Assessment of Export Efficiency Equations in the Southern Ocean Applied to Satellite‐Based Net Primary Production

Abstract

Carbon export efficiency (e‐ratio) is defined as the fraction of organic carbon fixed through net primary production (NPP) that is exported out of the surface productive layer of the ocean. Recent observations for the Southern Ocean suggest a negative e‐ratio versus NPP relationship, and a reduced dependency of export efficiency on temperature, different than in the global domain. In this study, we complement information from a passive satellite sensor with novel space‐based lidar observations of ocean particulate backscattering to infer NPP over the entire annual cycle, and estimate Southern Ocean export rates from five different empirical models of export efficiency. Inferred Southern Ocean NPP falls within the range of previous studies, with a mean estimate of 15.8 (± 3.9) Pg C yr−1 for the region south of 30 °S during the 2005–2016 period. We find that an export efficiency model that accounts for silica(Si)‐ballasting, which is constrained by observations with a negative e‐ratio versus NPP relationship, shows the best agreement with in situ‐based estimates of annual net community production (annual export of 2.7 ± 0.6 Pg C yr−1 south of 30 °S). By contrast, models based on the analysis of global observations with a positive e‐ratio versus NPP relationship predict annually integrated export rates that are ∼ 33% higher than the Si‐dependent model. Our results suggest that accounting for Si‐induced ballasting is important for the estimation of carbon export in the Southern Ocean.

Nutrient Controls on Export Production in the Southern Ocean

“…A similar mechanism of enhanced silica burial and carbon export induced by iron limiting conditions has been proposed for oceanic upwelling margins (Brzezinski et al, ; Pichevin et al, ). The meridional pattern of ANCP agrees well with a recent satellite‐based estimate of export production for the Southern Ocean (Arteaga et al, ; Figure a). This estimate is based on an export efficiency model constrained by a positive relationship between export efficiency and phytoplankton Si utilization in the Southern Ocean (Britten et al, ).…”

“…(a; Y axis left) Meridional pattern of ANCP estimates presented in Figure , calculated as the mean (± standard deviation) of 5° latitudinal bins (blue line with circles and shaded area), and satellite‐based carbon export estimates from Arteaga et al, (blue dashed line). ( Y axis right) Silicate to nitrate (Si:NO 3 ) ratio obtained from float nitrate measurements in the mixed layer and surface WOA13 silicate climatology subsampled at the same location and month as the float profiles (black continuous line).…”

Global Variability of Optical Backscattering by Non‐algal particles From a Biogeochemical‐Argo Data Set

“…Figure b shows the for each area, the ratios between the spatially resolved found at the surface and euphotic layers with the estimation for the bottom layer. Globally, is higher in the upper layer than the at the bottom from middle to low latitudes, while at the bottom is higher than at the surface in most productive seas such as the NASPG, SAZ, PFZ, and ASZ_SIZ areas (Alkire et al, ; Arteaga et al, ; Uitz et al, ). In these areas, is only a small fraction of the total b bp in surface waters (<20%; Figure c) as a consequence of the higher relative variability in the b bp and phytoplankton abundance (Alkire et al, ).…”

Southern Ocean Biogeochemical Float Deployment Strategy, With Example From the Greenwich Meridian Line (GO‐SHIP A12)

“…They remain essential for understanding the properties, observed surface fluxes, and trajectories of the floats over the several years since deployment (section ). Our own growing biogeochemical float data set has increasingly become part of this evaluation but of course was unavailable for PS89 (e.g., Arteaga et al, ; Briggs et al, ; Gray et al, ; Johnson, Plant, Dunne et al, ; Williams et al, ).…”

“…In formulating SOCCOM, it was appreciated that measuring and understanding regional Southern Ocean processes is central to model improvement and that the overwhelming lack of in situ observations, especially those that resolve seasonal cycles within the extensive ice‐covered regions, has impeded progress (Frölicher et al, ; Russell et al, ). Geographically diverse processes within SOCCOM's very large region include, among others, air‐sea carbon fluxes and budgets (Gray et al, ; Takahashi et al, ), biological processes (Ardyna et al, ; Arteaga et al, ; Carranza & Gille, ; Johnson, Plant, Dunne et al, ), water mass formation (Abernathey et al, ; Naveira‐Garabato et al, 2009; Ohshima et al, ; Sallée et al, ), mixing in the Antarctic Circumpolar Current (Ledwell et al, ; Naveira Garabato et al, , ), the overturning circulation (Marshall & Speer, ; Speer et al, ; Talley, ), and sea ice processes including its seasonal cycle and the associated freshwater cycle (Abernathey et al, ; Haumann et al, ). Dynamics of the Southern Ocean have commonly been oversimplified, although the tremendous diversity and nonzonality in Southern Ocean dynamical regimes are beginning to be appreciated (e.g., Masich et al, ; Sallée et al, ; Tamsitt et al, ; Thompson & Naveira Garabato, ).…”