Variability of Sea-Air Carbon Dioxide Flux in Autumn Across the Weddell Gyre and Offshore Dronning Maud Land in the Southern Ocean

Margaret Ojone Ogundare,Warren R Joubert,Agneta Fransson,Alakendra N Roychoudhury,Melissa Chierici

doi:10.3389/fmars.2020.614263

Abstract

Sea surface fugacity of carbon dioxide (fCO2ssw) was measured across the Weddell gyre and the eastern sector in the Atlantic Southern Ocean in autumn. During the occupation between February and April 2019, the region of the study transect was a potential ocean CO2 sink. A net CO2 flux (FCO2) of −6.2 (± 8; sink) mmol m–2 d–1 was estimated for the entire study region, with the largest average CO2 sink of −10.0 (± 8) mmol m–2 d–1 in the partly ice-covered Astrid Ridge (AR) region near the coast at 68°S and −6.1 (± 8) mmol m–2d–1 was observed in the Maud Rise (MR) region. A CO2 sink was also observed south of 66°S in the Weddell Sea (WS). To assess the main drivers describing the variability of fCO2ssw, a correlation model using fCO2 and oxygen saturation was considered. Spatial distributions of the fCO2 saturation/O2 saturation correlations, described relative to the surface water properties of the controlling variables (chlorophyll a, apparent oxygen utilization (AOU), sea surface temperature, and sea surface salinity) further constrained the interplay of the processes driving the fCO2ssw distributions. Photosynthetic CO2 drawdown significantly offsets the influence of the upwelling of CO2-rich waters in the central Weddell gyre and enhanced the CO2 sink in the region. FCO2 of −6.9 mmol m–2 d–1 estimated for the Weddell gyre in this study was different from FCO2 of −2.5 mmol m–2 d–1 in autumn estimated in a previous study. Due to low CO2 data coverage during autumn, limited sea-air CO2 flux estimates from direct sea-surface CO2 observations particularly for the Weddell gyre region are available with which to compare the values estimated in this study. This highlights the importance of increasing seasonal CO2 observations especially during autumn/winter to improving the seasonal coverage of flux estimates in the seasonal sea ice-covered regions of the Southern Ocean.

Highlights

The Southern Ocean defined here as south of 40◦S is wellknown for its key role in the sequestration of CO2 (Sabine et al, 2004; Takahashi et al, 2009) and accounts for about 43% of the global oceanic uptake of anthropogenic CO2 (Frölicher et al, 2015)
Temperature ranges within the Weddell Sea (WS), Astrid Ridge (AR), and Maud Rise region (MR) regions were −0.9 to 6.19◦C, −1.9–0.4◦C, and −0.1–14.0◦C, respectively (Table 2)
This study investigates the drivers of variability for fCO2ssw and estimates the FCO2 during autumn in the Atlantic sector of the Southern Ocean, spanning across the Weddell gyre and Antarctic Circumpolar Current (ACC)

Summary

Introduction

The Southern Ocean defined here as south of 40◦S is wellknown for its key role in the sequestration of CO2 (Sabine et al, 2004; Takahashi et al, 2009) and accounts for about 43% of the global oceanic uptake of anthropogenic CO2 (Frölicher et al, 2015). CO2 taken up from the atmosphere in the surface waters is sequestered into the deep ocean during deep-water formation and remained for a long time over several centuries (e.g., Kheshgi, 2004), climatically relevant. This deepwater is subsequently brought to the surface by upwelling; rich with CO2 begins to equilibrate with the atmosphere driving ocean CO2 outgassing but often counteracted by biological CO2 drawdown (e.g., Fransson et al, 2004; Metzl et al, 2006; Gruber et al, 2009). Biological uptake of CO2 in the surface Southern Ocean is enhanced at frontal structures as well in the Antarctic Circumpolar Current (ACC; e.g., Chierici et al, 2004; Ito et al, 2010) and the marginal sea ice zone (e.g., Froneman et al, 2004; Arrigo et al, 2008)

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