Tides and tidal currents in the Pearl River Estuary

Several cruises were conducted to investigate the variability in tidal levels, tidal flows, and water circulation in the Pearl River estuary during the dry and wet seasons in 1998. Unlike other temperate estuaries, the average tidal range was small in the offshore waters and increased gradually towards the estuary, reaching the maximum in the upstream part of the Pearl River estuary. The extent of the amplitude increment was different between the diurnal and semi-diurnal constituents. The percentage of sea level variance in the semi-diurnal band increased when the tide progressed upstream, but it reversed in the diurnal band. Thus, the accentuated sea level variance in the semi-diurnal band resulted in the increased amplitudes of semi-diurnal constituents which were larger than diurnal constituents. The distribution of the mean current in the estuary depicted an anti-clockwise circulation in the estuary: freshwater was dominant on the western side of the estuary, while salt water was dominant on the eastern side. The Pearl River estuary also showed a typical salt wedge circulation: a salt water intrusion largely occurred via the eastern channel in the estuary while river outflow dominated the western channel. In this study, long time series and large spatial coverage of tide and current observations allow in depth analysis of the temporal and spatial variations of tide and circulation, and the associated influence from open ocean in the Pearl River estuary.