INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 33: 1057–1069 (2013) Published online 11 April 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.3489 Model biases in long coupled runs of NCEP CFS in the context of Indian summer monsoon Hemantkumar S. Chaudhari,* Samir Pokhrel, Subodh K. Saha, Ashish Dhakate, R. K. Yadav, Kiran Salunke, Somnath Mahapatra, C. T. Sabeerali and Suryachandra A. Rao Climate and Global Modelling Division, Indian Institute of Tropical Meteorology, Pashan, Pune 411008, India ABSTRACT: This study examines the performance of National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) over the Indian monsoon region in 100 years long coupled run, in terms of biases of sea surface temperature (SST), rainfall and circulation. The study further explores the role of the feedback processes in maintaining these biases. The model simulates reasonable monsoon climatology during JJAS (June–September). It shows dry (wet) rainfall bias concomitant with cold (warm) SST bias over east (west) equatorial Indian Ocean. These biases of SST and rainfall affect both lower- and upper-level circulations in a feedback process, which in turn regulates the SST and rainfall biases by maintaining a coupled feedback process. A dry (wet) rainfall bias over east (west) Indian Ocean induces anomalous low level easterlies over tropical Indian Ocean and causes cold SST bias over east Indian Ocean by triggering evaporation and warm SST bias over west Indian Ocean through advection of warm waters. The persistent SST bias retains the zonal asymmetric heating and meridional temperature gradient resulting in a circum-global subtropical westerly jet core, which in turn magnifies the mid-latitude disturbances and decreases the Mascarene high. The decreased Mascarene high diminishes the strength of monsoon cross-equatorial flow and results in less upwelling as compared to that in the observation. It further increases the SST bias over the West Indian Ocean. The coupled interaction among SST, rainfall and circulation works in tandem through a closed feedback loop to maintain the model biases over tropical Indian Ocean. Copyright  2012 Royal Meteorological Society KEY WORDS Climate Forecast System (CFS); model bias; Indian summer monsoon; subtropical westerly jet Received 5 December 2011; Revised 10 March 2012; Accepted 18 March 2012 1. Introduction The National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) provides oper- ational prediction of the world’s climate including the Asian monsoon climate on different time scales. As described by Saha et al. (2006), the CFS (Version 1) shows important advances in operational prediction from the previous dynamical forecast efforts by demonstrat- ing a level of prediction skill comparable to statisti- cal methods, at least for the United States. CFS (Ver- sion 1) is utilized by various research institutes and universities for seasonal prediction purpose and opera- tional usage. CFS demonstrates skills in simulating El Ni˜ no-Southern Oscillation (ENSO) (Wang et al., 2005b), the climate over Africa (Thiaw and Mo, 2005), and the sub-seasonal features of the Asian summer mon- soon (Yang et al. 2008). South Asian summer monsoon variability and intraseasonal variability are investigated by Achuthavarier and Krishnamurthy (2010a, 2010b). * Correspondence to: H. S. Chaudhari, Indian Institute of Tropical Meteorology, Pashan, Pune 411008, India. E-mail: hemantkumar@tropmet.res.in Analysis of the retrospective ensemble predictions (hind- casts) of the NCEP CFS indicates that the model suc- cessfully simulates many major features of the Asian summer monsoon including the climatology and inter- annual variability of major precipitation centres and atmospheric circulation systems (e.g. Yang et al., 2008; Pattanaik and Kumar, 2010). CFS also depicts the inter- active oceanic – atmospheric processes associated with the precipitation anomalies reasonably well at different time leads. Thus, the CFS could be utilized as a better tool for the real-time prediction of Indian summer monsoon rainfall (ISMR). ISMR contributes about 80% of the annual rainfall over India and thus plays an important role in agricul- ture, water management and economic planning of this country. The Indian summer monsoon prevails over the Indian region for the four months, from June through September (JJAS). Numerical models have played a vital role in monsoon prediction. Indian summer monsoon is a deep moist baroclinic system. The following important components of Indian summer monsoon should be reflected in simulation of any numerical model: (1) monsoon trough over central India, (2) low level cross-equatorial flow over west Indian Ocean, (3) upper-level Tibetan high, (4) upper tropospheric tropical easterly jet (TEJ) over peninsular Copyright  2012 Royal Meteorological Society