Project Funding: 2014 - 2016

NRA: 2013 NASA: Carbon Cycle Science   

Funded by NASA

Abstract:
The Southern Ocean is a key region contributing to the regulation of atmospheric CO2 through the 'biological pump', in which carbon is fixed and exported out of the surface mixed layer. Deep waters become enriched in CO2 and depleted in O2 and later ventilate and equilibrate with atmospheric CO2 and O2 when they outcrop to the surface. The ~30% decrease in atmospheric CO2 during the Last Glacial Maximum (LGM) is commonly attributed in large part to a reduction in ventilation from the Southern Ocean. In the future, ocean biogeochemistry models tend to predict neutral to small increases in productivity and carbon export in the Southern Ocean, but the effect of these increases on the net air-sea CO2 flux and their interaction with future changes in deep ventilation are not well understood. Meanwhile some hindcast model simulations suggest that the current Southern Ocean CO2 sink may be weakening due to increased ventilation of naturally CO2-enriched waters around Antarctica. The ability to detect changes in ocean ventilation and export production is critical to accurate monitoring and prediction of changes in the Southern Ocean carbon sink. Remotely sensed ocean color data and atmospheric potential oxygen (APO) are valuable and complementary metrics for evaluating the Southern Ocean carbon cycle. Ocean color products provide high-resolution spatial coverage, while APO data provide regional integrals that helps constrain the satellite-derived NCP data. APO data further provide information about subsurface ventilation processes that are not captured by satellite data. Earth System Models (ESM) are widely used tools for predicting the future evolution of ocean carbon uptake. Output from a range of ESMs is publicly available through websites created for the IPCC 5th Assessment. In our preliminary calculations, six IPCC AR5 ocean biogeochemistry component models forced with the RCP8.5 scenario predict a 5-20% decrease in the amplitude of the APO seasonal cycle in the Southern Ocean, which is due in some cases to a reduction in ventilation of O2-depleted, CO2-enriched deepwater. We propose a synthesis of models, satellite ocean color data and atmospheric observations, including APO and other trace gases, to understand why this reduction in ventilation occurs in some models but not others, what it means for ocean carbon uptake, and which ESMs provide the most credible future projections. Specific tasks include: 1) Develop improved satellite ocean color based estimates of net community production (NCP) in the Southern Ocean, based on several recently published new formulations and evaluate against APO data. 2) Evaluate the ocean biogeochemistry components of a suite of IPCC Earth System Models in terms of their ability to reproduce observed seasonal and spatial variability in satellite-derived NCP and APO. Examine future changes in NCP and APO predicted by the ESMs forced with the RCP8.5 future scenario. 3) Implement techniques developed for coarse resolution ocean models in past work into the eddy-resolving ECCO2-Darwin optimization system to better understand the relationship between carbon export production (EP), NCP, and surface O2 fluxes associated with ocean productivity, ventilation and thermal forcing. The proposed project targets Theme 1 of the Carbon Cycle Science solicitation by focusing on the carbon cycle in the high latitude Southern Ocean. The research also addresses the NASA Earth Observations and Applications strategic needs for both Climate Monitoring and Research and Carbon Cycle Research: it synthesizes satellite ocean color data, surface-based observing systems and models to promote understanding of carbon exchange between the atmosphere and ocean in order to project with more confidence the future evolution of climate.

Publications:

Kahru, M., Lee, Z., Mitchell, B. G. 2017. Contemporaneous disequilibrium of bio-optical properties in the Southern Ocean. Geophysical Research Letters. 44(6), 2835-2842. DOI: 10.1002/2016GL072453

Kahru, M., Lee, Z., Mitchell, B. G., Nevison, C. D. 2016. Effects of sea ice cover on satellite-detected primary production in the Arctic Ocean. Biology Letters. 12(11), 20160223. DOI: 10.1098/rsbl.2016.0223

More details may be found in the following project profile(s):

• NASA Ocean Biology & Biogeochemistry (OBB) Project Profile