Active-Break Transitions of Monsoons Over India as Predicted by Coupled Model Ensembles
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Pure and Applied Geophysics
Active-Break Transitions of Monsoons Over India as Predicted by Coupled Model Ensembles D. R. PATTANAIK,1 A. K. SAHAI,2 R. PHANI MURALIKRISHNA,2 RAJU MANDAL,2 and AVIJIT DEY2 Abstract—The active-break cycle of monsoons is the most important phenomenon of the Indian summer monsoon, and its prediction in real time will help to review the ongoing monsoon conditions for providing outlooks to farmers and disaster managers. The monsoon seasons of 2017 and 2018 during July–August witnessed many active-break-active transitions, with long dry spells. The recently implemented Climate Forecast System version 2 coupled model-based ensemble prediction system at the India Meteorological Department was used to observe the performance of a real-time extended-range forecast (ERF) of active-break-active cycles of the monsoon at different spatial scales during 2017 and 2018. The results indicated that the operational ERF has good fidelity in predicting the active-break-active transitions of the monsoon during 2017 and 2018 based on large-scale indices including the monsoon intraseasonal oscillation and low-level circulation. Quantitatively, the forecast of all India and central India rainfall during the active, break and transition phases of the monsoon are very well captured with a lead time of 2–3 weeks. In particular, the long dry spell during first 3 weeks of August and the active last week of August 2017 associated with very heavy rainfall over western coastal states of India were very well captured, whereas the active phase over the southern peninsula during 10–16 August 2018 was slightly underestimated. At smaller spatial scales, the ERFs for 36 meteorological (met)-subdivisions of India for 2–3 weeks could provide useful guidance to farmers during different monsoon phases. Thus, the skilful real-time ERF of the monsoon is very useful to user communities. Keywords: Indian monsoon, active-break cycle, intraseasonal oscillation, extended-range forecast, CFSv2, multi-model ensemble.
1. Introduction The intraseasonal variability of Indian monsoon rainfall is characterized by ‘active’ and ‘break’ periods. The high rainfall over central India associated with the presence of a monsoon trough over the
1 India Meteorological Department, New Delhi, India. E-mail: [email protected] 2 Indian Institute of Tropical Meteorology, Pune, India.
northern plains and the weak or no rainfall over central India and high rainfall over northern India associated with the presence of a monsoon trough over the foothills of the Himalaya are termed as active periods or break periods, respectively (Ramamurthy 1969; Krishnan et al. 2000; Lawrence and Webster 2002; Webster and Hoyos 2004; Hoyos and Webster 2007). The frequency and duration of these active-break periods within a given monsoon season contribute to the seasonal mean and, thus, also modulate the interannual variability. Webster and Hoyos (2004) suggested that forecasts of precipitation on this intermediate time scale (between medium range and seasonal time scale
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