Abstract
AbstractThe accumulation of carbon within the Weddell Gyre and its exchanges across the gyre boundaries are investigated with three recent full‐depth oceanographic sections enclosing this climatically important region. The combination of carbon measurements with ocean circulation transport estimates from a box inverse analysis reveals that deepwater transports associated with Warm Deep Water (WDW) and Weddell Sea Deep Water dominate the gyre's carbon budget, while a dual‐cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean. Historical sea surface pCO2 observations, interpolated using a neural network technique, confirm the net summertime sink of 0.044 to 0.058 ± 0.010 Pg C yr−1 derived from the inversion. However, a wintertime outgassing signal similar in size results in a statistically insignificant annual air‐to‐sea CO2 flux of 0.002 ± 0.007 Pg C yr−1 (mean 1998–2011) to 0.012 ± 0.024 Pg C yr−1 (mean 2008–2010) to be diagnosed for the Weddell Gyre. A surface layer carbon balance, independently derived from in situ biogeochemical measurements, reveals that freshwater inputs and biological drawdown decrease surface ocean inorganic carbon levels more than they are increased by WDW entrainment, resulting in an estimated annual carbon sink of 0.033 ± 0.021 Pg C yr−1. Although relatively less efficient for carbon uptake than the global oceans, the summertime Weddell Gyre suppresses the winter outgassing signal, while its biological pump and deepwater formation act as key conduits for transporting natural and anthropogenic carbon to the deep ocean where they can reside for long time scales.
Highlights
The Southern Ocean (>44°S) plays a critical role in the global carbon cycle as the location of both substantial anthropogenic carbon uptake (~0.7 Pg C yrÀ1, approximately a third of the total global oceanic sink [Khatiwala et al, 2009; Mikaloff Fletcher et al, 2006]) and outgassing of upwelled natural dissolved inorganic carbon (DIC) (0.4 Pg C yrÀ1) [Mikaloff Fletcher et al, 2007]
The combination of carbon measurements with ocean circulation transport estimates from a box inverse analysis reveals that deepwater transports associated with Warm Deep Water (WDW) and Weddell Sea Deep Water dominate the gyre’s carbon budget, while a dual-cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean
Weddell Sea Deep Water (WSDW) transport across the section mainly takes place at locations where its movement is not restricted by topography, i.e., east of the South Scotia Ridge (SSR) and lower parts of the continental slope, some flow occurs through the deep passages of the SSR
Summary
The Southern Ocean (>44°S) plays a critical role in the global carbon cycle as the location of both substantial anthropogenic carbon uptake (~0.7 Pg C yrÀ1, approximately a third of the total global oceanic sink [Khatiwala et al, 2009; Mikaloff Fletcher et al, 2006]) and outgassing of upwelled natural dissolved inorganic carbon (DIC) (0.4 Pg C yrÀ1) [Mikaloff Fletcher et al, 2007] This is a consequence of its unique circulation: the lack of continental boundaries permits the existence of the Antarctic Circumpolar Current (ACC) that facilitates zonal exchange between the major ocean basins and meridionally allows the upwelling of DIC-rich Circumpolar Deep Water (CDW). Assessments of long-term trends are limited by poor data coverage [Lenton et al, 2013] except for a number of limited locations [Huhn et al, 2013; van Heuven et al, 2011]
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