ORCAS

The O₂/N₂ Ratio and CO₂ Airborne Southern Ocean (ORCAS) Study was an NSF sponsored airborne field campaign with research flights from Punta Arenas, Chile during January and February of 2016.  

The Southern Ocean plays a dominant role in the uptake of anthropogenic carbon yet this process is poorly represented by models and its future trajectory remains highly uncertain. ORCAS has advanced our understanding of the physical and biological controls on air-sea exchange of O₂ and CO₂ in the Southern Ocean. This was achieved through intensive airborne surveys of atmospheric O₂, CO₂, related gases, and ocean surface properties over diverse biogeochemical regions adjacent to the southern tip of South America and the Antarctic Peninsula. ORCAS utilized the NSF/NCAR HIAPER Gulfstream V (GV) aircraft with a suite of high-precision in situ and remote sensing instruments, combined with whole-air samplers on 14 flights over a period of 6 weeks in austral mid-summer. In addition to the core O₂, CO₂, and related gas measurements, the project included hyperspectral remote sensing of the ocean surface and characterization of the emissions of biogenic reactive gases over the Southern Ocean. The ORCAS observations were guided by and used to test a suite of ocean biogeochemistry models to improve our understanding of key processes and feedbacks in an undersampled yet climatically important part of the world.

The Southern Ocean plays a critical role in setting the overturning circulation for the global oceans, and determining the partitioning of heat and dissolved gases between the atmosphere and deep ocean. The region is particularly sensitive to climate forcing, and evidence suggests that it is already responding to observed changes. The Southern Ocean currently absorbs a significant amount of human emitted CO₂, but the future behavior of this sink is uncertain. Sparse observations and complex interacting physical and biological processes limit our understanding of biogeochemistry and climate feedbacks at high southern latitudes. The ORCAS measurements have added new observational constraints with unprecedented spatial coverage for Southern Ocean biogeochemical variables. In particular, the combination of atmospheric O₂ and CO₂ measurements has enabled disentangling key drivers of air-sea CO₂ exchange. ORCAS has delivered precise and zonally-representative measurements of the spatiotemporal evolution of summertime hemispheric-scale atmospheric O₂ and CO₂ over the Southern Ocean. These data have been used to challenge state-of-the-art Earth system models. The CO₂:O₂ ratios derived from vertical concentration gradients and fluxes derived from intensive boundary layer sampling have further elucidated the balance between thermal and biological forcing of CO₂ at regional scales. Collectively, the ORCAS measurements have improved understanding of the present day biogeochemical drivers of Southern Ocean air-sea CO₂ and related gas fluxes, laying the groundwork for better mechanistic representation of feedbacks to climate change.

ORCAS has generated a valuable and publicly available data set for use in improving Earth system models, which will ultimately support societal benefits of greater understanding of Southern Ocean biogeochemistry and more accurate climate projections for decision support. By crossing traditional disciplinary and methodological boundaries, ORCAS has addressed a broad need to more closely integrate biogeochemical observations and models to better make use of the measurements and accelerate model improvement.

The NASA Portable Remote Imaging Spectrometer (PRISM) was integrated on the NSF/NCAR GV research aircraft payload of the NSF-sponsored ORCAS campaign. PRISM provided Level 1 geometrically corrected and calibrated radiance and Level 2 water leaving reflectance. Complementary PRISM measurements during ORCAS provided data products that are critical to 1) improve understanding of biogeochemical processes in the Southern Ocean and their control on air-sea gas exchange, 2) serve as a test bed for ocean color and atmospheric correction algorithms, and 3) provide synergy with a host of ancillary bio-optical and biogeochemical data from collaborative projects.