Sea-level variability and vertical land motions in Singapore from tide gauge and GNSS observations

Sea-level rise poses a threat to Singapore's coastlines and coastal ecosystems. To strengthen coastal protection, it is crucial to understand sea-level variability and quantify the contributions from climate change and vertical land motions (VLM). Although the variability of relative sea level in the region has been studied, the impact of VLM and the temporal variations of seasonal sea-level cycles remain underexplored. This study examines the magnitude and causes of VLM using the most comprehensive and up-to-date dataset from an island-wide Global Navigation Satellite System (GNSS) network and integrates this information with relative sea-level measurements from tide gauges to isolate the effect of VLM. To investigate sea-level variability, we incorporate colored noise to derive rates and use forward stepwise multiple linear regression (MLR) analysis to examine temporal variations of seasonal sea-level cycles. Our results show that, from December 2004 to April 2012, island-wide land subsidence at rates up to -2.2 mm/yr was observed due to three great megathrust earthquakes in Sumatra. Except for this period of tectonic subsidence, the main island is generally stable with subsidence close to zero. On average, relative sea level in Singapore has risen at rates of 2.7 mm/yr from 1980 to 2020 and 3.6 mm/yr from 1993 to 2020. The annual sea-level cycle amplitude ranges from 5 to 15 cm, while the maximum semi-annual amplitude is approximately 4 cm, with both amplitudes exhibiting variations of 2 to 3 cm over time.