Synoptic variations of residual currents in the Huanghe (Yellow River)-derived distal mud patch off the Shandong Peninsula: Implications for long-term sediment transport

摘要

Huanghe (Yellow River) is one of the largest contributors of global terrestrial materials to the sea. A large amount of the Huanghe-derived sediment are responsible for forming not only the nearshore delta depositional system, but also a distal mud patch off the Shandong Peninsula, some 350 km east of the present-day river mouth. To better understand the spatial and temporal variations of sediment dynamics in the distal mud area, two field surveys employing the phase-averaging method (PAM) were conducted in September 2017 and January 2018, respectively along a 108-km East-West transect across the mud patch. The residual currents derived from the PAM surveys were featured by the southward current in the nearshore area and the northward current in the offshore region in summer, while a unidirectional current transported from south to north across the entire transit line in winter. These patterns, which deviate from the traditional understandings of circulations in magnitude and even in direction, are believed to result from episodic wind reversals and associated strengthening and weakening sea level pressure gradients. The northerly wind episodes in summer, works hand-in-hand with sea-level tilt by preceding southerly wind, to enhance the water exchange between the Bohai Sea and the Yellow Sea by intensifying both the southward flow along the coast and the northward flow of the South Yellow Sea water further offshore. In winter, episodic southerly wind causes relaxation or even reversal of the prevailing winter northerly wind that created the northward sea-level tilt in the Bohai Sea and the Yellow Sea, effectively enhances the South Yellow Sea water transport to the North Yellow Sea and the Bohai Sea. Consequently, a considerable amount of sediment previously exported out of the Bohai Sea to the mud patch actually return with these backflows. Since the synoptic time scale oscillations in the Bohai and Yellow seas are considerably frequent throughout summer and winter, the cumulative effects on the long-term sediment transport need to be further investigated. The observations in this study provide a base for new understanding of how episodic weather events impact the regional circulation and sediment dynamics on the Huanghe-derived distal mud patch.