Large Reemergence of Anthropogenic Carbon into the Ocean’s Surface Mixed Layer Sustained by the Ocean’s Overturning Circulation

Katsuya Toyama,Bruno Blanke,Olivier Aumont,Keith B Rodgers,Masao Ishii,Jorge L Sarmiento,Daniele Iudicone

doi:10.1175/jcli-d-16-0725.1

Abstract

We evaluate the output from a widely used ocean carbon cycle model to identify the subduction and obduction (reemergence) rates of anthropogenic carbon (Cant) for climatological conditions during the World Ocean Circulation Experiment (WOCE) era in 1995 using a new set of Lagrangian diagnostic tools. The principal scientific value of the Lagrangian diagnostics is in providing a new means to connect Cant reemergence pathways to the relatively rapid renewal time scales of mode waters through the overturning circulation. Our main finding is that for this model with 2.04 PgC yr−1 of uptake of Cant via gas exchange, the subduction and obduction rates across the base of the mixed layer (MLbase) are 4.96 and 4.50 PgC yr−1, respectively, which are twice as large as the gas exchange at the surface. Given that there is net accumulation of 0.17 PgC yr−1 in the mixed layer itself, this implies the residual downward Cant transport of 1.40 PgC yr−1 across the MLbase is associated with diffusion. Importantly, the net patterns for subduction and obduction transports of Cant mirror the large-scale patterns for transport of water volume, thereby illustrating the processes controlling Cant uptake. Although the net transfer across the MLbase by compensating subduction and obduction is relatively smaller than the diffusion, the localized pattern of Cant subduction and obduction implies significant regional impacts. The median time scale for reemergence of obducting particles is short (<10 yr), indicating that reemergence should contribute to limiting future carbon uptake through its contribution to perturbing the Revelle factor for surface waters.

Highlights

The ocean is a large reservoir of anthropogenic carbon (Cant), having absorbed approximately onethird of the anthropogenic carbon emissions over the industrial period (Mikaloff-Fletcher et al 2006; Le Quéré et al 2010)
We begin with a consideration of the instantaneous subduction and obduction rates
We find that approximately 19% of the total volume transport across the MLbase accounts for the ventilation of the main pycnocline over the time scales of a year and longer, while the remaining 81% (1613 ; 1614 Sv) is exchanged seasonally between the surface mixed layer and the seasonal pycnocline

Summary

Introduction

The ocean is a large reservoir of anthropogenic carbon (Cant), having absorbed approximately onethird of the anthropogenic carbon emissions over the industrial period (Mikaloff-Fletcher et al 2006; Le Quéré et al 2010). Earth system models and other modeling inferences suggest that in the future a variety of factors may contribute to making the oceans less effective in absorbing carbon from the atmosphere, Denotes content that is immediately available upon publication as open access. The exchange of Cant between the atmosphere and ocean interior is a two-step process. The first is associated with gas exchange across the air–sea interface, and the second is associated with the exchange of Cant across the base of the ocean’s mixed layer (MLbase), including upward (reemergence) and downward transport of Cant associated with obduction and subduction processes. The primary focus to date with the monitoring of ocean carbon uptake has been on the first of these, namely, using measurements of surface ocean pCO2 to infer uptake at the sea surface by gas exchange.

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