Multi-frequency sea level variations at the Quy Nhon port area in the period from 1987-2023

The Quy Nhon port in Vietnam is a key hub for trade and economic development in Binh Dinh province but is significantly impacted by sea-level (SL) variability, which can affect shipping and port operations. This study analyzes sea level variations at Quy Nhon port from 1987 to 2023 using monthly tide gauge data. The Empirical Mode Decomposition method applied to this dataset, identifies four dominant modes: an annual cycle (53.7% of variance, 23.7 cm amplitude) primarily driven by monsoon wind reversals, a semi-annual cycle (27.1%, 13.2 cm amplitude), and interannual (2.3 years, 7 cm amplitude) and decadal (10.7 years, 4 cm amplitude) variations. These latter modes are significantly anti-correlated with the El Niño-Southern Oscillation (ENSO, r = -0.34) and Pacific Decadal Oscillation (PDO, r = -0.45), indicating that warm (cold) phases of these climate modes correspond to lower (upper) SL. A long-term linear SL rise of 1.5–2.1 mm/yr was also observed. These findings provide key insights for SL forecasting and maritime safety, emphasizing the need to refine predictions and better understand underlying physical processes