East Asian Summer Monsoon Rainfall Anomalies in 2020 and the Role of Northwest Pacific Anticyclone on the Intraseasonal-to-Interannual Timescales

In summer 2020, extreme rainfall occurred over the middle and lower reaches of the Yangtze River Basin (YRB). The accumulated precipitation amount of this rainfall event broke the record since 1961. On the interannual timescale, seasonal mean rainfall anomalies in the YRB during summer 2020 are related to the tropical North Indian Ocean (NIO) sea surface temperature (SST) warming and a La Nina event in the equatorial eastern Pacific. Both the NIO SST warming and La Nina help to generate the anomalous tropospheric low-level Northwest Pacific anticyclone (NWP-AAC) via the Matsuno-Gill type response. On the subseasonal timescale, rainfall anomalies in the YRB exhibit two significant periods during summer 2020, intraseasonal (40-60 days) and quasi-biweekly (similar to 10 days). The former intraseasonal component is the focus of this study. The intraseasonal NWP-AAC, which is a part of tropical intraseasonal oscillation, plays a dominant role in connecting the East Asian summer monsoon (EASM) rainfall with tropical convection. Using the pattern regression, a real-time index (RTI) is proposed to represent the intraseasonal-to-interannual variability of the NWP-AAC. The RTI captures the quasi-biennial tendency of NWP-AAC as well as its intraseasonal variability. These suggest that the NWP-AAC is a common mode across timescales. Also, the RTI well depicts the EASM rainfall variability in 2020. Our results provide an alternative view of cross-scale EASM variability, with an emphasis on the conduit effect of NWP-AAC from intraseasonal to interannual timescale. Hence, the RTI could be utilized for subseasonal to seasonal climate monitoring in the Asian summer monsoon region.