The Penghu Channel is the main channel connecting the East and South China Seas, two of the largest marginal seas in the world. Located in the southeast of Taiwan Strait, the Penghu Channel is usually covered by the high salinity water from the South China Sea and the Kuroshio. However, we observed abnormal low-salinity water in the Penghu Channel during a cruise through the southern Taiwan Strait and northern South China Sea in August 2008. We argue that the normalized alkalinity is a good indicator for the identification of a river plume as it is not affected by rainwater. Using satellite-derived water transparency and chlorophyll images and field-measured alkalinity, the source of this low salinity water was found to be the intrusion of the Pearl River plume. A significant phytoplankton bloom across the entire Taiwan Strait occurred with the intrusion event. The intrusion was not a unique event, as we also found a strong jet-shaped Pearl River plume intruding into the Penghu Channel in the summer of 2009 from cloud-free satellite-derived images. Time series satellite data reveal that the Pearl River plume intrudes into the Penghu Channel in the summer of most years. Multiple data analysis and modeling simulation indicate that a large river discharge and strong southwesterly winds on the shelf may be responsible for the significant intrusion of the Pearl River plume into the Penghu Channel in summer. As the Pearl River plume has a high nutrient and dissolved inorganic carbon content, combined with the strong northward flows through the Penghu Channel, such intrusions may contribute to the nutrient dynamics and carbon budget of the East and northern South China Seas.
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