Subseasonal Prediction of Extreme Precipitation over Asia: Boreal Summer Intraseasonal Oscillation Perspective

The boreal summer intraseasonal oscillation (BSISO) is one of the most prominent modes in the tropical climate system. For better subseasonal prediction of extreme precipitation the relationship between BSISO activity and extreme precipitation events (days with daily precipitation exceeding the local 90th percentile) over Asia is investigated, especially the dependence of extreme precipitation occurrence on BSISO precipitation anomaly pattern (phase) and intensity (amplitude) in each month. At a given area and month, the probability of extreme precipitation changes from less than 10% to over 40%-50% according to BSISO phases, and it tends to be high when BSISO amplitude is large. The extreme precipitation probability estimated by BSISO activity is generally higher over ocean than over land. Over some land regions, however, occurrence of extreme precipitation is notably modulated by BSISO activity. In May, the extreme precipitation probability over southeastern China can reach about 30%-40% when BSISO precipitation anomaly arrives over the region. Similarly, in September the extreme precipitation probability over western China can reach 40%-50% when BSISO precipitation anomaly arrives there. The BSISO activity provides useful information in narrowing down the area and timing of high probability of extreme precipitation occurrence. Using real-time BSISO monitoring and forecast data provided by the Asia-Pacific Economic Cooperation (APEC) Climate Center, it is shown that 1) the best model (ECMWF) can predict the leading BSISO modes about 20 days ahead with bivariate correlation skills higher than 0.5 except in May, and 2) the empirical probability distributions of extreme precipitation that are based on BSISO activity can be captured by the BSISO forecasts for lead times longer than 2 weeks.