Characterization of far-field Mekong freshwater mass transport in the southern South China Sea using satellite gravimetry

Abstract

Estuarine freshwater discharge to the ocean and its oceanic transport represent a dynamic interaction between freshwater and ambient seawater. While a near-field estuarine freshwater mass propagation within 350 km away from estuary has been investigated via in-situ data and modeling, its far-field propagation remains unexplored. This paper, for the first time, explores the potential usage of Mekong estuarine-discharged freshwater mass in the ocean inferred from time-variable satellite gravimetry data (e.g. Gravity Recovery and Climate Experiment (GRACE)) to characterize spread pattern and transport paths, and propagation direction and time duration in the entire Sunda Shelf and southern South China Sea. Through a cross-correlation analysis, spatial distribution of maximum cross-correlation coefficient and time lag between in-situ Mekong estuarine runoff and gravimetry-inferred estuarine-discharged freshwater mass on the ocean surface were calculated. It was found that the gravimetry-inferred time lags were largely consistent with the freshwater ages derived by isotope analysis sampled from the campaign in September 2007. During 2005–2013, the long-term spatial patterns of the maximum cross-correlation coefficient generated over sliding five four-year time spans yielded eastward clockwise and westward counterclockwise freshwater transport pathways, in summer and winter respectively, in the Sunda Shelf and the southern South China Sea. The long-term spatial patterns of the time lag showed less than a month was required for the freshwater mass transporting in the northeast direction, while the time lag lengthened progressively from a month to four months, clockwise from the northeast to the southwest direction. This could be explained by the monthly ocean surface current averaged between 2005 and 2013, simulated by the Estimating the Circulation and Climate of the Ocean (ECCO) model. Using eight one-year time spans between 2005 and 2013, the short-term spatial patterns of the maximum cross-correlation coefficient and the time lag varied considerably, which were likely attributable to global climate variability. As usual, the western boundary current in the southwestern South China Sea should be enhanced in La Nina year. However, in the strong La Niña event during 2010–2011, the weakened southwest current in the western Sunda Shelf enabled faster southward freshwater mass transport, resulting in shorter time lag (within three months) than that of the long-term one.