Autonomous Biogeochemical Floats Detect Significant Carbon Dioxide Outgassing in the High‐Latitude Southern Ocean

Abstract

Although the Southern Ocean is thought to account for a significant portion of the contemporary oceanic uptake of carbon dioxide (CO2), flux estimates in this region are based on sparse observations that are strongly biased toward summer. Here we present new estimates of Southern Ocean air‐sea CO2 fluxes calculated with measurements from biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling project during 2014–2017. Compared to ship‐based CO2 flux estimates, the float‐based fluxes find significantly stronger outgassing in the zone around Antarctica where carbon‐rich deep waters upwell to the surface ocean. Although interannual variability contributes, this difference principally stems from the lack of autumn and winter ship‐based observations in this high‐latitude region. These results suggest that our current understanding of the distribution of oceanic CO2 sources and sinks may need revision and underscore the need for sustained year‐round biogeochemical observations in the Southern Ocean.

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

“…However, the implications for this discrepancy are potentially immense. An initial analysis of 35 floats over 3 years in the Southern Ocean found an annual uptake of only 0.08 ± 0.55 Pg C year −1 , instead of the~1 Pg C year −1 uptake calculated from ship-based estimates, due primarily to increased wintertime outgassing around the Polar Front [100]. This discrepancy represents approximately 50% of the annual contemporary global oceanic CO 2 uptake [1], and would pose a major challenge to our understanding of the global carbon budget by effectively eliminating the role of the Southern Ocean as a carbon sink.…”

“…The array of biogeochemical floats in the Southern Ocean is providing novel insights into air-sea CO 2 fluxes and biogeochemical processes. For example, large discrepancies in winter surface pCO 2 between float observations and climatologies based on shipboard observations [25] are consistently observed [98][99][100][101]. This is not particularly surprising because wintertime shipboard data are sparse.…”

“…Small systematic biases in estimated pCO 2 can have a large impact when scaled up to global air-sea flux estimates. The average difference between shipboard underway pCO 2 and float-estimated pCO 2 is 3.7 μatm pCO 2 (n = [35][36][37][38][39] [99,100]. However, it is unclear whether or not this represents a real systematic bias in the float estimates, or an artifact due to small sample size and spatiotemporal variability (± 1 day and ± 25 km) between float and ship measurements.…”

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

“…Each color represents sampling areas and abbreviations. * indicates data acquired in four regions below 30°S, which have been delineated by using temperature profiles (Gray et al, ): Sub‐Tropical Zone (STZ) with a temperature at 100 m above 11 °C; the Sub‐Antarctic Zone (SAZ) with a temperature at 400 m below 5 °C; the Polar Frontal Zone (PFZ) with the minimum temperature between 0 and 200 m above 2 °C; and the Antarctic Southern Zone and Seasonal Ice Zone (ASZ_SIZ) minimum temperature between 0 and 200 m below 2 °C.…”

Impact of Near‐Inertial Waves on Vertical Mixing and Air‐Sea CO2 Fluxes in the Southern Ocean

“…In recent years, the SO Carbon and Climate Observations and Modeling project deployed floats with the ability to estimate the carbon concentration at the surface and CO 2 exchange between the atmosphere and the ocean. The new observations revealed the wintertime CO 2 outgassing near the ACC (Gray et al, ), which had been underestimated by most numerical models. Hence it is important to evaluate whether NIWs are properly included in those numerical simulations in diagnosing the models' performance.…”