Revealing the relationship between ENSO and the annual cycle of the East Asia and western Pacific monsoon using a low-level circulation weather-type calendar

This paper proposes a novel approach to identifying the annual cycle and variability of the East Asia and western Pacific (EA-WP) monsoon. A low-level circulation weather type (WT) calendar is constructed based on nine WTs obtained from K-means cluster analysis for 46 years (1979–2024) of daily 850-hPa wind data. The WT daily occurrence frequency reveals climatological features of the monsoon annual cycle and seasonal progression. The WT-based summer and winter monsoon indices well represent key monsoon characteristics. A strong East Asian summer monsoon (EASM) corresponds to more frequent occurrences of WT featuring low-level anticyclonic circulation over the western North Pacific, while a strong East Asian winter monsoon (EAWM) corresponds to more frequent occurrences of WTs characterized by low-level northerly winds over the South China Sea and the Philippine Sea. The variability of EASM and EAWM is synchronized during the developing phase of ENSO events. Strong (weak) EA-WP monsoon years are marked by strong (weak) EASM followed by strong (weak) EAWM, whereas no clear relationship is found between EASM and its preceding EAWM. Approximately, 86% (76%) of the strong (weak) EA-WP monsoon years are associated with La Niña (El Niño) winters. The period from mid-July to late-August is identified as a critical window for the synchronization of monsoon and ENSO. Observational evidence suggests that strong and weak air-sea coupling represent two distinct states of EA-WP monsoon years. Under strong air-sea coupling, monsoon variability is significantly enhanced, with EA-WP monsoon and ENSO functioning as two interconnected components of a single coupled system.