characteristics of BSISO: the intensity, significant period, and propagation. The results show that the four AGCMs can reproduce boreal summer intraseasonal signals of precipitation; however their limitations are also evident. Compared with the Climate Prediction Center Merged Analysis of Precipitation (CMAP) data, the models underestimate the strength of the intraseasonal oscillation (ISO) over the eastern equatorial Indian Ocean (IO) during the boreal summer (May to October), but overestimate the intraseasonal variability over the western Pacific (WP). In the model results, the westward propagation dominates, whereas the eastward propagation dominates in the CMAP data. The northward propagation in these models is tilted southwest–northeast, which is also different from the CMAP result. Thus, there is not a northeast–southwest tilted rain belt revolution off the equator during the BSISO’s eastward journey in the models. The biases of the BSISO are consistent with the summer mean state, especially the vertical shear. Analysis also shows that there is a positive feedback between the intraseasonal precipitation and the summer mean precipitation. The positive feedback processes may amplify the models’ biases in the BSISO simulation. Key words: boreal summer intraseasonal oscillation, AGCM, simulation, feedback Citation: Zhao, C. B., T. J. Zhou, L. C. Song, and H. L. Ren, 2014: The boreal summer intraseasonal oscillation simulated by four Chinese AGCMs participating in the CMIP5 project. Adv. Atmos. Sci., 31(5), 1167–1180, doi: 10.1007/s00376-014- 3211-7. 1. Introduction The intraseasonal oscillation (ISO) is one of the most prominent large-scale sources of variability in the tropics and it undergoes a peculiar seasonal variation (Seo et al., 2005). While the strongly eastward propagating ISO (the Madden– Julian oscillation) is primarily observed in the boreal winter, the ISO in the boreal summer from May to October is dom- inated by the northward propagation over the Indian and western Pacific (Jiang et al., 2004). This boreal summer subseasonal mode significantly affects the active and break phases of the summer monsoon (Yasunari, 1979, 1980; Li et al., 2001). The wet and dry spells of the boreal summer intraseasonal oscillation (BSISO) strongly influence the ex- treme hydro-meteorological events, which cause about 80% of natural disasters (Lau and Waliser, 2005). The intrasea- sonal oscillation also has an effect on the formation, intensity, ∗ Corresponding author: ZHAO Chongbo Email: chongbozhao@mail.iap.ac.cn and course of the tropical cyclones (Nakazawa, 1986; Lieb- mann et al., 1994). Therefore, the BSISO is important for weather forecasting and climate prediction. Numerical weather prediction results have shown that a model capable of simulating ISO may have a better over- all forecasting ability (Li et al., 2006). However, attempts to simulate the BSISO have met with poor results (Slingo et al., 1996; Waliser et al., 2003; Lin et al., 2008; Sperber et al., 2013). This is because the BSISO shows a complex propagation process due to a prominent northeastward prop- agation associated with the monsoon over the Indian Ocean (IO) and the western Pacific (WP), as well as the formation of the summertime intertropical convergence zone off the equa- tor (Sikka and Gadgil, 1980; Lau and Chan, 1986; Wang and Rui, 1990; Annamalai and Sperber, 2005; Seo et al., 2007). How to reliably simulate the BSISO has been a challenge to the climate-modeling community. Over the Asian summer monsoon region, precipitation is an important forecast variable in model simulation. The outgoing longwave radiation (OLR) and the real-time multi- © Institute of Atmospheric Physics/Chinese Academy of Sciences, and Science Press and Springer-Verlag Berlin Heidelberg 2014