Observations at the air-sea interface: quantifying carbon and heat exchange in the Gulf Stream region using uncrewed surface vehicles

Author

Sarah Nickford, University of Rhode IslandFollow

Date of Award

2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Specialization

Physical Oceanography

Department

Oceanography

First Advisor

Jaime Palter

Abstract

The atmosphere and ocean communicate via the air-sea interface through the exchange of heat, mass, and momentum. This exchange is maximized in stormy regions such as western boundary currents, like the Gulf Stream in the North Atlantic Ocean. The Gulf Stream experiences frequent storms, many of which occur in the wintertime, bringing strong winds and cold air masses that invigorate exchange. This dissertation focuses on novel in-situ observations collected by Uncrewed Surface Vehicles (USVs) to study the bulk air-sea exchange of carbon dioxide (CO₂) and heat in the Gulf Stream region.

Much of our knowledge of global ocean CO₂ uptake is from the analysis of in-situ ocean CO₂ data stored in large repositories (with minimal wintertime observations) and machine learning models that use these data try to reconstruct global ocean CO₂ uptake. In Chapter 1, we analyze new observations collected in the Gulf Stream region by a USV in February 2019 in comparison to observations collected by a research vessel in January 2006. The sea surface temperatures (SSTs) progressively cooled on the south side of the Gulf Stream, leading the surface waters there to be undersaturated in the partial pressure of carbon dioxide (pCO₂) relative to the atmosphere. This undersaturation is not captured in coarse ocean pCO₂ reconstructions that are based on sparse data. We also identify that the progressive cooling of SSTs does not explain the resulting ocean pCO₂ signature alone. Pairing the USV observations with those collected by a profiling Argo float, we determined that the USV recorded active subtropical mode water formation by detecting the highest ocean pCO₂ values on the subtropical side of the Gulf Stream corresponding to some of the coolest temperatures, likely the result of deep vertical mixing of previous years subtropical mode water. We show that the Gulf Stream is an ideal spot for the ocean uptake of large quantities of atmospheric CO₂ because of the cooling temperatures, strong winds, and low Revelle factor.

Next, we evaluate the importance of collecting all of the parameters necessary to calculate the CO₂ exchange on the same platform, as opposed to using a combination of in-situ and data-based products. Chapter 2 uses data collected by five USVs in the Gulf Stream region to test this. We first evaluate the quality of the data between the individual USV platforms and against a regularly maintained mooring array consisting of three surface moorings that measure the same parameters as the USVs. The USVs compared extremely well against one another and showed close comparison to the surface moorings, aside from one CO₂ instrument that we suspect malfunctioned. By systematically replacing variables in the bulk CO₂ flux equation with publicly-available data products, we reveal that there is an 8% low bias in the ocean CO₂ uptake estimate when using ERA-5 wind speeds in place of in-situ. This underestimate is greater when atmospheric CO₂ and significant wave height are also replaced with a data product. We conclude that measuring the variables needed to calculate bulk fluxes is important to do on the same platform to reduce biases.

In Chapter 3, we use the USV observations to weigh in on a decades long discussion about near surface wind convergence at the Gulf Stream and assess its role in the enhancement of turbulent heat fluxes. Turbulent heat flux is the sum of the latent (due to humidity) and sensible (due to temperature) heat fluxes. We find, contrary to previous satellite and reanalysis studies, that near surface wind convergence is present at the strong SST front of the Gulf Stream on short spatio-temporal scales. We examine two case studies when convergence is present and identify that they exhibit differences partly due to the direction of the winds: one case has winds blowing predominately from the warm side of the Gulf Stream to the cold side, while the other has cold to warm winds. Large turbulent heat flux gradients are present at the Gulf Stream when the winds are blowing from cold to warm.

Recommended Citation

Nickford, Sarah, "Observations at the air-sea interface: quantifying carbon and heat exchange in the Gulf Stream region using uncrewed surface vehicles" (2023). Open Access Dissertations. Paper 1610.
https://digitalcommons.uri.edu/oa_diss/1610