Abstract:The impact of Madden-Julian oscillation (MJO) upon extreme rainfall in southern China was studied using the Real-time Multivariate MJO (RMM) index and daily precipitation data from high-resolution stations in China. The probability-distribution function (PDF) of November-March rainfall in southern China was found to be skewed toward larger (smaller) values in phases 2-3 (6-7) of MJO, during which the probability of extreme rainfall events increased (reduced) by 30-50% (20-40%) relative to all days in the same season. Physical analysis indicated that the favorable conditions for generating extreme rainfall are associated with southwesterly moisture convergence and vertical moisture advection over southern China, while the direct contributions from horizontal moisture advection are insignificant. Based on the above results, the model-based predictability for extreme rainfall in winter was examined using hindcasts from the Climate Forecast System version 2 (CFSv2) of NOAA. It is shown that the modulations of MJO on extreme rainfall are captured and forecasted well by CFSv2, despite the existence of a relatively small bias. This study suggests the feasibility of deriving probabilistic forecasts of extreme rainfall in southern China based on RMM indices.
The boreal summer intraseasonal oscillation (BSISO) is one of the dominant modes of intraseasonal variability of the tropical climate system, which has prominent northward propagation extending much further from the equator. The impacts of BSISO on extreme rainfall in eastern China were studied using the BSISO indices and daily rainfall data in China. We revealed that the responses of extreme rainfall to BSISO activity in eastern China are not spatially uniform. Under the influences of BSISO1, the probability‐distribution functions of rainfall in two southeastern China subregions: South China and Yangtze River Valley significantly skew toward larger values, respectively, in phases 4 and 8, and phases 3 and 4 with the probability of 90th extremes increased by 35–45% relative to May–August rainfall probability‐distribution function, showing southward propagation with the speed of 1.96°/phase. Under the BSISO2, the probability of the 90th extremes increased more than 40% in South China and Yangtze River Valley, respectively, during phases 4–5 and phases 6–7, showing northward propagation with a speed of 2.75°/phase. Physical analysis showed that the increased probability of extreme rainfall is associated with intensifying moisture convergence and upward moisture transport during BSISO active phases. The hindcasts from the Climate Forecast System version 2 have been used to evaluate the modulations of BSISO on extreme rainfall and associated predictability. It was shown that the Climate Forecast System version well reproduces the modulations of BSISO on extreme rainfall within 2 weeks. These results demonstrate the feasibility to develop medium‐to‐extended‐range probabilistic forecasts of extreme rainfall for southeastern China.
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