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 km3 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 ~30 m/day (~15 m/day) and generate temperature anomalies of −1.5°C (−1°C), which follow the eddies' trajectory along their pathway.
Plain Language Summary: Cross‐shelf material exchange has many implications for ecosystem dynamics. This study investigates the dynamics of oceanic processes governing cross‐shelf exchange within the East Auckland Current (EAuC) system. Using a data‐constrained simulation and a free run, we elucidate the role of the EAuC and its eddy variability (circular water movements) in driving water exchange between the continental shelf and the slope. Our findings highlight the impact of the EAuC on shoreward transport, with variability observed across its latitudinal extent. Notably, while the data‐constrained run exhibits a stronger EAuC than the free run, both simulations reveal similar statistics regarding small‐eddy behavior. We identify small cold‐core eddies as predominant features along the shelfbreak, forcing prolonged influence compared to warm‐core eddies. Furthermore, our investigation delineates the distinct characteristics of coastal and slope cold‐core eddies, elucidating their respective roles in facilitating offshore water mass transport.
Key Points:
• Data assimilation improves the representation of the East Auckland Current but does not change the statistics of eddies impacting cross‐shelf exchange.
• The East Auckland Current generates uplift of colder water onto the shelf in most regions of the shelfbreak and routinely sheds a frontal eddy at the same location along its pathway.
• Small eddies (radius <30 km) travel along the shelfbreak generating frequent and large uplift of slope waters onto the shelf, and they may also travel offshore exporting shelf waters to the deep ocean.