Long‑Term Relative Sea‑Level Variability and Its Relationship with Large‑Scale Climate Modes along the South Central Coast of Vietnam

Abstract

This study investigates long-term relative sea-level variability along the South Central Coast of Vietnam using tide-gauge observations spanning 40 years at Quy Nhon and 51 years at Cau Da, and examines its relationships with major large-scale climate modes. Sen's slope estimation indicates that relative sea level increased at both stations at rates of 1.91–2.36 mm yr−1, with trend significance at the 90% confidence level. Empirical Mode Decomposition separated the sea-level series into six components. Among them, Intrinsic Mode Functions (IMF) 3 and IMF4, with dominant periods of 2.13–2.37 and 5.33 years, respectively, represent interannual variability associated with El Niño–Southern Oscillation (ENSO), whereas IMF5, with a common period of 10.67 years at both stations, reflects decadal variability likely related to Pacific Decadal Oscillation (PDO). Composite analysis revealed that sea-level anomalies decreased by 2.9–3.2 cm during El Niño events and increased by 2.4–3.2 cm during La Niña events. Correlation analyses showed significant negative relationships between sea-level anomalies and the Oceanic Niño Index (ONI) and PDO, with response lags of 1–4 months and 1–2 months, respectively, whereas the Indian Ocean Dipole (IOD) exhibited only weak or statistically insignificant correlations. Partial correlation and multiple regression analyses further identified ENSO and PDO as the important large-scale climate drivers influencing interannual sea-level variability. ENSO exerted a stronger influence at Cau Da, while PDO contributed more substantially at Quy Nhon. Together, these climate modes explained approximately 22–25% of the observed interannual sea-level variance. These findings provide new insights into regional sea-level dynamics and support future assessments of sea-level change and coastal adaptation along the South Central Coast of Vietnam.