Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

K B Rodgers,G Madec,C De Boyer Montégut,D Iudicone,O Aumont,L Bopp,R Wanninkhof,R F Keeling,S E Mikaloff Fletcher,Y Plancherel

doi:10.5194/bg-11-4077-2014

Abstract

Abstract. Here we test the hypothesis that winds have an important role in determining the rate of exchange of CO2 between the atmosphere and ocean through wind stirring over the Southern Ocean. This is tested with a sensitivity study using an ad hoc parameterization of wind stirring in an ocean carbon cycle model, where the objective is to identify the way in which perturbations to the vertical density structure of the planetary boundary in the ocean impacts the carbon cycle and ocean biogeochemistry. Wind stirring leads to reduced uptake of CO2 by the Southern Ocean over the period 2000–2006, with a relative reduction with wind stirring on the order of 0.9 Pg C yr−1 over the region south of 45° S. This impacts not only the mean carbon uptake, but also the phasing of the seasonal cycle of carbon and other ocean biogeochemical tracers. Enhanced wind stirring delays the seasonal onset of stratification, and this has large impacts on both entrainment and the biological pump. It is also found that there is a strong reduction on the order of 25–30% in the concentrations of NO3 exported in Subantarctic Mode Water (SAMW) to wind stirring. This finds expression not only locally over the Southern Ocean, but also over larger scales through the impact on advected nutrients. In summary, the large sensitivity identified with the ad hoc wind stirring parameterization offers support for the importance of wind stirring for global ocean biogeochemistry through its impact over the Southern Ocean.

Highlights

Ocean interior properties over much of the global domain are largely set by late-winter mixed-layer characteristics, as these are the characteristics passed to the interior during restratification in early spring (Stommel, 1979)
We show that accounting for the additional effect of wind stirring greatly reduces contemporary carbon uptake in the Southern Ocean
The excessive winter mixed-layer depth in WSTIR indicates that the current www.biogeosciences.net/11/4077/2014/

Summary

Introduction

Ocean interior properties over much of the global domain are largely set by late-winter mixed-layer characteristics, as these are the characteristics passed to the interior during restratification in early spring (Stommel, 1979). For this reason, ocean modelers have long been interested in improving ocean mixing parameterizations that ameliorate the representation of winter mixed-layer depths, and thereby ventilation in their models. Our main focus is to conduct such an analysis for the case of carbon over the Southern Ocean, and to give more general consideration to other tracers related to ocean biogeochemistry This is considered within the context of the full seasonal cycle over the Southern Ocean, and the implications for trends in carbon uptake are explored

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