USAP-DC

The Antarctic Circumpolar Current (ACC) is the largest current on the planet, flowing west to east around Antarctica, forming a barrier that separates warmer waters to the north from colder waters to the south. Ocean eddies (like atmospheric storms) break through the ACC barrier, transferring heat across the ACC towards Antarctica. When warmer ocean waters intrude onto the Antarctic continental shelves, they contribute to glacial melt and ice shelf retreat. Over the past several decades, the Southern Ocean has warmed and winds have increased due to climate change. Somewhat surprisingly the ACC, though pushed by faster winds, has not accelerated; a faster current would present a stronger barrier to heat transfer. Instead, ocean eddies have increased. These eddies are concentrated at 6-7 "hot spots". Drake Passage is one of these hot spots. As the narrowest land gap on the entire circumpolar path of the ACC, Drake Passsage is an ideal monitoring spot. However, it is also one of the windiest and roughest stretches of water on the globe. The only ship that crosses Drake Passage year-round is the USAP supply vessel for Palmer Station, making it a unique platform to monitor the currents and temperature with a minimum of personnel and resources. The Drake Passage time series of upper ocean currents and temperature is now in its 24th year. The upper ocean temperature measurements have found significant warming in Drake Passage. The upper ocean current measurements have confirmed that the ACC has remained steady on average but have also revealed a complicated filamented current structure. Combining temperature and current measurements has provided a better understanding of heat transfer across the ACC by eddies. The time series has also provided valuable ground-truth for satellite measurements and for numerical model predictions looking at the entire ACC. Our studies are focused on examining low-frequency variability - seasonal, interannual, and decadal - in order to provide baselines from which to evaluate and interpret physical and biogeochemical changes occurring in the Southern Ocean.

Person	Role
Chereskin, Teresa	Investigator and contact
Sprintall, Janet	Co-Investigator

Antarctic Ocean and Atmospheric Sciences	Award # 2001646
Antarctic Ocean and Atmospheric Sciences	Award # 1542902	Previous Award High Resolution Underway Observations in Drake Passage: Ocean Physics The Antarctic Circumpolar Current (ACC) is the strongest ocean current in the world, with a flow rate more than 100 times greater than all the rivers on Earth combined. This project continues a long-term study of ocean properties and variability in the ACC by repeat transects across the Drake Passage from Punta Arenas to Palmer Station using the USAP supply ship RV/AS LMGould. This project will provide information about long term trends and variability in Southern Ocean fronts, surface water mass properties, heat, and salinity budgets. This study will contribute to understanding the response of the ACC to atmospheric forcing on climate relevant time scales, and its role in driving the meridional overturning circulation of the Southern Ocean. These data represent the only year-round shipboard measurements in the Southern Ocean. This project will continue high-resolution, near-repeat XBT/XCTD/ADCP transect sampling in Drake Passage in order to study modes of variability in ocean temperature, salinity, currents and backscatter in the Antarctic Circumpolar Current (ACC). The data will capture variability on time scales from seasonal to interannual and on spatial scales from current cores to eddies. With almost two decades of data now available, the primary science objectives are to analyze the Drake Passage time series and (1) describe and relate the seasonal and long-term ocean energy distribution to wind, buoyancy and topographic forcing and sinks, and (2) describe and relate seasonal and long-term changes in the ACC fronts, water masses and upwelling to biogeochemical variability. The continued data collection, quality control and dissemination of the Drake Passage data sets to the broader oceanographic community are critical components of the project.

USAP-2001646_1

DataManagementPlan_2019_final.pdf

Repository	Title (link)	Format(s)	Status
NCEI	Joint Archive for shipboard ADCP data	netCDF; ASCII	exist
NCEI	World Ocean Database	Not Provided	exist

1. Freeman, N. M., Munro, D. R., Sprintall, J., Mazloff, M. R., Purkey, S., Rosso, I., DeRanek, C. A., & Sweeney, C. (2019). The Observed Seasonal Cycle of Macronutrients in Drake Passage: Relationship to Fronts and Utility as a Model Metric. Journal of Geophysical Research: Oceans, 124(7), 4763–4783. (doi:10.1029/2019jc015052)
2. Chidichimo, M. P., Perez, R. C., Speich, S., Kersale, M., Sprintall, J., Dong, S., Lamont, T., Sato, O. T., Chereskin, T. K., Hummels, R., and Schmid, C., 2023. Energetic overturning flows, dynamic interocean exchanges, and ocean warming observed in the South Atlantic, Communications Earth & Environment, 4, 10. (doi:10.1038/s43247-022-00644-x)
3. Gutierrez-Villanueva, M. O., Chereskin, T. K., and Sprintall, J. 2020. Upper-ocean eddy heat flux across the Antarctic Circumpolar Current in Drake Passage from observations: time-mean and seasonal variability, J. Phys. Oceanogr., 50(9), 2507-2527. (doi:10.1175/JPO-D-19-0266.1)
4. Gutierrez-Villanueva, M. O., T. K. Chereskin, J. Sprintall, and J. A. Goff, 2022. Turbulent mixing and lee-wave radiation in Drake Passage: Sensitivity to topography. Journal of Geophysical Research-Oceans, 127. (doi:10.1029/2021JC018103)
5. Gutierrez-Villanueva, M. O., T. K. Chereskin, and J. Sprintall, 2023. Compensating transport trends in the Drake Passage frontal regions yield no acceleration in net transport. Nature Communications 14, 7792 (doi:10.1038/s41467-023-43499-2)