Abstract
Abstract. Sensitivities of the oceanic biological pump within the GISS (Goddard Institute for Space Studies ) climate modeling system are explored here. Results are presented from twin control simulations of the air–sea CO2 gas exchange using two different ocean models coupled to the same atmosphere. The two ocean models (Russell ocean model and Hybrid Coordinate Ocean Model, HYCOM) use different vertical coordinate systems, and therefore different representations of column physics. Both variants of the GISS climate model are coupled to the same ocean biogeochemistry module (the NASA Ocean Biogeochemistry Model, NOBM), which computes prognostic distributions for biotic and abiotic fields that influence the air–sea flux of CO2 and the deep ocean carbon transport and storage. In particular, the model differences due to remineralization rate changes are compared to differences attributed to physical processes modeled differently in the two ocean models such as ventilation, mixing, eddy stirring and vertical advection. GISSEH(GISSER) is found to underestimate mixed layer depth compared to observations by about 55% (10%) in the Southern Ocean and overestimate it by about 17% (underestimate by 2%) in the northern high latitudes. Everywhere else in the global ocean, the two models underestimate the surface mixing by about 12–34%, which prevents deep nutrients from reaching the surface and promoting primary production there. Consequently, carbon export is reduced because of reduced production at the surface. Furthermore, carbon export is particularly sensitive to remineralization rate changes in the frontal regions of the subtropical gyres and at the Equator and this sensitivity in the model is much higher than the sensitivity to physical processes such as vertical mixing, vertical advection and mesoscale eddy transport. At depth, GISSER, which has a significant warm bias, remineralizes nutrients and carbon faster thereby producing more nutrients and carbon at depth, which eventually resurfaces with the global thermohaline circulation especially in the Southern Ocean. Because of the reduced primary production and carbon export in GISSEH compared to GISSER, the biological pump efficiency, i.e., the ratio of primary production and carbon export at 75 m, is half in the GISSEH of that in GISSER, The Southern Ocean emerges as a key region where the CO2 flux is as sensitive to biological parameterizations as it is to physical parameterizations. The fidelity of ocean mixing in the Southern Ocean compared to observations is shown to be a good indicator of the magnitude of the biological pump efficiency regardless of physical model choice.
Highlights
The world’s oceans help regulate the amount of heat, CO2 and other greenhouse gases in the atmosphere that are responsible for the radiative imbalance of the planet
To cast the biological pump sensitivity in light of the physical model differences, we review some of the main findings of Romanou et al (2013) where a more detailed model evaluation was presented
The results show that both models
Summary
The world’s oceans help regulate the amount of heat, CO2 and other greenhouse gases in the atmosphere that are responsible for the radiative imbalance of the planet. Oceans play a crucial role in the global climate. There are two processes that control the air–sea exchanges of CO2: the solubility pump, and the biological (and carbonate) pump (Volk and Hoffert, 1985; Gruber and Sarmiento, 2002; Falkowski et al, 2003). About one-third of the anthropogenic long-term averaged CO2 emissions released into the atmosphere is absorbed by the ocean (Khatiwala et al (2009) and references therein) whereas shorter-term averages amount to one-fourth A. Romanou et al.: Ocean biological pump and recycling
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