Intermember Variability of the Summer Northwest Pacific Subtropical Anticyclone in the Ensemble Forecast

Abstract

Abstract The accurate prediction of the East Asian summer monsoon (EASM) remains a major challenge for the climate research community. The northwest Pacific (NWP) subtropical anticyclone (NWPSA) is the dominant feature of the EASM low-level circulation variability. This study identifies two coupled modes between intermember anomalies of the NWPSA and sea surface temperature (SST). The first mode features SST anomalies over the tropical Pacific. This tropical Pacific mode has little impact on East Asian climate. The second mode features a strong coupling between SST in the north Indian Ocean (NIO)–NWP and NWPSA, with large impacts on East Asia. This resembles the Indo–western Pacific Ocean capacitor (IPOC) mode of interannual variability. Major differences exist in temporal evolution of the intermember SST spread between the equatorial Pacific and NIO. In the equatorial Pacific, the intermember SST spread grows gradually with lead time, while the spread of SST and low-level zonal wind grow rapidly from May to June in the NIO. The rapid growth over the NIO is due to positive feedback arising from the coupling between intermember anomalies of SST and winds. In post–El Niño summer, the intermember spread in equatorial Pacific SST forecast represents the variations in the timing of the El Niño phase transition. The late decay of El Niño relates to SST cooling and an anomalous cyclonic circulation over the South China Sea (SCS) but with little impact on East Asian climate. Thus, a better representation of the IPOC mode of regional ocean–atmosphere interaction over the NIO–NWP holds the key to improving the reliability of seasonal forecast of East Asian climate.