QUantifying Interocean fluxes in the Cape Cauldron Hotspot of Eddy kinetic energy
The Cape Basin in the southeast Atlantic is a global hotspot of eddy kinetic energy, fed by a leakage of waters from the subtropical Indian Ocean via the Agulhas Current. A proportion of warm and salty Agulhas waters are vigorously stirred and mixed into the cooler and fresher Atlantic by co-interacting rings and eddies. The basin has been dubbed the Cape Cauldron. Until now the focus of physical studies in the Cape Cauldron has largely been on discrete, deep-reaching anticyclonic Agulhas rings, which can often be tracked for many months, sometimes years, using satellite altimetry. These rings and the warm and salty water masses they entrap have been studied extensively to estimate the magnitude of inter-ocean exchange, called Agulhas leakage, between the Indian and Atlantic Oceans. Yet, studies suggest that most Agulhas leakage is found outside these rings. We hypothesize that a large proportion of the Indian Ocean waters that leak into the Atlantic are to be found in submesoscale features generated by the mesoscale strain field. Observations of these features are lacking, as are estimates of the fluxes they affect. To fill this gap we propose to:
(1) Observe and characterize submesoscale features generated by the mesoscale eddy strain field within the Cape Cauldron for the first time.
(2) Make novel estimates of Agulhas leakage fluxes with these new observations, using theoretical eddy diffusivity and eddy flux frameworks.
(3) Relate diffusivity and fluxes to new and existing satellite altimeter observations to infer variability in Agulhas leakage.
The QUICCHE (QUantifying Interocean fluxes in the Cape Cauldron Hotspot of Eddy kinetic energy) campaign will collect new observations within the Cape Basin, targeting several dynamical regimes, like filamentation and eddy interaction, to measure the related submesoscale features and estimate diffusivities. From these measurements we will quantify eddy heat and salt fluxes, similar to the way poleward heat flux has been estimated in the Southern Ocean. We will use both Eulerian and Lagrangian instrumentation, including moorings, an undulating CTD system, gliders, drifters, profiling floats, uncrewed surface vehicles and microstructure turbulence profilers, capturing time and space scales from hours to seasons and from 1 to 100 km.
The QUICCHE observations dovetail with the daily SWOT crossover in the Cape Basin that will provide information about surface ocean dynamics at unprecedented time and space scales. Moored measurements will be collected for a full year at the SWOT cross-over, augmented by a glider to cover the upper water column and Sailbuoys/Wave Glider to measure air-sea fluxes. Our observations will also help evaluate SWOT, by providing in situ measurements of sea surface height and stratification. We will infer broader time-scales of variability by parameterizing our measurements in terms of ongoing satellite observations.
Ocean models and satellite data suggest that Agulhas leakage is increasing with climate change and that the associated trend in salt flux into the Atlantic may help stabilize the Atlantic meridional overturning circulation. Agulhas leakage also impacts the adjacent Benguela Upwelling system, with increasing EKE expected to flush more nutrients and organisms offshore, causing a reduction in productivity. So far, the turbulent nature of leakage has confounded in situ observational evidence for these trends and influences. Here we propose an entirely novel approach to this important problem, using an eddy flux framework. Along the way we will be the first to characterize stirring and mixing in the unique eddy-dominated flow regime of the Cape Basin.
Principal investigators: Lisa M, Beal (RSMAS, USA), Kathleen Donohue (Univ. of Rhode Island, USA), Chris Roman (Univ. of Rhode Island, USA), Yueng Lenn (Bangor University, Wales), and Sebastiaan Swart (Univ. Gothenburg, Sweden).
Contact point for the study site: Lisa M Beal (firstname.lastname@example.org)