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Meanders formed where the Antarctic Circumpolar Current (ACC) interacts with topography have been identified as dynamical hot spots, characterized by enhanced eddy energy, momentum transfer, and cross-front exchange. However, few studies have used observations to diagnose the dynamics of ACC standing meanders. We use a synoptic hydrographic survey and satellite altimetry to explore the momentum and vorticity balance of a Subantarctic Front standing meander, downstream of the Southeast Indian Ridge. Along-stream anomalies of temperature in the upper ocean (150–600 m) show along-stream cooling entering the surface trough and along-stream warming entering the surface crest, while warming is observed from trough to crest in the deeper ocean (600–1500 m). Advection of relative vorticity is balanced by vortex stretching, as found in model studies of meandering currents. Meander curvature is sufficiently large that the flow is in gradient wind balance, resulting in ageostrophic horizontal divergence. This drives downwelling of cooler water along isopycnals entering the surface trough and upwelling of warmer water entering the surface crest, consistent with the observed evolution of temperature anomalies in the upper ocean. Progressive along-stream warming observed between 600 and 1500 m likely reflects cyclogenesis in the deep ocean. Vortex stretching couples the upper and lower water column, producing a low pressure at depth between surface trough and crest and cyclonic flow that carries cold water equatorward in the surface trough and warm water poleward in the surface crest (poleward heat flux). The results highlight gradient–wind balance and cyclogenesis as central to dynamics of standing meanders and their critical role in the ACC momentum and vorticity balance. Significance Statement: The Antarctic Circumpolar Current (ACC) in the Southern Ocean is a nearly zonal current that encircles Antarctica. It acts as a barrier between warmer water equatorward and colder water poleward. In a few regions where the current encounters strong topographic changes, the current meanders and opens a pathway for heat to travel across the ACC toward Antarctica. We surveyed a meander in the ACC and examined the along-stream change of temperature. In the upper ocean, temperature changes are caused by a vertical circulation, bringing cool water down when entering the surface trough (the part of the meander closest to the equator), and warm water up when exiting the surface trough and entering the surface crest. At depth, cold water is transported equatorward in the surface trough and warm water poleward in the surface crest, leading to a net transport of heat poleward. This study highlights the importance of the secondary circulation within a meander for generating cross-ACC flows and moving heat toward Antarctica. [ABSTRACT FROM AUTHOR]

Most theories of the Southern Ocean and more specifically the Antarctic Circumpolar Current(ACC) are based on zonally symmetric models that lack information about thecomplexity of the ACC’s frontal structure and the presence of several significanttopographic obstructions to the flow. When the current interacts with strong topographystanding meanders, or stationary Rossby waves, can occur. Standing meanderssteepen in response to a stronger current as a result of increasing wind stress upstreamthat steepen the isopycnals. This leads to instabilities in the flow generating moreeddies, that flatten the isopycnals, slow down the ACC and lead to stronger polewardheat fluxes. This has implications for warming of the waters around Antarctica,leading to melting of sea ice and basal melt of ice shelves. We surveyed a standingmeander in the Subantarctic Front downstream of the Southeast Indian Ridge with 11cross-frontal transects, comprising 99 CTD profiles to ∼ 1500 dbar, to examinethe alongstream change of watermass properties in the upper ocean. We describethe change in properties relative to gravest empirical mode (GEM) mean fields oftemperature and salinity. Over the ridge and upstream of the meander, streamlinesconverge speeding up the flow and merging inter-frontal zones. Into the trough, warmand salty anomalies relative to the GEM mean field are shown in the intermediatewaters (γn < 27.6 kgm−3) and cold and fresh anomalies in the upper deep waters(27.2 < γn < 27.8). In between the trough and the crest, the flow is deceleratedallowing for mixing of the frontal zones, which leads into the trough to warmer andsaltier anomalies over the water column down to ∼ 1500 dbar. The meridionalgradient in the temperature suggests a net poleward heat transport, also referred to as'eddy stirring’, and explains the warm anomalies in the upper water column inboth the trough and the crest of the meander. The cyclonic rotation of the troughsuggests upwelling, according to the 'eddy pump’ mechanism, and explains the coldanomaly in the lower water column, whereas the anticyclonic rotation suggestsdownwelling and explains the warm anomaly in the lower water column of the crest. [ABSTRACT FROM AUTHOR]