Coastal water level trends and ENSO-related variability in the northeastern Mekong Delta (1979–2024)

Abstract

Coastal water levels in the northeastern Mekong Delta exhibit pronounced long-term trends and interannual variability linked to large-scale climate forcing. Using tide-gauge observations at the Vam Kenh station (1979–2024) and CMEMS (Copernicus Marine Environment Monitoring Service) satellite altimetry (1993–2024), we assess long-term, seasonal, and ENSO-related water level variability. The results reveal a statistically significant and evidence of recent acceleration inferred from sub-period trends in mean relative water level, with rates exceeding approximately 6 mm yr⁻¹ over the last two decades (2005–2024), compared to about 5 mm yr⁻¹ over the full 46-year record. Seasonal variability shows modulation of the rise by the monsoon cycle, with faster increases during the northeast monsoon and slower rises during the southwest monsoon. Interannual variability is clearly modulated by the El Niño–Southern Oscillation, with strong La Niña events elevating water levels by up to about 5 cm in anomaly magnitude, while El Niño phases are generally associated with negative anomalies. Atmospheric pressure acts as a secondary control, contributing approximately 5–20 % of interannual variance through the inverse barometer effect. Tide-gauge and CMEMS data exhibit strong agreement (r = 0.95), supporting their combined use for coastal monitoring. Overall, the results highlight accelerating relative rise in a subsiding deltaic environment, with important implications for coastal vulnerability.