Using Lagrangian coherent structures to investigate upwelling and physical process in the Gladstone coastal region

Coastal upwelling plays an important role in the exchange of nutrients and pollutants. Understanding of upwelling and its predictors allows for important management of nutrient-enriched coastal waters. This study presents an analysis of coastal upwelling with a focus on Gladstone, Queensland (Australia). Particularly, Lagrangian coherent structures (LCSs) are used to quantify the spatio-temporal variability of upwellings, and to reveal the relationships between the surface mixing and physical properties of water masses and oceanographic activity in the Gladstone region. Four different seasonal selections of water surface velocity data (for summer, autumn, winter and spring) are compared with wind, sea surface temperature (SST), sea surface salinity and density data to investigate the coastal upwelling area. The surface mixing is calculated based on the backward Finite-Time Lyapunov Exponent (FTLE) diagnostic approach of LCSs. The strong seasonality of upwelling largely follows seasonal patterns of winds. Results highlight how upwelling behaviour varied with the intensity of cyclones. The core conclusion is that upwelling dynamics, estimated using classic means such as SST, have a strong correlation with the FTLE metric of LCSs. The potential upwelling hotspot shows that on average, it has 17.3% of its area above the FTLE threshold, compared to a non-upwelling area which has a corresponding percentage of 0.64%. This relationship makes it possible to define different upwelling locations, and properties of upwellings (e.g., seasonal variability, cyclone effect) as a function of FTLE, a derivative of the surface velocity field. Therefore, LCSs can be used to predict surface mixing and upwelling behaviour in coastal water systems.