ABSTRACT

The Agulhas Current is the western boundary current of the South Indian Ocean subtropical gyre and, as such, it dominates the heat budget of the Indian Ocean basin. Variability in sea surface temperature and heat content is central to many aspects of climate over the Indian Ocean basin, such as East African rainfall, tropical cyclone activity, and sea level rise. More than 50% of decadal Sea Surface Temperature variability over the Indian Ocean is explained by uniform, basinwide warming or cooling and it is hypothesized that this could be linked to changes in the meridional overturning circulation. Understanding the links between Indian Ocean overturning, meridional heat transport and sea surface temperature is currently hampered by lack of observations. Estimates of the Indian Ocean meridional overturning circulation have been made less than a handful of times over the past 50 years. Yet, one can expect its short-term variability to exceed the span of these few estimates, just as it does in the Atlantic. The poleward heat transport that exits the Indian Ocean via the Agulhas Current feeds both northward into the Atlantic, where it is believed to influence meridional overturning and hence climate in that basin, and southward into the Southern Ocean. Measuring Agulhas variability and investigating its relationship to wind forcing and Agulhas leakage proxies will lead to a better understanding of how these phenomena are linked in the real ocean. An integral component of this project is to provide skills-transfer to South African scientists and technicians so that they can lead and sustain an Agulhas array in the longer term. Resource sharing, capacity building, an open data policy, and collaborative analyses will promote greater productivity and better quality research from the western Indian Ocean oceanographic community in general. The principal investigator will be a visiting Professor at the University of Cape Town in 2015 and 2018 and teach a 2-week Honors module on large-scale dynamical oceanography, as part of this project. A U.S. graduate student, a postdoctoral researcher and an early career scientist will be trained.

The Agulhas System Climate Array (ASCA) project is a collaboration between South African, Dutch, and US scientists to implement sustained observations of the Agulhas Current, as part of the Global Ocean Observing System. This five-year phase for ASCA will establish measurements of velocity, temperature, and salinity variability in the Current, including its influence on coastal waters. These measurements will be synthesized with satellite and Argo data to achieve the following major scientific objectives:

(1) Characterize the relationship between volume and temperature transport in the Agulhas Current.
(2) Quantify the seasonal variability in basin-wide overturning and heat transports of the Indian Ocean, by synthesizing ASCA measurements with Argo and satellite data over the ocean interior.
(3) Investigate the annual and interannual variability of the Agulhas Current in the context of wind forcing and changes in retroflection and leakage.

Three ASCA research cruises (2016, 2018 and 2019) will take place aboard South African research vessels, during which the international team will deploy and maintain a linear array of seven full-depth current meter moorings with additional temperature and salinity sensors interspersed with four bottom-mounted Current and Pressure-sensing Inverted Echo Sounders. Finally, three coastal moorings will be added to the array at the shelf break and inshore, all in less than 200 m of water. These are led by the South African partners, whose motivation is to study the regional impact of Agulhas Current variability on coastal currents, upwelling, and the annual sardine run. During deployment and recovery of the mooring, hydrographic sections across the Agulhas Current near 34◦S will be collected. The sampling plan is based on moored velocity observations that have been successfully collected before in this location under the Agulhas Current Time-series experiment funded by NSF. This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.

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