Sea-level changes over the last six decades in the South China Sea

Abstract

Spatial and temporal changes in mean and extreme sea levels and their forcing mechanisms have been examined over the last six decades in the South China Sea (SCS). A total of 53 tide gauge records and 22 years of satellite altimetry observations, along with oceanographic and meteorological datasets, have been used. The first objective examined changes in mean sea level trends. It was found that the subregion basin sea level trends from altimetry for the period 1993-2014 varied between 2.9 ± 0.8 mm/yr in the northern SCS and 6.2 ± 5.7 mm/yr in the Celebes Sea. In the northern SCS, the halosteric variation significantly contributes to the observed altimetry trends, which are up to 3 mm/yr. The separation of the local sea level trends into various contributors involves high uncertainties especially due to vertical land motion (~3 mm/yr). The second objective assessed changes in inter-annual mean sea level variability. The inter-annual sea level variability is larger in the Celebes Sea and accounts for up to 70% of the monthly sea level variability. Changes in steric and wind stress contribute to more than half of inter-annual sea level variability in the Malacca Strait and the eastern basin of the SCS. The inter-annual sea level variability in these regions is associated with the El Niño Southern Oscillation (ENSO) and the monsoon. The third objective evaluated changes in the mean seasonal sea level cycle. It was found that the contribution of seasonal cycle is larger on the continental shelf of the Gulf of Thailand, accounting for up to 92% of monthly sea level variability. Wind forcing is dominant on the shelf areas of the SCS with the local winds showed annual amplitudes of up to 27 cm. In the deep basin regions and the shallow Malacca Strait, the steric component is the major contributor with the maximum annual amplitudes reaching 15 cm. Significant temporal variability of the annual and semi-annual harmonics is up to 63% and 45% of the maximum amplitude, 15 cm and 11 cm, respectively. The fourth objective explored changes in extreme sea level. Significant increases in extreme sea levels were observed at more than half of the stations analysed and were found to be mainly driven by changes in mean sea level. In the Malacca Strait, the inter-annual variability in extreme sea level is found to be associated with ENSO and the monsoon. Overall, this study provides improved understanding of changes in mean and extreme sea levels and their associate mechanisms that occurred over different timescales. Changes in these sea level components need to be taken into consideration for mean and extreme sea levels projections in this region.

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ORCID for Ivan Haigh: orcid.org/0000-0002-9722-3061

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