Research
Episodic circulation and exchange in a wave-driven coral reef lagoon
HENCH, J. L., J. J. LEICHTER, AND S. G. MONISMITH, 2008. Episodic circulation and exchange in a wave-driven coral reef and lagoon system. Limnology & Oceanography, 53 (6): 2681-2694. Abstract
We examine the role of wave-driven circulation relative to wind and buoyancy forcing in a coral reef lagoon system. Circulation measurements in Paopao Bay, Moorea during austral summer show the importance of waves in driving flows over the reef crest, through the lagoon, and out the reef pass. Tides were comparatively weak, due to proximity to amphidromic points, and exhibited an unusual spring-neap cycle where the major solar tide modulates the major lunar tide, with the overall tidal phase staying approximately constant. Wind had only a secondary effect compared to surface waves. A simple fluid mass balance indicated rapid flushing of the shallow back reef and export through the reef pass, and a reef capture zone width of ca. 2.3 km. The reef pass circulation dynamics exhibit two-layer baroclinic exchange flow when waves are small that is suppressed during large wave events. The unusually weak tidal forcing provided an opportunity to more closely investigate wave-driven circulation dynamics. As expected theoretically, there was a wave-driven set-up of the free surface across the shallow lagoon, which drives a highly frictional flow, evident by a large drag coefficient CD = 0.1. Diverging from extant theory, the observed set-up varied strongly with significant wave height and period. Overall, the circulation and exchange between this coral reef system and the adjacent open ocean is largely determined by episodic remote forcing events, and differs significantly from periodic tidal exchange mechanisms.