Physics-Based Predictions of the Month-by-Month Summer Western North Pacific Anomalous Anticyclone

Abstract This study identified the distinct predictors of the western North Pacific anomalous anticyclone (WNPAC) in June, July, and August and revealed their independent influencing mechanisms on the formation of the summer WNPAC in each corresponding month. For the June WNPAC (AC_Jun), the North Atlantic warming (NAW) in April–May induces subsequently enhanced convection over the Indian Ocean and suppressed the convection over the tropical central Pacific, leading to AC_Jun via the Gill-type response. Meanwhile, the warming tendency over the western North Pacific from February to April, which reflects the slow decay of El Niño, results in AC_Jun through Indian Ocean warming. For the July WNPAC (AC_Jul), the cooling tendency from April to June over the central and eastern tropical Pacific is the sole predictor, which results in AC_Jul through the wind–evaporation–SST feedback. For the August WNPAC (AC_Aug), the tropical Atlantic warming (TAW) in June and July and the Bering Strait sea ice slow melting (BSSM) from April to May are the two predictors. While TAW has a similar influence on AC_Aug to that of NAW on AC_Jun, BSSM makes for AC_Aug by strengthening the westerly jet and subsequently atmosphere–ocean coupling over the North Pacific. Using these predictors, three physics-based empirical models (PEMs) for the seasonal predictions of AC_Jun, AC_Jul, and AC_Aug are conducted, which all outperform the National Centers for Environmental Prediction Climate Forecast System (CFS), shedding light on improving the seasonal prediction of the East Asian summer climate. Significance Statement In this article, we distinguish the different origins of the western North Pacific anomalous anticyclone (WNPAC) in June–August and establish three physics-based seasonal prediction models for the WNPAC in each summer month. It is unraveled that the June WNPAC is primarily associated with Atlantic warming and slow decay of El Niño in spring, the July WNPAC is mainly related to the fast decay of El Niño from spring to early summer, and the August WNPAC is heralded by early summer tropical Atlantic warming and spring sea ice concentration melting over the Bering Strait. The month-by-month physics-based empirical models using these predictors outperform the National Centers for Environmental Prediction (NCEP) Climate Forecast System for predicting both the monthly and seasonal mean of the WNPAC indices, thus providing a new perspective for improving the seasonal prediction of East Asian summer climate.