Sea surface temperature as a tracer to estimate cross‐shelf turbulent diffusivity and flushing time in the Great Barrier Reef lagoon

Abstract

AbstractAccurate parameterization of spatially variable diffusivity in complex shelf regions such as the Great Barrier Reef (GBR) lagoon is an unresolved issue for hydrodynamic models. This leads to large uncertainties to the flushing time derived from them and to the evaluation of ecosystem resilience to terrestrially derived pollution. In fact, numerical hydrodynamic models and analytical cross‐shore diffusion models have predicted very different flushing times for the GBR lagoon. Nevertheless, scarcity of in situ measurements used previously in the latter method prevents derivation of detailed diffusivity profiles. Here detailed cross‐shore profiles of diffusivity were calculated explicitly in a closed form for the first time from the steady state transects of sea surface temperature for different sections of the GBR lagoon. We find that diffusivity remains relatively constant within the inner lagoon (<∼20 km) where tidal current is weak, and increases linearly with sufficiently large tidal amplitude in reef‐devoid regions, but increases dramatically where the reef matrixes start and fluctuates with reef size and density. The cross‐shelf profile of steady state salinity calculated using the derived diffusivity values agrees well with field measurements. The calculated diffusivity values are also consistent with values derived from satellite‐tracked drifters. Flushing time by offshore diffusion is of the order of 1 month, suggesting the important role of turbulent diffusion in flushing the lagoon, especially in reef‐distributed regions. The results imply that previous very large residence times predicted by numerical hydrodynamic models may result from underestimation of diffusivity. Our findings can guide parameterization of diffusivity in hydrodynamic modeling.