Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 141: 2760 – 2775, October 2015 A DOI:10.1002/qj.2562 Influence of extratropical sea-surface temperature on the Indian summer monsoon: an unexplored source of seasonal predictability Rajib Chattopadhyay, a * R. Phani, a C. T. Sabeerali, a A. R. Dhakate, a K. D. Salunke, a S. Mahapatra, a A. Suryachandra Rao a and B. N. Goswami b a Indian Institute of Tropical Meteorology, Pune, India b Earth and Climate Science Department, Indian Institute of Science Education and Research, Pune, India *Correspondence to: R. Chattopadhyay, Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pune-411008, India. E-mail: rajib@tropmet.res.in Based on extensive analysis of observations and a series of climate model experiments, here we establish that slow variations of northern hemispheric extratropical sea-surface temperature (SST) anomalies can augment seasonal predictability of the south Asian monsoon. The SST conditions and performance of the south Asian monsoon during 2013 boreal summer months (June–September) led us to hypothesize that the strong extratropical SST anomalies in the North Pacific and North Atlantic in conjunction with weak tropical SST anomalies (weak La Ni˜ na) were responsible for the above-normal rainfall over India during 2013. We also argue that the 2013 SST pattern and above-normal monsoon condition are not unique but occurred on several occasions in the past. Further, we show that there is a complementary pattern of strong extratropical SST and weak tropical SST that is associated with below-normal south Asian monsoon rainfall. We also show that the extratropical SST pattern in the Northern Hemisphere is associated with a low-frequency interdecadal mode of variability indicating potential predictability associated with such extratropical SST forcing. Extensive experiments with an atmospheric general circulation model forced by such SST conditions elucidate the mechanism through which the extratropical SSTs influence the Indian monsoon. The SST anomalies affect the north–south temperature gradient and lead to a local displacement of the jet stream, setting up a quasi-stationary wave. Such a stationary wave, in turn, affects the tropospheric temperature (TT) over southern Eurasia, influencing the north–south TT gradient in the region and thereby the Indian monsoon. Our discovery of this additional source of potential predictability together with the fact that the new-generation coupled ocean–atmosphere models are capable of predicting the extratropical SST anomalies brightens the prospect of south Asian monsoon prediction. Key Words: Indian summer monsoon; extratropical sea-surface temperature; seasonal prediction; NCEP-CFSv2 Received 5 August 2014; Revised 23 February 2015; Accepted 26 March 2015; Published online in Wiley Online Library 15 May 2015 1. Introduction The prediction of the seasonal mean south Asian monsoon rainfall, a critical input for policy makers, has remained a grand challenge problem for the climate community. While the skill of prediction of seasonal mean rainfall has increased significantly in large parts of the deep Tropics (Wang et al., 2005), it remains poor over the Asian monsoon region. Although the skill of seasonal mean rainfall forecasting over the Indian subcontinent has shown noticeable improvement over the past decade, it remains significantly below the potential limit of predictability (Rajeevan et al., 2012). The fact that the potential limit on seasonal predictability of rainfall over the Asian monsoon region is lower than for the rest of the Tropics (Goswami, 1998; Goswami et al., 2006b) makes it a challenge to achieve useful skill in seasonal prediction of south Asian monsoon rainfall. Is there any hope of improving this situation? Are we missing something that could add to the predictive signal? Emerging from the pioneering work of Charney and Shukla (1981), the seasonal predictability in the Tropics has been set on a solid foundation by a large body of work during the past three decades (Shukla and Paolino, 1983; Palmer and Anderson, 1994; Shukla, 1998; Webster et al., 1998). Modulation of the annual cycle by slow coupled ocean–atmosphere modes such as El Ni˜ no and Southern Oscillation (ENSO) is the basis for this predictability. The predictive signal for the south Asian monsoon also arises largely from ENSO through the ENSO–monsoon teleconnection. Extensive attempts have been made to exploit c  2015 Royal Meteorological Society