An official website of the United States government
Abstract Carbon export driven by submesoscale, eddy‐associated vertical velocities (“eddy subduction”), and particularly its seasonality, remains understudied, leaving a gap in our understanding of ocean carbon sequestration. Here, we assess mechanisms controlling eddy subduction's spatial and seasonal patterns using 15 years of observations from BGC‐Argo floats in the Southern Ocean. We identify signatures of eddy subduction as subsurface anomalies in temperature‐salinity and oxygen. The anomalies are spatially concentrated near weakly stratified areas and regions with strong lateral buoyancy gradients diagnosed from satellite altimetry, particularly in the Antarctic Circumpolar Current's standing meanders. We use bio‐optical ratios, specifically the chlorophyllato particulate backscatter ratio (Chl/bbp) to find that eddy subduction is most active in the spring and early summer, with freshly exported material associated with seasonally weak vertical stratification and increasing surface biomass. Climate change is increasing ocean stratification globally, which may weaken eddy subduction's carbon export potential.
more »
« less
Southern Ocean Phytoplankton Blooms Observed by Biogeochemical Floats
Abstract The spring bloom in the Southern Ocean is the rapid‐growth phase of the seasonal cycle in phytoplankton. Many previous studies have characterized the spring bloom using chlorophyll estimates from satellite ocean color observations. Assumptions regarding the chlorophyll‐to‐carbon ratio within phytoplankton and vertical structure of biogeochemical variables lead to uncertainty in satellite‐based estimates of phytoplankton carbon biomass. Here, we revisit the characterizations of the bloom using optical backscatter from biogeochemical floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling and Southern Ocean and Climate Field Studies with Innovative Tools projects. In particular, by providing a three‐dimensional view of the seasonal cycle, we are able to identify basin‐wide bloom characteristics corresponding to physical features; biomass is low in Ekman downwelling regions north of the Antarctic Circumpolar Current region and high within and south of the Antarctic Circumpolar Current.
more »
« less
- Award ID(s):
- 1658001
- PAR ID:
- 10375351
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 124
- Issue:
- 11
- ISSN:
- 2169-9275
- Page Range / eLocation ID:
- p. 7328-7343
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)Abstract Over the last ten years, satellite and geographically constrained in situ observations largely focused on the northern hemisphere have suggested that annual phytoplankton biomass cycles cannot be fully understood from environmental properties controlling phytoplankton division rates (e.g., nutrients and light), as they omit the role of ecological and environmental loss processes (e.g., grazing, viruses, sinking). Here, we use multi-year observations from a very large array of robotic drifting floats in the Southern Ocean to determine key factors governing phytoplankton biomass dynamics over the annual cycle. Our analysis reveals seasonal phytoplankton accumulation (‘blooming’) events occurring during periods of declining modeled division rates, an observation that highlights the importance of loss processes in dictating the evolution of the seasonal cycle in biomass. In the open Southern Ocean, the spring bloom magnitude is found to be greatest in areas with high dissolved iron concentrations, consistent with iron being a well-established primary limiting nutrient in this region. Under ice observations show that biomass starts increasing in early winter, well before sea ice begins to retreat. The average theoretical sensitivity of the Southern Ocean to potential changes in seasonal nutrient and light availability suggests that a 10% change in phytoplankton division rate may be associated with a 50% reduction in mean bloom magnitude and annual primary productivity, assuming simple changes in the seasonal magnitude of phytoplankton division rates. Overall, our results highlight the importance of quantifying and accounting for both division and loss processes when modeling future changes in phytoplankton biomass cycles.more » « less
-
Abstract Previous studies suggest that meridional migrations of the Antarctic Circumpolar Current may have altered wind-driven upwelling and carbon dioxide degassing in the Southern Ocean during past climate transitions. Here, we report a quantitative and continuous record of the Antarctic Circumpolar Current latitude over the last glacial-interglacial cycle, using biomarker-based reconstructions of surface layer temperature gradient in the southern Indian Ocean. The results show that the Antarctic Circumpolar Current was more equatorward during the ice ages and shifted ~6° poleward at the end of glacial terminations, consistent with Antarctic Circumpolar Current migration playing a role in glacial-interglacial atmospheric carbon dioxide change. Comparing the temporal evolution of the Antarctic Circumpolar Current mean latitude with other observations provides evidence that Earth’s axial tilt affects the strength and latitude range of Southern Ocean wind-driven upwelling, which may explain previously noted deviations in atmospheric carbon dioxide concentration from a simple correlation with Antarctic climate.more » « less
-
Abstract This study develops a new regional model of the Southern Ocean including an improved representation of the iron biogeochemistry and ecosystem component, nesting within a biogeochemical ocean state estimate, and benchmarking with a suite of observations. The regional domain focuses on the Udintsev Fracture Zone (UFZ) in the central Pacific sector of the Southern Ocean. The UFZ is characterized by the deep gap between the Pacific‐Antarctic Ridge and the East Pacific Rise, which is one of the key “choke points” of the Antarctic Circumpolar Current where major Southern Ocean fronts are constrained within close proximity to this topographic feature. It is also a region of elevated mesoscale eddy activity, especially downstream of the UFZ. The model reproduces observed partial pressure of carbon dioxide in the surface water (pCO2) remarkably well from seasonal to interannual timescales relative to prior studies (r = 0.89). The seasonality of pCO2is difficult to simulate correctly because it is a small residual between the opposing influences of temperature and carbon. This model represents an intermittent double peak pattern of pCO2; one driven by the summertime high temperature and another from the wintertime high of dissolved inorganic carbon. The model also captures the spatial and temporal structure of the regional net primary production with respect to the satellite ocean color products (r = 0.57). The model is further validated by comparing it with biogeochemical float observations from the Southern Ocean Carbon and Climate Observations and Modeling project, revealing the model performance and challenges to accurately represent physical and biogeochemical properties in frontal regions.more » « less
-
Abstract Despite its importance for the global cycling of carbon, there are still large gaps in our understanding of the processes driving annual and seasonal carbon fluxes in the high‐latitude Southern Ocean. This is due in part to a historical paucity of observations in this remote, turbulent, and seasonally ice‐covered region. Here, we use autonomous biogeochemical float data spanning 6 full seasonal cycles and with circumpolar coverage of the Southern Ocean, complemented by atmospheric reanalysis, to construct a monthly climatology of the mixed layer budget of dissolved inorganic carbon (DIC). We investigate the processes that determine the annual mean and seasonal cycle of DIC fluxes in two different zones of the Southern Ocean—the Sea Ice Zone (SIZ) and Antarctic Southern Zone (ASZ). We find that, annually, mixing with carbon‐rich waters at the base of the mixed layer supplies DIC which is, in the ASZ, either used for net biological production or outgassed to the atmosphere. In contrast, in the SIZ, where carbon outgassing and the biological pump are weaker, the surplus of DIC is instead advected northward to the ASZ. In other words, carbon outgassing in the southern Antarctic Circumpolar Current (ACC), which has been attributed to remineralized carbon from deep water upwelled in the ACC, is also due to the wind‐driven transport of DIC from the SIZ. These results stem from the first observation‐based carbon budget of the circumpolar Southern Ocean and thus provide a useful benchmark to evaluate climate models, which have significant biases in this region.more » « less
