EMBRACE-OCE-Seed: Variability and trends in Agulhas Current water mass properties

The Agulhas Current is western boundary current of the South Indian Ocean subtropical gyre. Due to its strong, warm, and salty flow, the Agulhas dominates the Indian Ocean heat and freshwater budgets. Variability in the Agulhas Current impacts regional weather over Southern Africa and can alter global climate through the transport of warm and salty waters to the Atlantic Ocean, which impacts the Atlantic meridional overturning circulation.

Under climate change, the Indian Ocean has warmed rapidly while the Agulhas Current has broadened and cooled, while maintaining a constant volume transport. This suggests that the full depth structure of the temperature, salinity, and velocity of the current need to be considered to understand ongoing changes. The goal of this project is to quantify the variability and changes in transports of different water masses within the Agulhas Current from six high-accuracy repeat hydrographic crossings that occurred over 1987-2023.

This analysis will also serve as a framework for understanding changes that may occur in other western boundary currents, such as the Gulf Stream and Kuroshio. The student-led Graduate Research Fellowship Program (GRFP) workshop at the University of Alaska Fairbanks will be supported, and a graduate student will be trained. The graduate student will participate in the GRFP workshop and submit a fellowship application under mentorship from the principal investigator.

Diagnosing changes to the heat and freshwater fluxes of the ocean is essential for understanding how ocean circulation changes are altering climate. It is often assumed that the changing ocean circulation can be monitored through volume transports. However, the integrated volume transport of the Agulhas Current has not changed over recent decades, but the horizontal and vertical structure of the current velocity has changed.

Therefore, a thorough investigation of changing heat and freshwater fluxes must consider the changing transports of different water masses -- not only the integrated transport. This project will directly estimate trends in Agulhas Current water mass transports from existing publicly available, high quality repeat hydrographic crossings. The Agulhas Current impacts the Atlantic meridional overturning circulation, and thus global climate, primarily through its freshwater flux.

Understanding water mass transport trends will elucidate whether a trend in freshwater flux may be present without a trend in volume transport. Physical drivers of water mass transport trends will be determined using a heave/spice decomposition. These analyses will provide a crucial point of comparison for models and reanalyses.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.