An Alternative to Static Climatologies: Robust Estimation of Open Ocean CO2 Variables and Nutrient Concentrations From T, S, and O2 Data Using Bayesian Neural Networks.

Bittig, Henry C., Steinhoff, Tobias, Claustre, Hervé, Fiedler, Björn, Williams, Nancy L., Sauzède, Raphaëlle, Körtzinger, Arne and Gattuso, Jean-Pierre (2018) An Alternative to Static Climatologies: Robust Estimation of Open Ocean CO2 Variables and Nutrient Concentrations From T, S, and O2 Data Using Bayesian Neural Networks. Open Access Frontiers in Marine Science, 5 . Art.Nr. 328. DOI 10.3389/fmars.2018.00328.

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Abstract

This work presents two new methods to estimate oceanic alkalinity (AT), dissolved inorganic carbon (CT), pH, and pCO2 from temperature, salinity, oxygen, and geolocation data. “CANYON-B” is a Bayesian neural network mapping that accurately reproduces GLODAPv2 bottle data and the biogeochemical relations contained therein. “CONTENT” combines and refines the four carbonate system variables to be consistent with carbonate chemistry. Both methods come with a robust uncertainty estimate that incorporates information from the local conditions. They are validated against independent GO-SHIP bottle and sensor data, and compare favorably to other state-of-the-art mapping methods. As “dynamic climatologies” they show comparable performance to classical climatologies on large scales but a much better representation on smaller scales (40–120 d, 500–1,500 km) compared to in situ data. The limits of these mappings are explored with pCO2 estimation in surface waters, i.e., at the edge of the domain with high intrinsic variability. In highly productive areas, there is a tendency for pCO2 overestimation due to decoupling of the O2 and C cycles by air-sea gas exchange, but global surface pCO2 estimates are unbiased compared to a monthly climatology. CANYON-B and CONTENT are highly useful as transfer functions between components of the ocean observing system (GO-SHIP repeat hydrography, BGC-Argo, underway observations) and permit the synergistic use of these highly complementary systems, both in spatial/temporal coverage and number of observations. Through easily and robotically-accessible observations they allow densification of more difficult-to-observe variables (e.g., 15 times denser AT and CT compared to direct measurements). At the same time, they give access to the complete carbonate system. This potential is demonstrated by an observation-based global analysis of the Revelle buffer factor, which shows a significant, high latitude-intensified increase between +0.1 and +0.4 units per decade. This shows the utility that such transfer functions with realistic uncertainty estimates provide to ocean biogeochemistry and global climate change research. In addition, CANYON-B provides robust and accurate estimates of nitrate, phosphate, and silicate. Matlab and R code are available at https://github.com/HCBScienceProducts/.
Introduction

Document Type: Article
Funder compliance: info:eu-repo/grantAgreement/EC/H2020/633211
Keywords: carbon cycle, GLODAP, marine carbonate system, surface pCO2 climatology, Revelle buffer factor increase, machine learning, nutrient estimation
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.3389/fmars.2018.00328
ISSN: 2296-7745
Projects: AtlantOS, OCEANET, remOcean, SOCCOM, SOCAT, IOCCP, SOLAS, IMBER
Date Deposited: 20 Sep 2018 09:34
Last Modified: 01 Feb 2019 15:05
URI: http://oceanrep.geomar.de/id/eprint/44345

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