Geostrophic Pumping, Inflows and Upwelling in Barrier Reefs

Abstract

The communication between shallow and deep oceans via gaps in the separating barrier reefs is examined using a simplified two-layer analytical model. Attention is focused on the flow resulting from a sea-level difference between the ocean and the lagoon. Such a difference imposes a pressure gradient along the gap which, in turn, forces a flow into the lagoon. The coral reefs, which extend all the way to the surface and are exposed to the atmosphere at low tide, are presented by two portions of an infinitely long wall. A group of passages, whose combined width is not very small compared to the Rossby radius, is represented by a single gap separating the two portions of the wall. The fully nonlinear model is inviscid, hydrostatic and nondiffusive. Nonlinearity is essential because (i) the flow in the passages is rather fast, and (ii) the depth variations are of order unity. Steady solutions for the upstream and downstream fields are constructed analytically using the momentum equation in an integrated form, the Bernoulli integral and conservation of potential vorticity. It is found that, surprisingly, the transport through the gap is independent of the gap's width. Upstream, the oceanic water approaches the gap only from one direction; upon reaching the gap, the approaching current splits into two branches. One continues to flow in the oceanic basin and never enters the gap whereas the other passes through the gap and penetrates into the lagoon. When the sea-level difference between the ocean and the lagoon exceeds a critical value, water below the oceanic thermocline is pulled up and forced into the lagoon. This nutrient-rich upwelled water forms a boundary current that hugs the barrier reef on the right hand side in the Northern Hemisphere. We term this new type of upwelling and suction ?geostrophic pumping? because it is a result of the geostrophic flow away from the gap. A possible application of this geostrophic pumping to the upwelling and inflow in the Great Barrier Reef is briefly discussed. The model provides a plausible explanation for the health of the coral on the lagoon side where, without such an inflow, the nutrients would have been depleted.