ENSO-influenced Mekong plume extension revealed by causality between estuarine water level and GRACE-derived oceanic height

Abstract

Understanding river plume extension is essential for resolving freshwater connectivity and biogeochemical fluxes in the open ocean. However, it remains poorly understood due to the limited detectable transport ranges of isotope measurements, numerical models, and satellite methods. Rather than cross-correlating runoff with GRACE-derived oceanic equivalent water height (OEWH) as in previous studies, this study cross-correlates estuarine water level—served as a runoff proxy—with GRACE-derived OEWH/Ocean bottom pressure. Granger and Liang causality tests are further applied to examine whether estuarine output potentially drives the GRACE-derived oceanic height changes, thereby validating the inferred spatiotemporal extent of far-field plume transport across the Sunda Shelf Sea (SSS) and South China Sea (SCS). Isolating interannual variability via multi-wavelet analyses, we found that while La Niña tends to enhance offshore extension across SSS and SCS, alternating El Niño-La Niña phases stagnate transport in central SSS. Comparison with isotope-estimated plume ages confirms the method’s potential applicability. Notably, the inferred Mekong plume might extend to ∼18°N in SCS that represents an exceptionally long range of the plume transport. These findings provide a potential alternative for determining extended plume transport duration patterns and highlight the role of climate variability in reshaping estuary-to-shelf transport.