Wave and tidal flushing in a near-equatorial mesotidal atoll

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• 作者：Mirella B. Costa ; Eduardo C. Macedo ; Arnoldo Valle-Levinson ; Eduardo Siegle
• 关键词：Mesotidal atolls ; Lagoon circulation ; Flushing ; Wave pumping
• 刊名：Coral Reefs
• 出版年：2017
• 出版时间：March 2017
• 年：2017
• 卷：36
• 期：1
• 页码：277-291
• 全文大小：
• 刊物类别：Earth and Environmental Science
• 刊物主题：Freshwater & Marine Ecology; Oceanography;
• 出版者：Springer Berlin Heidelberg
• ISSN：1432-0975
• 卷排序：36

文摘

Most of the atolls found worldwide are under microtidal regimes, and their circulation mechanisms are widely documented and well known. Here, we describe the flushing mechanisms of a small-sized mesotidal atoll, based on water-level, wave and current data obtained during two different periods (total of 60 d). Rocas is the only atoll in the South Atlantic Ocean and is built primarily of coralline algae. Two reef passages connect the atoll lagoon to the ocean. Synchronous current profilers were deployed at the two reef passages, one inside and one outside the atoll, to characterize the influence of tides and waves on the circulation. Results showed that wind waves drove a setup on the exposed side of the atoll and that currents were predominately downwind, causing outflow at both reef passages. Waves breaking on the windward side supplied water to the atoll causing the lagoon water level to rise above ocean water level, driving the outflow. However, unlike microtidal atolls, at Rocas Atoll the water level drops significantly below the reef rim during low tides. This causes the reef rim to act as a barrier to water pumping into the lagoon by waves, resulting in periodic activation of the wave pumping mechanism throughout a tidal cycle. As result, inflow occurs in the wider passage during 27% of each tidal cycle, starting at low tides and reversing direction during mid-flood tide when the water level exceeded approximately 1.6 m (while overtopping the atoll’s rim). Our findings show that tides play a direct role in driving circulation on a mesotidal atoll, not only by modulating wave setup but also by determining the duration of wave pumping into the lagoon.