Sea‐Level Change in Hong Kong From Tide Gauges, GNSS, and Satellite Altimetry

Abstract

As a densely populated, highly urbanized coastal city, Hong Kong is particularly vulnerable to sea‐level rise, which amplifies flood risk and threatens coastal infrastructure, economies, and ecosystems. We combine observations from tide gauges, conventional gridded satellite altimetry, coastal‐retracked altimetry, and Global Navigation Satellite Systems (GNSS) to characterize sea‐level variability, estimate satellite‐era trends, and evaluate local relative sea‐level trends and the contribution of vertical land motion (VLM). Seasonal variability is dominated by annual cycle, with reduced amplitudes and a phase lead at the Tsim Bei Tsui (TBT) tide gauge near the Pearl River Estuary. Interannual variations are coherent across stations; the leading mode is anticorrelated with the Niño‐4 index ( = −0.52), with maximum anticorrelation when sea level lags Niño‐4 by approximately 3 months ( = −0.58). Despite this coherence, linear relative sea‐level trends derived from tide‐gauge records differ substantially (3.47 ± 0.30 mm/yr to −0.14 ± 0.25 mm/yr for 1999 – 2024), indicating strong site dependence from local processes. In contrast, geocentric sea‐level rise from gridded altimetry near Hong Kong (within 100 km) is relatively uniform, with an area‐mean trend of 3.18 ± 0.17 mm/yr. GNSS at the Quarry Bay (QUB) tide‐gauge site reveals pronounced non‐linear VLM and a large VLM residual when offshore gridded altimetry is used as the geocentric reference for altimetry–tide‐gauge‐derived VLM. These findings highlight that robust attribution and projection of sea‐level change in Hong Kong require improved nearshore altimetry, sustained co‐located geodetic control, and datum stability assessment at tide‐gauge sites.