Collaborative Research: Water mass variability and connectivity in a strengthening Southern Hemisphere Super-Gyre

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The recently identified Southern Hemisphere Super-Gyre (SHSG) is a nested system of the southern subtropical gyres. It sweeps out of the Tasman Sea, transports waters westward, and feeds the upper limb of the Atlantic thermohaline circulation. The Sub-Antarctic Mode Water(SAMW) and Antarctic Intermediate Water (AAIW) are prominently involved in the SHSG inter-ocean connection. Through this connection, the SAMW/AAIW can be distributed among the three ocean basins. Recent analysis has revealed a two-decade-long SHSG spin-up, and an increase in the inter-ocean Tasman and Agulhas Leakages. The project will study the time-varying nature of SAMW/AAIW and their inter-ocean link to a strengthening SHSG. The SAMW/AAIW are large volume water masses that take up substantial quantities of atmospheric gases, which are important for life and climate - such as oxygen and CO2. They are two significant water masses that are involved in the inter-ocean connection of the SHSG. Strengthening of SHSG has been correlated to atmospheric modes, e.g., Southern Annular Mode (SAM). However, recovery of the ozone layer is predicted to weaken the SAM. Thus, whether the SHSG and the Tasman and Agulhas Leakages continue strengthening or not is an open question. Also unknown is how variability in these circulations link to variability in SAMW/AAIW and their inter-ocean connectivity. Data and models will be used to assess connectivity of the southern gyres and their correlations with atmospheric modes (SAM, El Nino Southern Oscillation (ENSO), and Indian Ocean Dipole (IOD)). In addition, estimates from the observations will be useful as tests for model outputs. The research will promote education by the training of a postdoctoral fellow at UCLA in the analysis of observational data and model outputs. It will enhance infrastructure for research and education by establishing collaborations between different disciplines (physical and chemical oceanography) and different institutions (UCLA and University of Miami).This work would be the first systematic study of the SAMW/AAIW and their variability in a presently strengthening SHSG. Variability in the SAM, together with over a decade of Argo data and recent model advances make study of the SAMW/AAIW and their link to the SHSG inter-ocean connection possible and timely. The overall objectives are: (1) To estimate the annually averaged SAMW/AAIW properties and subduction rates in the three southern subtropical oceans, (2) To evaluate how much of the SAMW/AAIW is transported from one southern subtropical gyre into another, and how their pathways and volume transports vary with time and correlate with the SHSG spin-up, and (3)To explore the causes of water mass variability in the southern subtropical oceans with particular attention to variability associated with buoyancy fluxes, the SHSG spin-up and atmospheric modes. To achieve these objectives, available satellite and Argo data will be analyzed, and the results will be used to inform output from ocean general circulation models and simulated passive/adjoint tracers.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.