High-resolution Neural Network Demonstrates Strong CO2 Source-Sink
Juxtaposition in the Coastal Zone
Abstract
Coastal oceans may play an important role in regulating the
concentration of carbon dioxide in the atmosphere. Quantification of
carbon fluxes at this highly dynamic land-ocean interface will aid in
monitoring, reporting, and verification for marine carbon dioxide
removal. Here, we use a two-step neural network approach to generate
basin-wide estimates from sparse observational data in the coastal
Northeast Pacific Ocean at an unprecedented spatial resolution of 1/12°
with coverage in the nearshore (0 - 25 km offshore). We compiled partial
pressure of carbon dioxide (pCO2) observations as well as a range of
predictor variables including satellite-based and physical oceanographic
reanalysis products. With the predictor variables representing processes
affecting pCO2, we created non-linear relationships to interpolate
observations from 1998-2019. Compared to in situ shipboard and mooring
observations, our coastal pCO2 product captures broad spatial patterns
and seasonal cycle variability well. A sensitivity analysis identifies
that the parameters responsible for the neural network’s ability to
capture regional pCO2 variability agrees with mechanistic processes.
Using wind speed and atmospheric CO2, we calculated air-sea CO2 fluxes.
We report an anticorrelation between net annual air-sea CO2 flux and
air-sea CO2 flux seasonal amplitude and suggest the relationship is
driven by regional processes. We show the inclusion of nearshore net
outgassing fluxes lowers the overall regional net flux. Overall, our
results suggest that the region is a net sink (-0.7 mol m-2 yr-1) for
atmospheric CO2 with trends indicating increasing oceanic uptake due to
strong connectivity to subsurface waters.