Subantarctic Mode Water (SAMW) originates from the high latitude Southern Hemisphere. Measurements of radiocarbon and other chemical tracers clearly demonstrate that SAMW is present in the deeper water of the Pacific Equatorial Undercurrent (EUC) [Toggweiler et al., 1991]. More recent WOCE and CLIVAR measurements also show the low radiocarbon signal in the tropical Pacific. These data indicate that the directly ventilated Pacific thermocline is not insulated from deeper levels. Instead, the data show that the thermocline has properties derived from significant diapycnal exchange with deeper waters. Currently no consensus exists on the mechanism responsible for these observations. This study will investigate the mechanisms governing the supply of mode water nutrients (primarily NO3 and SiO3) to the equatorial thermocline. Its first hypothesis concerns the fate of mode water in the Pacific. For the lower thermocline, it is argued that an upward diapycnal transport is responsible for the supply of mass and nutrients from below. This primarily occurs over the South Pacific and is a consequence of the tremendous expanse of the subtropical region. This diapycnal nutrient transport mechanism is distinct from explicit vertical diffusion of nutrients, but it is sustained by physical diffusive processes and thus related. For the upper thermocline, it is expected that a distinct combination of mechanisms is responsible, namely: the impact of solar radiation penetrating below the base of the mixed layer, direct air-sea heat exchange between the atmosphere and the sea surface, and local mixing in the upper part of the EUC. The second hypothesis is that summer mixed layer depths largely control the formation and export of mode water nutrients from the Southern Ocean. A preliminary sensitivity study indicates that increased Southern Ocean MLD leads to increased pre-formed mode water nutrients. Deeper summer mixed layer depths inhibit NO3 and SIO3 consumption leaving higher winter nutrient concentrations at times of mode water subduction. The goal here is not only to identify the dynamical controls on mode water nutrients, but also to investigate the fate of pre-formed nutrients in the low-latitude thermocline and how this is influenced by Southern Ocean physics. These hypotheses will be tested in a water mass framework using Eulerian and Lagrangian diagnostics. This new theoretical approach allows one to identify the interplay of processes including transport, mixing, and biological cycling and analyze their impact on the nutrient supply. A global non- eddying ocean model will be used. It has significantly higher vertical resolution than is typical of ocean climate models, but still allows convergence to steady state for sensitivity studies. This does not imply that mesoscale and sub-mesoscale processes aren?t important. Rather, these efforts to understand vertical processes are complementary. The data analysis component of the proposed work will use oceanic ∆14C measurements and "conservative" nutrient proxy tracers to constrain upwelling pathways and transformation rates in the real ocean. Additional analysis with a full suite of biogeochemical tracers will be carried out to better constrain the large-scale vertical fluxes operating in the Pacific. Broader Implications: Although the proposed work is largely focused on the physical mechanisms controlling nutrient supply to the EUC, the work will have implications for scientific understanding of both Pacific and Southern Ocean ecosystems and biogeochemistry. The work will also benefit the training of a postdoctoral researcher, and thereby in the preparation of the next generation of earth scientists to engage in interdisciplinary research. If possible, the PIs will also involve undergraduate student(s) in this work as summer interns.
