15 May 2023
 | 15 May 2023
Status: this preprint is open for discussion.

Estimating Marine Carbon Uptake in the Northeast Pacific Using a Neural Network Approach

Patrick J. Duke, Roberta C. Hamme, Debby Ianson, Peter Landschützer, Mohamed M. M. Ahmed, Neil C. Swart, and Paul A. Covert

Abstract. The global ocean takes up nearly a quarter of anthropogenic CO2 emissions annually, but the variability of this uptake at regional scales remains poorly understood. Here we use a neural network approach to interpolate sparse observations, creating a monthly gridded seawater partial pressure of CO2 (pCO2) data product from January 1998 to December 2019, at 1/12° × 1/12° spatial resolution, in the Northeast Pacific open ocean. The data product (ANN-NEP; NCEI Record ID: BGSH2HNRP) was created from pCO2 observations within the 2021 version of the Surface Ocean CO2 Atlas (SOCAT), and a range of predictor variables acting as proxies for processes affecting pCO2 to create non-linear relationships to interpolate observations at a spatial resolution four times greater than leading global products and with better overall performance. In moving to a higher resolution, we show that the internal division of training data is the most important parameter for reducing overfitting. Using our pCO2 product, wind speed, and atmospheric CO2, we evaluate air-sea CO2 flux variability. On sub-decadal to decadal timescales, we find that the upwelling strength of the subpolar Alaskan Gyre, driven by large-scale atmospheric forcing, acts as the primary control on air-sea CO2 flux variability (r2 = 0.93, p < 0.01). In the northern part of our study region, divergence with atmospheric CO2 is enhanced by increased local wind stress curl, enhancing upwelling and entrainment of naturally CO2-rich subsurface waters, leading to decade-long intervals of strong winter outgassing. During recent Pacific marine heatwaves from 2013 on, we find enhanced atmospheric CO2 uptake (by as much as 45 %) due to limited wintertime entrainment. Our product estimates long-term surface ocean pCO2 increase at a rate below the atmospheric trend (1.4 ± 0.1 μatm yr−1) with the slowest increase in the center of the subpolar gyre where there is strong interaction with subsurface waters. This mismatch suggests the Northeast Pacific Ocean sink for atmospheric CO2 may be increasing.

Patrick J. Duke et al.

Status: open (until 05 Jul 2023)

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  • RC1: 'Comment on egusphere-2023-870', Marine Fourrier, 02 Jun 2023 reply

Patrick J. Duke et al.

Patrick J. Duke et al.


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Short summary
The ocean is both impacted by climate change and helps mitigate its affects through taking up carbon from the atmosphere. We used a machine learning approach to investigate what controls open ocean carbon uptake in the Northeast Pacific. Marine heatwaves that lasted 2–3 years increased uptake, while the Alaskan Gyre controlled uptake over 10-year time periods. The trend from 1998–2019 suggests carbon uptake in the Northeast Pacific open ocean is increasing.