On the cross-shelf exchange driven by frontal eddies along a western boundary current during austral winter 2007

Abstract

Western boundary currents are characterised by high mesoscale eddy activity including that of cyclonic frontal eddies (CFEs). These form frequently adjacent to the shelf-break, export coastal water and affect biological productivity. Here, we study the physical properties of eddies identified within the intensification zone of the East Australian Current (EAC) through the analysis of remotely sensed data, the application of an ocean circulation model and the evaluation of Lagrangian simulations during austral winter 2007. We examine the spatial and temporal evolution of identified eddies, investigate their formation mechanism, and quantify the contribution made to cross-shelf exchanges. The mesoscale eddies are tracked from generation to decay using an eddy detection and tracking method. In this case study, we identify two CFEs referred to as C1 and C2 with lifetimes of 11 and 38 days and radii of 45 and 70 km, respectively, and one anticyclonic eddy (ACE) with a lifetime of 62 days and a radius of 60 km. Both CFEs interact with the ACE leading to quasi-stationary dipole-eddies located in the vicinity of the shelf. This is the first time dipole-eddies are reported for the intensification zone of the EAC. The location of CFEs characterised by negative sea surface height anomaly (SSHA) and clockwise rotation coincides with remotely-sensed sea surface temperature (SST) and chlorophyll-a (Chl-a) anomalies. CFEs C1 and C2 appear to contribute in different ways to the cross-shelf transport along the shelf-break. CFE C1 drives a stronger onshore transport, while CFE C2 appears to enhance the offshore transport. The estimated export of shelf water associated with CFE C2 is estimated with about 1.23 Sv (~106 km3 day−1). It renews the regional shelf water in about 7 days. The lifetime of CFE C2 is about 38 days, thus the eddy had the potential to flush the shelf at least five times. The CFEs intensify the cross-shelf exchange that is due to the wind-driven cyclonic circulation of the Fraser Gyre. This seasonally occurring gyre dominates the local shelf during austral autumn and winter. The results from this study demonstrate the importance of CFEs in exporting cooler and Chl-a enriched shelf water into the warmer and oligotrophic waters of the southward-flowing EAC.