Air-sea gas exchange

Southern Ocean oxygen fluxes

The Southern Ocean is an important part of the carbon cycle, responsible for a significant fraction of the oceanic uptake of anthropogenic carbon and for the export of nutrients in mode waters that fuel productivity in much of the rest of the ocean. It also the location of a large number of Argo oxygen floats deployed over the past two decades.

High quality, air-calibrated biogeochemical floats have been deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) and a recent reanalysis of Argo oxygen data by Drucker and Riser (2016) have yielded an unparalleled opportunity to understand the linkage between seasonal oxygen changes and the carbon cycle.

In an early study (Bushinsky et al., 2017), we analyzed 9 years of Argo oxygen data to directly calculate air-sea oxygen fluxes over the entire Southern Ocean for the first time. We found an oxygen uptake double that of prior estimates (Gruber et al., 2001; Resplandy et al., 2015), primarily driven by ventilation in the southernmost regions of the Southern Ocean. Our current work involves understanding how these fluxes relate to newly estimated carbon fluxes also derived from SOCCOM Argo floats.

Flux parameterizations

Understanding how gas exchanges between the air and the water and what we can use to estimate this flux is incredibly important to our understanding of upper ocean processes. Air-sea flux parameterizations are used in everything from global climate models to simple box models because it is very difficult to measure the actual air-sea flux and we instead must use something we can measure, like wind speed, and estimate the flux with a parameterization. All of my upper ocean data analyses rely on air-sea flux parameterizations, which is why I am intrigued by testing and improving our understanding these relationships. Bubbles generated by waves and fractured sea ice are two of the factors that I am trying to better understand in my work.

The parameterization of air-sea gas fluxes has long been an area of study in oceanography. Standard parameterizations used in most climate models have a single diffusive term that scales as a function of wind speed. However, relatively insoluble gases are impacted by breaking waves that inject bubbles into the ocean. Oxygen, argon, and nitrogen are all substantially impacted by bubble injection and many air-sea flux parameterizations include bubble components in an attempt to deal with this issue.

The air-sea flux of oxygen is the largest term in the upper ocean oxygen balance. In order to identify the flux parameterization that best captures the bubble injection of oxygen we tested a range of air-sea flux parameterizations, in part by using my upper ocean gas model and comparing the results to mooring-derived nitrogen concentrations (Emerson and Bushinsky, 2016). We found that the Liang et al. (2013) parameterization best reproduced both mooring nitrogen data and water column noble gas measurements. Subsequently, we tuned the bubble component of the Liang et al. (2013) parameterization against wintertime periods when high wind speeds drive strong bubble injection (Yang, Emerson, and Bushinsky, 2017).