Estuary-shelf interactions off the Changjiang Delta during a dry-wet seasonal transition

Abstract

Estuary-shelf interaction is very complex and vital for maintaining healthy ecosystem in the estuaries and adjacent seas. The topic has been intensively studied through cruise observations and numerical models, but less by long-term observations at a fixed station. We present here for the first time roughly 2-months ADCP and CTD continuous data from the Xiaoqushan pilot station of seafloor observatory, aiming to deliberate river plume frontal processes off the Changjiang Estuary in response to river discharge transition from the dry to wet season in 2009. Multiple lines of evidence attest that inner river plume is always in a bottom-trapped form, but isohalines and isothermals are very sensitive to seasonal variations in freshwater input, wind climate and surface heating. Semidiurnal oscillations of salinity and temperature are good tracers for seasonal variations in plume frontal processes. In the dry season, both salinity and temperature have weak semidiurnal oscillations due to minimal cross-shore straining under strong mixing conditions near the inner plume front where isohaline and isothermal are nearly vertical with small horizontal salinity and temperature gradient. They are suddenly interrupted by a sharp increase of semidiurnal salinity oscillations owing to the arrival of newly established plume front with a less steep isocline slope but a maximum horizontal salinity gradient of 0.8 psu/km. The response time of inner plume front to the first discharge increasing pulse is roughly two weeks, but should change a lot when examined at different plume locations and under varied river-discharge increasing magnitudes and rates. The frontal processes mirror well the river discharge fluctuations in the transitional period which lasts for a few weeks before entering a typical wet season. The latter features strongest semidiurnal temperature oscillations linked with summer surface heating, and highly depressed semidiurnal salinity oscillations because the main plume path shifts from southward to northeastward due to alternating southerly to northerly prevailing winds. The three stages of inner plume frontal processes can be more clearly demarcated by the wavelet power spectral and the amplitudes of semidiurnal/diurnal oscillations of salinity and temperature through wavelet transform and the associated band-pass filtering methods. The seasonal plume frontal transition has been vividly portrayed for the first time through secular observation at the fixed station in the river plume region, significantly improving our understanding their controlling factors and mechanisms.