Assessing the Trapping Dynamics of Madagascar Cyclonic Eddies Through Non-Standard Argo Float Experiments and Numerical Lagrangian Particle Tracking

This study investigates the 3-D Lagrangian evolution of Madagascar cyclonic eddies and their interaction with the Agulhas Current, combining targeted Argo float experiments, satellite altimetry data, and ocean modeling, following three in situ experiments. The region of interest spans from southwest Madagascar, where the South East Madagascar Current detaches from the continental shelf and generates dipoles, to the KwaZulu-Natal Bight, where the Agulhas Current flows southward.

The first two experiments, conducted in April and July 2013, deployed eight Argo floats configured to measure temperature and salinity at high temporal resolutions (daily and five-daily) and at varying park depths (300, 500, 650, and 1,000 m). These deployments assessed float retention within two cyclonic eddies that propagated southwestward over 130 days at an average speed of 11 km/day, undergoing growth, maturity, and decay phases before interacting with the Agulhas Current. A third experiment, conducted from May to September 2022, deployed two Euro-Argo ERIC-managed Core Argo floats southwest of Madagascar to further explore eddy dynamics. These floats drifted at non-standard depths of 650 m and 800 m, with adaptive cycle intervals (daily, 2-daily, and 5-daily) based on the eddy's proximity to the Agulhas Current. This experiment also captured the eastward propagation of the cyclonic eddy and its interaction with the current. In all three experiments, the floats exited the eddy when positioned below the depth at which the eddy's nonlinearity ratio dropped below 1. Complementary numerical simulations used an eddy identification and tracking algorithm with the GLORYS12V1 reanalysis product. Virtual particle releases and Lagrangian tracking at depths matching the above Argo float parking levels replicated the field experiments. Numerical results aligned with observations, showing that cyclonic eddies exhibited greater trapping depths during their mature phase and shallower depths during the growth and decay phases.

By integrating targeted float experiments, satellite data, and numerical simulations, this study provides a comprehensive understanding of eddy trapping dynamics southwest of Madagascar and their role in transporting heat, salt, and biogeochemical properties into the Agulhas Current. These findings demonstrate the potential of GLORYS12V1 combined with numerical Lagrangian particle tracking to address observational gaps in traditionally undersampled regions and underscore the benefits of combining ad hoc Argo configurations and numerical simulations for studying 3-D eddy dynamics.