The Polar Front in Drake Passage: A composite‐mean stream‐coordinate view

The Polar Front (PF) is studied using 4 years of data collected by a line of current‐ and pressure‐recording inverted echo sounders in Drake Passage complemented with satellite altimetry. The location of the PF is bimodal in latitude. A northern and southern PF exist at separate times, separated geographically by a seafloor ridge—the Shackleton Fracture Zone—and hydrographically by 17 cm of geopotential height. Expressed in stream coordinates, vertical structures of buoyancy are determined with a gravest empirical mode analysis. Baroclinic velocity referenced to zero at 3500 dbar, width, and full transport (about 70 Sv) of the jets are statistically indistinguishable; the two jets alternate carrying the baroclinic transport rather than coexisting. Influences of local bathymetry and deep cyclogenesis manifest as differences in deep reference velocity structures. Downstream reference velocities of the PF‐N and PF‐S reach maximum speeds of 0.09 and 0.06 m s−1, respectively. Buoyancy fields are indicative of upwelling and poleward residual circulation at the PF. Based on potential vorticity and mixing lengths, the northern and southern PF both act as a barrier to cross‐frontal exchange while remaining susceptible to baroclinic instability

Eddy Heat Flux Across the Antarctic Circumpolar Current Estimated from Sea Surface Height Standard Deviation

Eddy heat flux (EHF) is a predominant mechanism for heat transport across the zonally unbounded mean flow of the Antarctic Circumpolar Current (ACC). Observations of dynamically relevant, divergent, 4 year mean EHF in Drake Passage from the cDrake project, as well as previous studies of atmospheric and oceanic storm tracks, motivates the use of sea surface height (SSH) standard deviation, H*, as a proxy for depth‐integrated, downgradient, time‐mean EHF ( ) in the ACC. Statistics from the Southern Ocean State Estimate corroborate this choice and validate throughout the ACC the spatial agreement between H* and seen locally in Drake Passage. Eight regions of elevated are identified from nearly 23.5 years of satellite altimetry data. Elevated cross‐front exchange usually does not span the full latitudinal width of the ACC in each region, implying a hand‐off of heat between ACC fronts and frontal zones as they encounter the different hot spots along their circumpolar path. Integrated along circumpolar streamlines, defined by mean SSH contours, there is a convergence of in the ACC: 1.06 PW enters from the north and 0.02 PW exits to the south. Temporal trends in low‐frequency [EHF] are calculated in a running‐mean sense using H* from overlapping 4 year subsets of SSH. Significant increases in downgradient [EHF] magnitude have occurred since 1993 at Kerguelen Plateau, Southeast Indian Ridge, and the Brazil‐Malvinas Confluence, whereas the other five hot spots have insignificant trends of varying sign

​​Observing Antarctic Bottom Water in the Southern Ocean​

Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system

Recent recovery of Antarctic Bottom Water formation in the Ross Sea driven by climate anomalies

Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation, ventilates the abyssal ocean and sequesters heat and carbon on multidecadal to millennial timescales. AABW originates on the Antarctic continental shelf, where strong winter cooling and brine released during sea ice formation produce Dense Shelf Water, which sinks to the deep ocean. The salinity, density and volume of AABW have decreased over the last 50 years, with the most marked changes observed in the Ross Sea. These changes have been attributed to increased melting of the Antarctic Ice Sheet. Here we use in situ observations to document a recovery in the salinity, density and thickness (that is, depth range) of AABW formed in the Ross Sea, with properties in 2018–2019 similar to those observed in the 1990s. The recovery was caused by increased sea ice formation on the continental shelf. Increased sea ice formation was triggered by anomalous wind forcing associated with the unusual combination of positive Southern Annular Mode and extreme El Niño conditions between 2015 and 2018. Our study highlights the sensitivity of AABW formation to remote forcing and shows that climate anomalies can drive episodic increases in local sea ice formation that counter the tendency for increased ice-sheet melt to reduce AABW formation