Modelling ocean–lagoon interaction during upwelling processes in the South West of New Caledonia

• Simulating upwelling events with a 3D coupled physical-biogeochemical model.
• Model reproduced well physical/biogeochemical patterns of the upwelling process.
• Rich upwelled waters were found to be able to enter the lagoon.
• Optimal windows for recruitment depend on wind, tide and mesoscale circulation.

Since 2005, wind-driven ‘sporadic’ coastal upwelling events have been identified off the southwestern reef of New Caledonia. Several studies have described the main physical processes and induced surface patterns using 1D and 3D modelling, as well as in situ measurements. Previous models were applied at the mesoscale without taking into account the lagoon. Using a recently developed 3D coupled physical-biogeochemical model that considers the complex ocean-lagoon interface, we aim to understand better the impact of the upwelling on the lagoon. The model was found to be in good agreement with measured data reported in previous publications about two upwelling events. However, in general, levels of surface chlorophyll-a were overestimated by the model in the upwelling area when compared to ocean colour data and several hypotheses have been proposed to explain the discrepancy. We then tracked rich upwelled water using a forward Lagrangian transport analysis. Upwelled waters from the upper nutricline were found to be able to reach the South West lagoon. An anti-cyclonic eddy was detected near the upwelling area, potentially responsible, in part, for the rich water intrusions into the lagoon.