QUICCHE: QUantifying Interocean fluxes in the Cape Cauldron Hotspot of Eddy kinetic energy

PIs: L. Beal, K. Donohue, Y. Lenn, S. Swart, C. Roman

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.  Recent 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. (2) Make novel estimates of Agulhas leakage with new observations, using theoretical eddy diffusivity and eddy flux frameworks. (3) Relate diffusivity and fluxes to new (SWOT) and existing satellite altimeter observations to infer variability in Agulhas leakage.

Funding: National Science Foundation (NSF), Swart funded by Wallenberg Academy Fellowship

PI: S. Swart

The seas surrounding Antarctica are where vast amounts of heat and carbon exchange between the atmosphere and the deep ocean. The physical processes in the Southern Ocean that underpin these exchanges ultimately determine the rate of climate change and therefore mitigation measures. Despite outstanding progress in observational techniques, there are extremely few observations, which has led to arguably the largest knowledge ‘blind spot’ in global ocean-climate research and predictability.

Specifically, new evidence suggests we urgently require to understand highly energetic upper ocean flows and instabilities (called submesoscale eddies and fronts, which evolve at scales of 0.1-10 km and hours-days). Through enhanced vertical exchange of properties, these phenomena change upper ocean mixing and stratification, thereby amplifying heat and carbon exchange at the air-sea interface. The changes in stratification by submeoscale processes can directly alter the transport of these climate-acute properties to the ocean interior, where they are stored at centennial timescales. Critically, contemporary understanding of these processes occurring in the sea ice regions surrounding Antarctica are severely poor due to a dearth of field data. This, in turn, has led to global climate models experiencing the greatest biases of key processes in the Southern Ocean.

To undertake this scientific challenge, we will coordinate state-of-the-art field observations and fit-for-purpose modelling experiments, including deploying under-ice capable ocean robots from ice breaker expeditions. A new ocean topography satellite mission will provide unprecedented high-resolution ‘surface views’ of the submesoscaleprocesses. These cornerstone observations will be combined with models, of varying complexity, to provide new knowledge on how sensitive ocean-ice processes are to our changing climate and thereby improve climate prediction.

Funding: Prolongation of Wallenberg Academy Fellowship – Knut and Alice Wallenberg Foundation – 2021-2026

PI: M. du Plessis; Host: S. Swart; Collaborators: S. Speich, B. Ward

In SPICE, our aim is to enhance our knowledge of oceanic heat and carbon uptake in the Southern Ocean in order to reduce uncertainties in future global climate model projections. Using ocean observations from state-of-the-art autonomous platforms and high-resolution numerical simulations, we will 1) quantify the variability of heat and carbon air-sea fluxes in the Southern Ocean and 2) better understand how ocean submesoscale processes modulate heat and carbon exchange between the atmosphere and ocean interior. These findings will enhance our understanding of both contemporary and future climate and will provide a benchmark for future research to critically assess climate models – an exercise vital to improving forecasts and thus mitigating against the effects of our changing climate.