Over the past two decades, several large observational climate programs in the Indian Ocean have collected a comprehensive chemical and physical oceanographic data set. In this project, all available Indian Ocean data will combined to investigate carbon cycling and decadal variability in the Indian Ocean. The timing of the project is such that the proposed work will supplement an ongoing NSF-funded project designed to determine the three-dimensional Indian Ocean circulation from hydrographic observations.
This project has two main goals: To perform a comprehensive, basin-wide study to determine the strength of the biological carbon pump in the Indian Ocean, and to investigate changes in ventilation time scales at the southern entry point to the Indian Ocean along 32°S. The proposed research effort consists of an extensive analysis of ventilation ages derived from analysis of transit time distributions (TTDs) and the corresponding mean ages. This age information will be used to estimate subsurface rates of oxygen utilization, denitrification, and calcium carbonate dissolution, and the amount of organic and inorganic carbon export from the surface ocean needed to support these rates. In addition, at 32°S, a time series of ventilation ages will be constructed based on four occupations of this section. This will provide time scales for the ventilation variability that can be seen in other properties and that may be related to the Southern Annual Mode (SAM). Correlations with meridional transport variations inferred from the three-dimensional Indian Ocean circulation estimates will also be examined.
The data analysis will be complemented by offline numerical simulations with an isopycnal ocean circulation model that includes tracers and basic biogeochemistry. In addition to sensitivity studies regarding the data methods used, a tracer adjoint for the offline code will be developed. With the adjoint, it will be possible to determine whether the source region of waters in the thermocline at 32°S changes during different states of the SAM and how changes in remote forcing may contribute to tracer age variability observed at 32°S.
Intellectual Merit: Inferring surface ocean carbon export rates from subsurface tracer data is an entirely data-based, "bottom-up approach" that provides estimates on a large scale. A novel tracer, sulfur hexafluoride, will be used in a dual tracer approach with chlorofluorocarbons to constrain transit time distributions and mean ventilation ages. The tracer adjoint model provides a tool to run tracers "backwards" to trace anomalies back to their surface origin.
Broader Impacts: The proposed work will contribute to the analyses of data from the CLIVAR/Carbon Repeat Hydrography Program and the study of decadal climate variability. The work is interdisciplinary, involving ocean physics and biogeochemistry, and will contribute to the study of the global carbon cycle. Studies of the Indian Ocean are timely because of the development of the Indian Ocean Global Ocean Observing System. All three investigators are involved in outreach activities at all educational levels from high school through mentoring women at the early states of their careers. A graduate student will be involved in the proposed work and receive training in ocean modeling and analyses of transient tracer data.
This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) Program.
