Award: OCE-1829796
Award Title: Decadal trends in oceanic anthropogenic CO2 from the CLIVAR and GO-SHIP d13C datasets and in an ocean biogeochemistry model
Outcomes Report
Abstract The availability of ocean δ13C and DIC data from WOCE, CLIVAR and GO-SHIP provide the opportunity to quantify, directly from observations, the evolution of the anthropogenic DIC and DIC13 signal on regional and global scales in the ocean. At the same time, implementation of the anthropogenic DIC and DIC13 perturbations into an ocean model, driven by interannually varying winds, yields simulations of the DIC and DIC13 evolution that can be compared with observations to help identify likely processes that cause interdecadal shifts in the rate of this evolution. The combination of observational and model analysis tracking the anthropogenic DIC13 and DIC perturbations will improve our insight into the ocean?s important role in modulating specifically by establishing basin-wide budgets of ocean uptake and storage of anthropogenic CO2. Outcomes This grant funded a postdoctoral researcher Mariona Claret's training in modeling and interpretation of 13C in the ocean. An efficient gas-exchange and Iron, Light, and Nutrient controlled oceanic C-cycling scheme was adapted to include 13C and incorporated into the ocean component of an Earth System Model, ESM2M. The model (BLING-13C) was driven to steady state followed by the anthropogenic 13C and CO2 perturbations. The air-sea CO2 and 13C flux fields from this model were used in an atmosphere-terrestrial biosphere inversion to quantify spatial patterns and the seasonal cycle in terrestrial carbon uptake [Joyce et al., 2021]. Dr. Claret found that BLING-13C?s mean 13C is sensitive to model resolution and is critically sensitive to the rate and schematics of North Atlantic Deep and Antarctic Bottom Water formation. Dr. Claret found that meridional and inter-basin trends in 13C in the ocean can be used to constrain the wind-speed dependence of air-sea exchange, as has been attempted before with 14C. A companion manuscript to Claret et al. (2021) (manuscript in preparation) demonstrates improved model fidelity to oceanic 13C changes in response to the anthropogenic perturbation in comparison with prior MOM-based 13C models. We used BLING_13C to test basin-scale anthropogenic 13C-budgets from the Pacific and North Atlantic subsampling the model output in the same (few) locations that are available in the data and found the approach to be accurate despite limited observations. Dr. Claret has constructed basin-wide budgets of the air-sea d13C uptake and accumulation of the d13C perturbation for the South Atlantic and Indian ocean basins based on CLIVAR and GOSHIP 13C comparisons with the WOCE 13C dataset. These efforts are detailed in the manuscript in preparation for Global Biogeochemical Cycles by Claret et al. GO-SHIP re-occupations of CLIVAR and WOCE (including TTO and GEOSECS, where appropriate) hydrographic sections that include the d13C tracer in the North Atlantic, South Atlantic, South Indian, and South Pacific Oceans indicate that ocean uptake of anthropogenic CO2 has been slowing in these regions. In addition, while the air sea disequilibrium increased (became more negative) during the 1990s to 2005, the air-sea disequilibrium stopped growing during the CLIVAR (2005) to GO-SHIP era (~ 2015). Both of these observations imply either a slowdown in upper ocean overturning or in air-sea gas exchange. The model was used to evaluate the extent to which ocean d13C observations may be aliased by variability in the ocean. A model variant driven by the Common Ocean Reference Experiments (CORE-2 IAF) interannual wind and buoyancy forcing simulation reveals that the d13C changes in the model, and likely the ocean, are very sensitive to the subtle changes in circulation. The model?s inter-decadal trends in sea surface d13C indicate, for example, that the decade long d13C time series collected in the Drake Passage could be strongly aliased by inter-decadal trends in this location. The Drake Passage and Hawaii time series measurements, partially supported by this grant, and the decadal trends in the oceanic 13C Suess effect from CLIVAR and GOSHIP, are to be included in a synthesis manuscript by the original PIs on this grant, Rolf Sonnerup and Paul Quay. Papers Carter, B. R., et al. (2019), Pacific anthropogenic carbon between 1991 and 2017, Glob. Biogeochem. Cycles, 33 (5), 597-617, doi: 10.1029/2018GB006154. Claret M. et al. (2021) A next generation ocean carbon isotope model for climate studies 1: Steady state controls on ocean 13C, Global Biogeochemical Cycles, 35, e2020GB006757, doi: 10.1029/2020GB006757. Joyce, P., et al. (2021) How Robust is the Apparent Break-Down of Northern High-Latitude Temperature Control on Spring Carbon Uptake? Geophysical Research Letters, 48, e2020GL091601, doi: 10.1029/2020GL091601. Claret M. et al. (in prep) A next generation ocean carbon isotope model for climate studies 2: Response to the anthropogenic perturbation, Global Biogeochemical Cycles Quay, P. D., et al. (in prep) decadal trends in oceanic uptake of the 13C perturbation from time-series and CLIVAR and GOSHIP measurements, for Global Biogeochemical Cycles. Presentations (all pre-covid) 2018 Claret, OCB 2018 Claret, Spring Symposium of UW Program of Climate Change 2018 Claret, M., Ocean Sci. Meet. Last Modified: 03/20/2023 Submitted by: Brendan R Carter