How useful is the extra-tropical Sea Surface Temperature in predicting the seasonal mean monsoon over South Asia?

Modulation of the annual cycle by slow coupled ocean-atmosphere modes such as the El Nino and Southern Oscillation (ENSO) forms the basis for the predictability of seasonal mean monsoon using statistical and dynamical models (Charney and Shukla 1981; Palmer and Anderson 1994; Shukla and Paolino 1983; Shukla 1998). Current research so far stresses on the predictable signal that arises largely from the ENSO through the ENSO-monsoon teleconnection. Extensive attempts have been made to exploit this teleconnection in empirical as well as dynamical seasonal prediction models in the past few decades. Unfortunately, this teleconnection is relatively weak over the south Asian monsoon region compared to some other parts of the tropics. In order to push the skill of south Asian monsoon forecasts, it is, therefore, imperative to explore any other source seasonal predictability.Based on earlier evidences of the influence of slowly varying multi-decadal components of north Atlantic and northern hemispheric extra-tropical SST(Burns et al. 2003; Gupta et al. 2003; Goswami et al. 2006; Wang et al. 2009; Luo et al. 2011), we made an extensive analysis of observations and a series of climate model experiments using Climate Forecast System Model version 2 from National Centre for Environmental Prediction, US (NCEP-CFSv2) and the atmospheric component of the same model (NCEP-GFS) and we establish that slow variations of northern hemispheric extra-tropical sea surface temperature (SST) anomalies can augment the seasonal predictability of south Asian monsoon. The skilful real-time forecast of SST using NCEP-CFSv2 during June-September (JJAS) 2013 and the performance of the south Asian monsoon during 2013 (Fig.1) 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-Nina) were responsible for the above normal rainfall over India during 2013. 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 AGCM forced by such SST elucidate the mechanism through which the extratropical SSTs influence the Indian monsoon and show that they 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, affect the tropospheric temperature (TT) over the 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 coupled ocean-atmosphere models are becoming capable of predicting these SST anomalies brightens the prospect of south Asian monsoon prediction. References Burns, S. J., D. Fleitmann, A. Matter, J. Kramers, and A. A. Al-Subbary, 2003: Indian Ocean Climate and an Absolute Chronology over Dansgaard/Oeschger Events 9 to 13. Science, 301, 1365–1367. Charney, J. G., and J. Shukla, 1981: Predictability of monsoons. Monsoon Dynamics,, pp. 99–109, Cambridge University Press. Goswami, B. N., M. S. Madhusoodanan, C. P. Neema, and D. Sengupta, 2006: A physical mechanism for North Atlantic SST influence on the Indian summer monsoon. Geophysical Research Letters, 33. Gupta, A. K., D. M. Anderson, and J. T. Overpeck, 2003: Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean. Nature, 421, 354–357. Luo, F., S. Li, and T. Furevik, 2011: The connection between the Atlantic Multidecadal Oscillation and the Indian Summer Monsoon in Bergen Climate Model Version 2.0. Journal of Geophysical Research: Atmospheres, 116. Palmer, T. N., and D. L. T. Anderson, 1994: The prospects for seasonal forecasting—A review paper. Quarterly Journal of the Royal Meteorological Society, 120, 755–793. Shukla, J., 1998: Predictability in the Midst of Chaos: A Scientific Basis for Climate Forecasting. Science, 282, 728–731. Shukla, J., and D. A. Paolino, 1983: The Southern Oscillation and Long-Range Forecasting of the Summer Monsoon Rainfall over India. Monthly Weather Review, 111, 1830–1837. Wang, Y., S. Li, and D. Luo, 2009: Seasonal response of Asian monsoonal climate to the Atlantic Multidecadal Oscillation. Journal of Geophysical Research: Atmospheres, 114.