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1. Upper-Ocean Eddy Heat Flux across the Antarctic Circumpolar Current in Drake Passage from Observations: Time-Mean and Seasonal Variability

   https://doi.org/10.1175/jpo-d-19-0266.1  

   Gutierrez-Villanueva, Manuel O.; Chereskin, Teresa K.; Sprintall, Janet (September 2020, Journal of Physical Oceanography)

   null (Ed.)

   Abstract Eddy heat flux plays a fundamental role in the Southern Ocean meridional overturning circulation, providing the only mechanism for poleward heat transport above the topography and below the Ekman layer at the latitudes of Drake Passage. Models and observations identify Drake Passage as one of a handful of hot spots in the Southern Ocean where eddy heat transport across the Antarctic Circumpolar Current (ACC) is enhanced. Quantifying this transport, however, together with its spatial distribution and temporal variability, remains an open question. This study quantifies eddy heat flux as a function of ACC streamlines using a unique 20-yr time series of upper-ocean temperature and velocity transects with unprecedented horizontal resolution. Eddy heat flux is calculated using both time-mean and time-varying streamlines to isolate the dynamically important across-ACC heat flux component. The time-varying streamlines provide the best estimate of the across-ACC component because they track the shifting and meandering of the ACC fronts. The depth-integrated (0–900 m) across-stream eddy heat flux is maximum poleward in the south flank of the Subantarctic Front (−0.10 ± 0.05 GW m −1 ) and decreases toward the south, becoming statistically insignificant in the Polar Front, indicating heat convergence south of the Subantarctic Front. The time series provides an uncommon opportunity to explore the seasonal cycle of eddy heat flux. Poleward eddy heat flux in the Polar Front Zone is enhanced during austral autumn–winter, suggesting a seasonal variation in eddy-driven upwelling and thus the meridional overturning circulation. 

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2. What Controls the Partition between the Cold and Warm Routes in the Meridional Overturning Circulation?

   https://doi.org/10.1175/JPO-D-21-0308.1  

   Rousselet, Louise; Cessi, Paola; Mazloff, Matthew R. (January 2023, Journal of Physical Oceanography)

   Abstract The origins of the upper limb of the Atlantic meridional overturning circulation and the partition among different routes has been quantified with models at eddy-permitting and one eddy-resolving model or with low-resolution models assimilating observations. Here, a step toward bridging this gap is taken by using the Southern Ocean State Estimate (SOSE) at the eddy-permitting 1/6° horizontal resolution to compute Lagrangian diagnostics from virtual particle trajectories advected between 6.7°S and two meridional sections: one at Drake Passage (cold route) and the other from South Africa to Antarctica (warm route). Our results agree with the prevailing concept attributing the largest transport contribution to the warm route with 12.3 Sv (88%) (1 Sv ≡ 10 6 m 3 s −1 ) compared with 1.7 Sv (12%) for the cold route. These results are compared with a similar Lagrangian experiment performed with the lower-resolution state estimate from Estimating the Circulation and Climate of the Ocean. Eulerian and Lagrangian means highlight an overall increase in the transport of the major South Atlantic currents with finer resolution, resulting in a relatively larger contribution from the cold route. In particular, the Malvinas Current to Antarctic Circumpolar Current (MC/ACC) ratio plays a more important role on the routes partition than the increased Agulhas Leakage. The relative influence of the mean flow versus the eddy flow on the routes partition is investigated by computing the mean and eddy kinetic energies and the Lagrangian-based eddy diffusivity. Lagrangian diffusivity estimates are largest in the Agulhas and Malvinas regions but advection by the mean flow dominates everywhere. 

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3. The Impact of Parameterized Lateral Mixing on the Antarctic Circumpolar Current in a Coupled Climate Model

   https://doi.org/10.1175/JPO-D-19-0249.1  

   Ragen, Sarah; Pradal, Marie-Aude; Gnanadesikan, Anand (April 2020, Journal of Physical Oceanography)

   This study examines the impact of changing the lateral diffusion coefficient A Redi on the transport of the Antarctic Circumpolar Current (ACC). The lateral diffusion coefficient A Redi is poorly constrained, with values ranging across an order of magnitude in climate models. The ACC is difficult to accurately simulate, and there is a large spread in eastward transport in the Southern Ocean (SO) in these models. This paper examines how much of that spread can be attributed to different eddy parameterization coefficients. A coarse-resolution, fully coupled model suite was run with A Redi = 400, 800, 1200, and 2400 m 2 s −1 . Additionally, two simulations were run with two-dimensional representations of the mixing coefficient based on satellite altimetry. Relative to the 400 m 2 s −1 case, the 2400 m 2 s −1 case exhibits 1) an 11% decrease in average wind stress from 50° to 65°S, 2) a 20% decrease in zonally averaged eastward transport in the SO, and 3) a 14% weaker transport through the Drake Passage. The decrease in transport is well explained by changes in the thermal current shear, largely due to increases in ocean density occurring on the northern side of the ACC. In intermediate waters these increases are associated with changes in the formation of intermediate waters in the North Pacific. We hypothesize that the deep increases are associated with changes in the wind stress curl allowing Antarctic Bottom Water to escape and flow northward. 

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4. Expedition 383 Scientific Prospectus: Dynamics of the Pacific Antarctic Circumpolar Current (DYNAPACC)

   https://doi.org/10.14379/iodp.sp.383.2018  

   Lamy, F; Winckler, G; Alvarez_Zarikian, CA (August 2018, International Ocean Discovery Program)

   The Antarctic Circumpolar Current (ACC) is the world’s strongest zonal current system that connects all three major ocean basins of the global ocean and therefore integrates and responds to global climate variability. Its flow is largely driven by strong westerly winds and constricted to its narrowest extent in the Drake Passage. Transport of fresh and cold surface and intermediate water masses through the Drake Passage (cold-water route) strongly affect the Atlantic Meridional Overturning Circulation (AMOC) together with the inflow of Indian Ocean water masses (warm-water route). Both oceanographic corridors are critical for the South Atlantic contribution to AMOC changes. In contrast to the Atlantic and Indian sectors of the ACC, and with the exception of drill cores from the Antarctic continental margin and off New Zealand, deep-sea drilling records of the Pacific sector of the ACC lack information on its Cenozoic paleoceanography. To advance our knowledge and understanding of Plio-Pleistocene atmosphere-ocean-cryosphere dynamics in the Pacific and their implications for regional and global climate and atmospheric CO2, International Ocean Discovery Program Expedition 383 proposes the recovery of 180 to 500 m long high-resolution Plio-Pleistocene sediment sequences at (1) three primary sites located on a cross-frontal transect in the central South Pacific (CSP) between the modern Polar Front (Site CSP-3A) and the Subantarctic Zone (Sites CSP-1A and CSP-2B), (2) two sites (CHI-1C and CHI-4B) at the Chilean margin, and (3) one site from the pelagic eastern South Pacific (ESP; Site ESP-1A) close to the entrance to the Drake Passage. The planned sites represent a depth transect from ~1100 m at the Chilean margin (Site CHI-4B) to >5000 m in the Bellingshausen Sea (Site CSP-3A) that will allow investigation of Plio-Pleistocene changes in the vertical structure of the ACC—a key issue for understanding the role of the Southern Ocean in the global carbon cycle. All of the six primary and eight alternate sites were surveyed with seismic lines in 2009–2010 and most recently in 2016. The sites are located at latitudes and water depths where sediments will allow the application of a wide range of siliciclastic, carbonate, and opal-based proxies to address our objectives of reconstructing, with unprecedented stratigraphic detail, surface to deep ocean variations and their relation to atmosphere and cryosphere changes through stadial-to-interstadial, glacial-to-interglacial, and warmer-than-present time intervals. 

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5. Expedition 383 Scientific Prospectus: Dynamics of the Pacific Antarctic Circumpolar Current (DYNAPACC)

   https://doi.org/10.14379/iodp.sp.383add.2018  

   Lamy, F.; Winckler, G.; Alvarez Zarikian, C.A. (August 2018, Scientific prospectus)

   null (Ed.)

   The Antarctic Circumpolar Current (ACC) is the world’s strongest zonal current system that connects all three major ocean basins of the global ocean and therefore integrates and responds to global climate variability. Its flow is largely driven by strong westerly winds and constricted to its narrowest extent in the Drake Passage. Transport of fresh and cold surface and intermediate water masses through the Drake Passage (cold-water route) strongly affect the Atlantic Meridional Overturning Circulation (AMOC) together with the inflow of Indian Ocean water masses (warm-water route). Both oceanographic corridors are critical for the South Atlantic contribution to AMOC changes. In contrast to the Atlantic and Indian sectors of the ACC, and with the exception of drill cores from the Antarctic continental margin and off New Zealand, deep-sea drilling records of the Pacific sector of the ACC lack information on its Cenozoic paleoceanography. To advance our knowledge and understanding of Plio-Pleistocene atmosphere-ocean-cryosphere dynamics in the Pacific and their implications for regional and global climate and atmospheric CO2, International Ocean Discovery Program Expedition 383 proposes the recovery of 180 to 500 m long high-resolution Plio-Pleistocene sediment sequences at (1) three primary sites located on a cross-frontal transect in the central South Pacific (CSP) between the modern Polar Front (Site CSP-3A) and the Subantarctic Zone (Sites CSP-1A and CSP-2B), (2) two sites (CHI-1C and CHI-4B) at the Chilean margin, and (3) one site from the pelagic eastern South Pacific (ESP; Site ESP-1A) close to the entrance to the Drake Passage. The planned sites represent a depth transect from ~1100 m at the Chilean margin (Site CHI-4B) to >5000 m in the Bellingshausen Sea (Site CSP-3A) that will allow investigation of Plio-Pleistocene changes in the vertical structure of the ACC—a key issue for understanding the role of the Southern Ocean in the global carbon cycle. All of the six primary and eight alternate sites were surveyed with seismic lines in 2009–2010 and most recently in 2016. The sites are located at latitudes and water depths where sediments will allow the application of a wide range of siliciclastic, carbonate, and opal-based proxies to address our objectives of reconstructing, with unprecedented stratigraphic detail, surface to deep ocean variations and their relation to atmosphere and cryosphere changes through stadial-to-interstadial, glacial-to-interglacial, and warmer-than-present time intervals. 

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