Abstract
Abstract. Southern Ocean organic carbon export plays an important role in the global carbon cycle, yet its basin-scale climatology and variability are uncertain due to limited coverage of in situ observations. In this study, a neural network approach based on the self-organizing map (SOM) is adopted to construct weekly gridded (1° × 1°) maps of organic carbon export for the Southern Ocean from 1998 to 2009. The SOM is trained with in situ measurements of O2 / Ar-derived net community production (NCP) that are tightly linked to the carbon export in the mixed layer on timescales of one to two weeks and with six potential NCP predictors: photosynthetically available radiation (PAR), particulate organic carbon (POC), chlorophyll (Chl), sea surface temperature (SST), sea surface height (SSH), and mixed layer depth (MLD). This nonparametric approach is based entirely on the observed statistical relationships between NCP and the predictors and, therefore, is strongly constrained by observations. A thorough cross-validation yields three retained NCP predictors, Chl, PAR, and MLD. Our constructed NCP is further validated by good agreement with previously published, independent in situ derived NCP of weekly or longer temporal resolution through real-time and climatological comparisons at various sampling sites. The resulting November–March NCP climatology reveals a pronounced zonal band of high NCP roughly following the Subtropical Front in the Atlantic, Indian, and western Pacific sectors, and turns southeastward shortly after the dateline. Other regions of elevated NCP include the upwelling zones off Chile and Namibia, the Patagonian Shelf, the Antarctic coast, and areas surrounding the Islands of Kerguelen, South Georgia, and Crozet. This basin-scale NCP climatology closely resembles that of the satellite POC field and observed air–sea CO2 flux. The long-term mean area-integrated NCP south of 50° S from our dataset, 17.9 mmol C m−2 d−1, falls within the range of 8.3 to 24 mmol C m−2 d−1 from other model estimates. A broad agreement is found in the basin-wide NCP climatology among various models but with significant spatial variations, particularly in the Patagonian Shelf. Our approach provides a comprehensive view of the Southern Ocean NCP climatology and a potential opportunity to further investigate interannual and intraseasonal variability.
Highlights
The Southern Ocean plays an important role in the global carbon cycle
In the absence of the basinscale net community production (NCP) observation, we compare the spatial pattern of the constructed NCP with the satellite-measured particulate organic carbon (POC) and Chl because the former accounts for 80–90 % of NCP in the Southern Ocean (Hansell and Carson 1998; Allison et al, 2010) and the latter is often used to derive phytoplankton biomass and net primary production (NPP)
We note that exact agreement is not expected, given that the in situ derived NCP used for comparison were obtained by various methods that access different temporal and spatial scales of carbon export and that sometimes include different processes
Summary
The Southern Ocean plays an important role in the global carbon cycle. The current annual global ocean uptake of atmospheric carbon dioxide (CO2) is about 2 petagrams (Pg) of carbon, half of which is taken up by the vast Southern Ocean south of 30◦ S (Takahashi et al, 2012). Atmospheric CO2 absorbed by the ocean can be transferred from the surface to the deep ocean via various physical, chemical, and biological mechanisms associated with the solubility and biological pumps (Volk and Hoffert, 1985; Carlson et al, 2010). Chang et al.: Neural network-based estimates of Southern Ocean NCP
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