Eddy-Driven Cross-Shelf Exchange and Variability in the East Auckland Current

Abstract

Using an ocean reanalysis and freely evolving numerical simulation, eddy-driven cross-shelf exchange in the East Auckland Current (EAuC) system was investigated. The EAuC is stronger in the reanalysis than the free run and has a more evident impact on cross-shelf exchange. Despite differences in the EAuC strength, both simulations produced similar small-eddy (radius 30 km) statistics, which supports the robustness of the reanalysis considering that data assimilation could generate unrealistic variability. These 1-year simulations revealed mechanisms by which the EAuC and its eddy variability drive water exchange between the continental shelf and the slope that depend on its proximity to the shelfbreak. At times and locations where the EAuC is attached to the continental slope, onshore bottom Ekman transport with variability between 4 and 60 days results in cross-isobath exchange. However, EAuC-driven bottom Ekman transport is smaller than the total volume that crosses the shelfbreak (1%–12%), which is largely controlled by submesoscale variability. An eddy-tracking algorithm was used to identify, classify, and analyze eddy-driven impact on cross-shelf exchange in both runs. Cyclonic eddies generated cross-shelf exchange by exporting up to 291 ${\text{km}}^3$ of shelf waters to the open ocean. Coastal cyclones are small eddies (radius 18 km) formed on the continental shelf via shear instability and have submesoscale characteristics (Rossby and Richardson numbers O(1)) and are more efficient in generating cross-shelf exchange than larger eddies. Slope cyclones form in deeper waters ($>$200 m) and have transitional characteristics between submesoscale and the large mesoscale eddies (radius $>$50 km). These larger mesoscale eddies did not have an impact on cross-shelf exchange. Coastal (slope) eddies pull water from the slope toward the continental shelf with vertical speeds of $\sim$30 m/day ($\sim$15 m/day) and generate temperature anomalies of −1.5°C (−1°C), which follow the eddies' trajectory along their pathway.