The importance of contemporaneous wind and pCO2 measurements for regional air-sea CO2 flux estimates

Abstract

Few observational platforms are able to sustain direct measurements of all the key variables needed in the bulk calculation of air-sea carbon dioxide (CO₂) exchange, a capability newly established for some Uncrewed Surface Vehicles (USVs). Western boundary currents are particularly challenging observational regions due to strong variability and dangerous sea states but are also known hot spots for CO₂ uptake, making air-sea exchange quantification in this region both difficult and important. Here, we present new observations collected by Saildrone USVs in the Gulf Stream during the winters of 2019 and 2022. We compared Saildrone data across co-located vehicles and against the Pioneer Array moorings to validate the data quality. We explored how CO₂ flux estimates differ when all variables needed to calculate fluxes from the bulk formulas are simultaneously measured on the same platform, relative to the situation where in situ observations must be combined with publicly-available data products. We systematically replaced variables in the bulk formula with those often used for local and regional flux estimates. The analysis revealed that when using the ERA-5 reanalysis wind speed in place of in situ observations, the ocean uptake of CO₂ is underestimated by 8%; this underestimate grows to 9% if the NOAA Marine Boundary Layer atmospheric CO₂ product and ERA-5 significant wave height are also used in place of in situ observations. Overall our findings point to the importance of collecting contemporaneous observations of wind speed and ocean pCO₂ to reduce biases in estimates of regional CO₂ flux, especially during high wind events.

Key Points

• Compared to Saildrone wind speeds, the ERA-5 reanalysis appears to underestimate the highest wind speed events (>10 m s⁻¹)
• Co-located ocean pCO₂ and wind speed observations results in larger uptake of CO₂ in this region by 9%
• Using a sea-state dependent gas transfer velocity leads to a 28% greater ocean uptake of CO₂ compared to using a wind-only equation

Plain Language Summary

The North Atlantic Ocean absorbs a large amount of carbon dioxide (CO₂) from the atmosphere. The ocean CO₂ uptake mainly occurs during the winter months in a region where a swift ocean current, called the Gulf Stream, carries warm waters from the tropics to northern latitudes. Understanding how much CO₂ this ocean region absorbs is important for constraining global scale assessments of the sources and sinks of CO₂. In this paper, we reflect on the methods that are typically used to calculate the exchange of CO₂ between the ocean and atmosphere. Using new data collected by uncrewed surface vehicles, that resemble small sailboats, in the North Atlantic Ocean in the vicinity of the Gulf Stream, we find that the estimate of the exchange of CO₂ is larger when directly measured wind speeds are used in the calculation in comparison to wind speeds from a widely used data product. This result signifies the importance of co-located sensors on the same observing platform.

Nickford S., Palter J. B. & Mu L., 2024. The importance of contemporaneous wind and pCO2 measurements for regional air-sea CO2 flux estimates. JGR Oceans 129(6): e2023JC020744. doi: 10.1029/2023JC020744. Article.

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