Dynamics of Agulhas Current and Interocean Exchange

The Agulhas Current is the swift western boundary flow of the South Indian Ocean that retroflects south of Africa, shedding mesoscale eddies (Agulhas Rings) and leaking a portion of its waters into the South Atlantic. This interocean exchange, known as Agulhas leakage, transports heat, salt and biogeochemical tracers, thereby influencing the Atlantic meridional overturning circulation (AMOC) and global climate. The dynamics of the retroflection and leakage are modulated by mesoscale and submesoscale instabilities, air–sea interactions, large-scale wind forcing and subtropical gyre circulation. Variability in the Agulhas system occurs on seasonal to multidecadal timescales, affecting ocean mixing, nutrient supply to oligotrophic regions and the redistribution of water masses between ocean basins. Elucidating these processes is critical for understanding past and future shifts in climate and marine productivity.

Research from Nature Portfolio

Recent studies have used large-ensemble ocean–climate simulations to demonstrate that under continued greenhouse-gas forcing the upper limb of the AMOC may weaken not through reduced import of Indian and Pacific waters but via enhanced recirculation in a distorted South Atlantic subtropical gyre, altering the pathway of Agulhas leakage towards local recycling. Seminal work constructing a multidecadal time series of Agulhas leakage has revealed a robust correlation with the Atlantic multidecadal oscillation, with leakage leading by approximately 15 years. This finding underscores the role of Southern Hemisphere westerly winds in modulating interocean exchange and provides a predictive link to North Atlantic climate variability.

Dynamics of Agulhas Current and Interocean Exchange publication trend

The graph below shows the total number of articles in dynamics of agulhas current and interocean exchange across all publications each year (not limited to Nature Index journals).

Technical terms

Agulhas leakage: The flux of Indian Ocean waters that bypasses the southern tip of Africa into the South Atlantic.

Retro­flection: The process by which a current turns sharply back on itself, as the Agulhas Current does near the Cape of Good Hope.

Eddy kinetic energy (EKE): The energy associated with turbulent mesoscale motions in the ocean.

Mesoscale eddy: A rotating water mass, typically 10–100 km across, that transports heat, salt and momentum.

Western boundary current: A strong, narrow current flowing along the western edge of an ocean basin.

Ekman pumping: Vertical movement of water induced by wind-driven divergence or convergence via the Ekman transport mechanism.

References

1. Projected Atlantic overturning slow-down is to be compensated by a strengthened South Atlantic subtropical gyre. Communications Earth & Environment (2023).
2. The role of eddy-wind interaction in the eddy kinetic energy budget of the Agulhas retroflection region. Environmental Research Letters (2023).
3. Instabilities Across the Agulhas Current Enhance Upward Nitrate Supply in the Southwest Subtropical Indian Ocean. AGU Advances (2023).
4. Inclination Trend of the Agulhas Return Current Path in Three Decades. Remote Sensing (2023).
5. Atlantic multi-decadal oscillation covaries with Agulhas leakage. Nature Communications (2015).
6. An observed 20-year time series of Agulhas leakage. Ocean Science (2014).