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.

河口-陆架相互作用非常复杂，对于维持河口和邻近海域的健康生态系统至关重要。该主题已通过巡航观测和数值模型进行了深入研究，但较少通过固定站的长期观测进行研究。我们首次在这里展示海底天文台小曲山试验站大约2个月的ADCP和CTD连续数据，旨在研究长江口外的河羽前沿过程，以应对2009年枯水期向湿润期的河流排放过渡。 。多条证据表明，内河羽流总是处于底部陷井的形式，但是等盐和等温线对淡水输入，风气候和地表加热的季节性变化非常敏感。盐度和温度的半日振荡是羽锋过程中季节性变化的良好示踪剂。在干旱季节，由于在内部羽状流锋附近的强混合条件下，最小的跨岸应变，盐度和温度都具有较弱的半日振荡，那里的等盐卤和等温线几乎是垂直的，水平盐度和温度梯度较小。由于新建立的羽状锋的到来，半倾斜盐度振荡急剧增加，中断了陡峭的等斜线坡度，但最大水平盐度梯度为0.8 psu / km，这使它们突然中断。内部羽流前沿对第一个放电增加脉冲的响应时间大约为两周，但是，在不同的羽流位置以及在不同的河水排放条件下进行检查时，应该有很大的变化，从而增加了幅度和速率。在进入典型的湿润季节之前，过渡过程持续数周，因此前缘过程很好地反映了河流的流量波动。后者的特征是与夏季表面加热相关的最强的半日温度振荡，以及高度压抑的半日盐度振荡，这是由于主要风向从南风向北风交替而从南向东北移动。通过小波变换和相关的带通滤波方法，小波功率谱以及盐度和温度的半日/日振荡的振幅可以更清楚地划分出内部羽状锋面过程的三个阶段。