资源环境科技发展态势分析平台

Two of the most important hydroclimatic features of the Indian summer monsoon (ISM) rainfall are its onset/demise and intraseasonal oscillations (ISOs) manifested by the active-break cycles. In this study, we aim to understand the quantitative association between these two phenomena. An objective definition of local onset/demise of the ISM based on more than a century-long India Meteorological Department rain gauge observation is taken into consideration. Using multichannel singular spectrum analysis we isolate northward-propagating low-frequency (20-60 days; LF-ISO) and northwestward propagating high-frequency (10-20 days; HF-ISO) ISOs from the daily ISM rainfall. Our results suggest that a large number of local onset (59%) and demise (62%) events occur during positive developing phases and positive decaying phases of two ISOs, respectively, with phase locking between LF-ISO and HF-ISO being particularly important. Local onset is largely associated with favorable phases of ISOs across India except for LF-ISO over eastern India and HF-ISO over western Ghats and central India (CI). We find that local demise is more coherent with the ISO phases, especially with HF-ISO across the domain. We performed a case study to understand large-scale association with the onset of the ISM over CI. In 44 of total 58 cases (1948-2005), when CI onset occurred during favorable LF-ISO or HF-ISO phase, they are either linked with a northward propagation of convection from the equator in LF-ISO time scale (28 cases) or westward propagating structures from the western Pacific in HF-ISO time scale (27 cases).