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

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.