Roles of Atmospheric Intraseasonal Oscillations in Causing Sea Level Extremes Off the Indonesian Coasts of the Indian Ocean in Recent Decades

– William Kamp, Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder –

Résumé :

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 the Regional Ocean Modeling System (ROMS) and a Bayesian dynamic linear model (DLM) 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, ISO-induced SLAs exceed the 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 causing the HEXs. The effects of the ISOs exhibit strong seasonality, with stronger influence on HEXs during boreal spring (March-May) than winter (November-January). The MJO is the dominant cause of ISO-dominated HEXs. The spring MJOs are associated with stronger, more equatorially symmetric convective anomalies in the eastern Indian Ocean compared to winter which drive stronger zonal winds across the equatorial basin, leading to more HEX events in spring.