Impacts of Indian Ocean SST biases on the Indian Monsoon: as simulated in a global coupled model

Abstract

International audienceIn this study, the impact of the ocean-atmosphere coupling on the atmospheric mean state over the Indian Ocean and the Indian Summer Monsoon (ISM) is examined in the framework of the SINTEX-F2 coupled model through forced and coupled control simulations and several sensitivity coupled experiments. During boreal winter and spring, most of the Indian Ocean biases are common in forced and coupled simulations, suggesting that the errors originate from the atmospheric model, especially a dry islands bias in the Maritime Continent. During boreal summer, the air-sea coupling decreases the ISM rainfall over South India and the monsoon strength to realistic amplitude, but at the expense of important degradations of the rainfall and Sea Surface Temperature (SST) mean states in the Indian Ocean. Strong SST biases of opposite sign are observed over the western (WIO) and eastern (EIO) tropical Indian Ocean. Rainfall amounts over the ocean (land) are systematically higher (lower) in the northern hemisphere and the south equatorial Indian Ocean rainfall band is missing in the control coupled simulation. During boreal fall, positive dipole-like errors emerge in the mean state of the coupled model, with warm and wet (cold and dry) biases in the WIO (EIO), suggesting again a significant impact of the SST errors. The exact contributions and the distinct roles of these SST errors in the seasonal mean atmospheric state of the coupled model have been further assessed with two sensitivity coupled experiments, in which the SST biases are replaced by observed climatology either in the WIO (warm bias) or EIO (cold bias). The correction of the WIO warm bias leads to a global decrease of rainfall in the monsoon region, which confirms that the WIO is an important source of moisture for the ISM. On the other hand, the correction of the EIO cold bias leads to a global improvement of precipitation and circulation mean state during summer and fall. Nevertheless, all these improvements due to SST corrections seem drastically limited by the atmosphere intrinsic biases, including prominently the unimodal oceanic position of the ITCZ (Inter Tropical Convergence Zone) during summer and the enhanced westward wind stress along the equator during fall

Benefits of Increasing the Model Resolution for the Seasonal Forecast Quality in EC-Earth

“…In the Indian Ocean, in agreement with Prodhomme et al (2014), the SST cooling, associated with the increase of oceanic resolution, leads to a decrease of the excessive oceanic precipitation (Figs. 1c,f).…”

“…In summer, in the Indian Ocean, SRes exhibits a warm bias in the Somali upwelling that is common to other coupled models (Prodhomme et al 2014). In the summer hemisphere (JJA in the Northern Hemisphere and DJF in the Southern Hemisphere) SRes exhibits a large cold bias in the midlatitudes.…”

“…Precipitation is excessive in the Indian Ocean in both winter and summer, especially over the Maritime Continent [consistent with Neale and Slingo (2003)] and for the monsoon precipitations [as in Prodhomme et al (2014)]. In the Pacific, the double ITCZ bias is also found for precipitation in winter, although in the summer the ITCZ is shifted northward (Lin 2007;Bellucci et al 2010).…”

“…In addition, the monsoon and the monsoon onset date are known to be strongly influenced by ENSO (Boschat et al 2011;Prodhomme et al 2015); therefore improvement of ENSO representation might lead to improvement in the Indian monsoon. Figures 6a-c show three different monsoon indices: the Indian summer monsoon dynamical index (IMDI; zonal wind at 850 hPa averaged in the region 58-158N, 408-808E minus the zonal wind averaged in the region 208-308N, 708-908E; Wang et al 2001), the extended Indian monsoon rainfall (58-308N, 708-958E), and the Indian summer monsoon rainfall (continental precipitation over the region 58-308N, 708-958E).…”

Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient

“…2), which is 0.5°C greater than the warming over the warm pool region 28 . Even though a warming trend over the Indian Ocean may result in an increased supply of moisture locally over the ocean, it is not necessary that the surplus moisture is carried on to the peninsula 29 . The sustained Indian Ocean warming in the west has led to a greater westward extension of this Indian Ocean warm pool in recent decades 21,28 .…”