Behavior of a wave‐driven buoyant surface jet on a coral reef

A wave‐driven surface‐buoyant jet exiting a coral reef was studied in order to quantify the amount of water reentrained over the reef crest. Both moored observations and Lagrangian drifters were used to study the fate of the buoyant jet. To investigate in detail the effects of buoyancy and alongshore flow variations, we developed an idealized numerical model of the system. Consistent with previous work, the ratio of alongshore velocity to jet velocity and the jet internal Froude number were found to be important determinants of the fate of the jet. In the absence of buoyancy, the entrainment of fluid at the reef crest creates a significant amount of retention, keeping 60% of water in the reef system. However, when the jet is lighter than the ambient ocean water, the net effect of buoyancy is to enhance the separation of the jet from shore, leading to a greater export of reef water. Matching observations, our modeling predicts that buoyancy limits retention to 30% of the jet flow for conditions existing on the Moorea reef. Overall, the combination of observations and modeling we present here shows that reef‐ocean temperature gradients can play an important role in reef‐ocean exchanges. Key Points Reef‐ocean temperature gradients are important in regulating reef‐ocean exchange Substantial recirculation of wave‐driven jets exiting reef passes can occur Alongshore velocity is important in determining amount of jet recirculation