The Antarctic Circumpolar Current has highly energetic mesoscale phenomena, but their impacts on phytoplankton biomass, productivity, and biogeochemical cycling are not understood well. We analyze satellite observations and an eddy-rich ocean model to show that they drive chlorophyll anomalies of opposite sign in winter versus summer. In winter, deeper mixed layers in positive sea surface height (SSH) anomalies reduce light availability, leading to anomalously low chlorophyll concentrations. In summer with abundant light, however, positive SSH anomalies show elevated chlorophyll concentration due to higher iron level, and an iron budget analysis reveals that anomalously strong vertical mixing enhances iron supply to the mixed layer. Features with negative SSH anomalies exhibit the opposite tendencies: higher chlorophyll concentration in winter and lower in summer. Our results suggest that mesoscale modulation of iron supply, light availability, and vertical mixing plays an important role in causing systematic variations in primary productivity over the seasonal cycle.
Bibliographical noteFunding Information:
The altimeter products were produced and distributed by AVISO (http://www.aviso.altimetry.fr/), as part of the Ssalto ground processing segment. The CHL observations are available through NASA MEaSUREs Ocean Color Product Evaluation Project (ftp://ftp.oceancolor.ucsb.edu/). The observational data for the depth of the mixed layer are from Dong et al. (2008) and for iron concentration are from Tagliabue et al. (2014). Computational facilities have been provided by the Climate Simulation Laboratory, which is managed by CISL at NCAR. NCAR is supported by the National Science Foundation. The CESM source code is freely available at http://www2.cesm. ucar.edu. The analysis code for the Fe budget averaged over the mixed layer is available at https://github.com hajsong/tracerbudget. H. S., J. M., and D. J. M. were supported by the NSF MOBY project (OCE-1048926). D. J. M. also acknowledges support from NSF (OCE-1048897) and NASA (NNX13AE47G). In addition, P. G. acknowledges support from NSF (OCE-1558809) and NASA (NNX13AE47G and NNX16AH9G).
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All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)