Weiqing Han (Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, United States); Lei Zhang (Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, United States); Shoichiro Kido (Application Laboratory (APL), Research Institute for Value‐Added‐Information Generation (VAiG), Japan Agency for Marine‐Earth Science and Technology (JAMSTEC), Japan); Julian McCreary (University of Hawaii, Honolulu, United States)

Coastal flooding induced by sea surface High EXtreme (HEX) events is an increasing risk to human society and infrastructure as both urban growth in coastal areas and anthropogenic sea level rise continue, especially for island nations like Indonesia. This paper investigates the role of atmospheric intraseasonal oscillations (ISOs), which are dominated by the Madden-Julian Oscillation (MJO), in forcing HEXs on the coasts of Indonesia bordering the Indian Ocean. We use satellite altimetry data from 1993-2021 and tide gauge observations to detect HEXs, and modeling experiments using both the Regional Ocean Modeling System and a Bayesian dynamic linear model to understand the forcing and processes. We find that intraseasonal sea level anomalies (SLAs) are significant contributors to HEXs in coastal Indonesia. In 32% of the 56 HEX events detected, the amplitude of ISO-induced SLAs exceeds that of seasonal-to-decadal SLAs. Surface wind stress associated with atmospheric ISOs is the major forcing for intraseasonal SLAs, and both the remote zonal wind stress from the Indian Ocean equator and alongshore wind stress at the Indonesian coasts play important roles in driving the HEXs. Effects of the ISOs exhibit strong seasonality, with a stronger influence on HEXs during boreal spring (March-May) than winter (December-February). The MJO is the dominant cause of ISO-dominated HEXs. The spring MJOs are associated with stronger convective anomalies over the eastern Indian Ocean equator compared to winter, which drive stronger zonal winds across the equatorial basin that lead to more HEX events in spring.