Radiation and Energy Flux Transfers under Monsoon conditions over Heron Reef, Southern Great Barrier Reef, Australia

Coral reefs cover approximately 2.8 to 6.0 x 10⁵ km² of the Earth's tropical and sub-tropical oceans and support the livelihoods of > 500 million people. Their lower thermal capacity compared to the open ocean means that they are subject to more rapid and greater heating (cooling) and higher (lower) sea surface temperatures (SST) than adjacent oceanic sites. Differences in surface roughness and albedo over coral reefs due to modification of waves by the reef rim, shallow waters and underlying benthos affect radiation transfers and exchanges of heat, moisture and trace gases across their air-sea interface. Accordingly, the meteorology of coral reefs is distinct from the adjacent ocean.

In this paper we present results from the measurement of radiation and energy flux exchanges using the eddy covariance method over Heron Reef, a 27 km² lagoonal platform reef on the Southern Great Barrier Reef under monsoon conditions. Eddy covariance systems were mounted on pontoons and anchored over the reef ensuring a constant measurement height of 2.2 m above the water surface. Results show mean daily latent heat flux over the reef flat ranged from 52 to 77% of net radiation, while sensible heat flux ranged from 4 to 12% of net radiation with remaining energy going into heating of the reef and overlying water. By comparison, over the deep lagoon, the sum of convective fluxes exceeded mean daily net radiation resulting in a net cooling of the water. The drag coefficient across Heron Reef ranged from 1.5 x 10^-3 to 2.3 x 10^-3. These results highlight the spatial variability in energy exchanges across coral reefs and their very different energy balance compared to the open ocean.