Poleward heat flux across the southern flank of the Antarctic Circumpolar Current (a case study in the Fawn Trough Kerguelen Plateau)

Les principaux mécanismes transportant la chaleur vers le pôle dans l'Océan Austral (OA) font encore l objet d un débat. L idée admise stipule que le flux de chaleur vers le pôle à travers le Courant Circumpolaire Antarctique (CCA) est principalement dû aux tourbillons à méso-échelle, et que le flux de chaleur par le courant moyen est négligeable. Le flux de chaleur vers le pôle par ces deux méthodes a été estimé au niveau du passage Fawn Trough (FT), Plateau de Kerguelen. Il a été montré que le flux de chaleur tourbillonnaire dans la partie sud de l OA est négligeable, tandis que celui par courant moyen le dépasse par deux ordres de grandeur. Les résultats suggèrent un nouveau mécanisme de transfert de chaleur par le courant moyen à travers le front sud du CCA. Ceci est dû à la rotation inhabituelle des courants, dans le sens inverse des aiguilles d une montre, avec la profondeur décroissante. Cette rotation est associée à une remontée importante des eaux, engendrée par des forts courants de fond qui s'écoulent à travers la topographie en pente du passage du FT. Les implications circumpolaires de ces observations locales sont discutées en termes de bilan de vorticité intégré sur la verticale, qui suggère que les six accidents topographiques le long du flanc sud du CCA transporteraient suffisamment de chaleur vers le pôle, afin d'équilibrer la perte océanique de chaleur dans la région subpolaire. Comme l'activité tourbillonnaire sur le flanc sud du CCA est trop faible pour transporter suffisamment de chaleur vers le pôle, la structure non-équivalente barotrope de l'écoulement moyen dans les passages topographiquement resserrés devrait suffire à accomplir la tâche requise.The major mechanisms of the poleward oceanic heat flux in the Southern Ocean (SO) are still in debate. The long standing belief stipulates that the poleward heat flux across the Antarctic Circumpolar Current (ACC) is mainly due to mesoscale transient eddies, and the cross-stream heat flux by time-mean flow is insignificant. The poleward heat flux due to the time-mean flow and to transient eddies is estimated across the the Southern ACC Front (SACCF) at the Fawn Trough (FT) Passage, Kerguelen Plateau, and the impact of each mechanism on the global heat balance in the SO is analysed. It is shown that the eddy heat flux in this southern part of the ACC is negligible, while that due to the mean flow is overwhelming by two orders of magnitude. Results suggest then a new mechanism of the cross-stream poleward heat flux by time-mean flow across the southern ACC front. This is due to the unusual anticlockwise turning of currents with decreasing depth, which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough. The circumpolar implications of these local observations are discussed in terms of the depth-integrated linear vorticity budget, which suggests that the six topographic features along the southern flank of the ACC equivalent to the Fawn Trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region. As eddy activity on the southern flank of the ACC is too weak to transport sufficient heat poleward, the non-equivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Time-Mean Flow as the Prevailing Contribution to the Poleward Heat Flux across the Southern Flank of the Antarctic Circumpolar Current: A Case Study in the Fawn Trough, Kerguelen Plateau

International audienceThe major mechanisms of the oceanic poleward heat flux in the Southern Ocean are still in debate. The long-standing belief stipulates that the poleward heat flux across the Antarctic Circumpolar Current (ACC) is mainly due to mesoscale transient eddies and the cross-stream heat flux by time-mean flow is insignificant. This belief has recently been challenged by several numerical modeling studies, which stress the importance of mean flow for the meridional heat flux in the Southern Ocean. Here, this study analyzes moored current meter data obtained recently in the Fawn Trough, Kerguelen Plateau, to estimate the cross-stream heat flux caused by the time-mean flow and transient eddies. It is shown that the poleward eddy heat flux in this southern part of the ACC is negligible, while that from the mean flow is overwhelming by two orders of magnitude. This is due to the unusual anticlockwise turning of currents with decreasing depth, which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough. The circumpolar implications of these local observations are discussed in terms of the depth-integrated linear vorticity budget, which suggests that the six topographic features along the southern flank of the ACC equivalent to the Fawn Trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region. As eddy activity on the southern flank of the ACC is too weak to transport sufficient heat poleward, the nonequivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task

Variability of the Antarctic Circumpolar Current transport through the Fawn Trough, Kerguelen Plateau

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Heat fluxes across the Antarctic Circumpolar Current in Drake Passage: Mean flow and eddy contributions

International audienceIn contrast to a long-standing belief, observations in the Antarctic Circumpolar Current (ACC) show that mean velocity vectors rotate with depth, thus suggesting a possible importance of the time-mean flow for the local poleward heat transport. The respective contributions of the eddy and mean flows to the heat flux across the ACC in Drake Passage (DP) are investigated using recently acquired and historical time series of velocity and temperature from a total of 24 current meter moorings and outputs of a high-resolution (1/12°) model with realistic topography. Only 11 out of the 24 depth-integrated eddy heat flux estimates are found to be significant, and they are poleward. Model depth-integrated eddy heat fluxes have similar signs and amplitudes as the in situ estimates at the mooring sites. They are mostly poleward or nonsignificant, with amplitude decreasing to the south. The cross-stream temperature fluxes caused by the mean flow at the moorings have a sign that varies with location and corresponds to the opposite of the vertical velocity estimates. The depth-integrated temperature fluxes due to the mean flow in the model exhibit small spatial scales and are of opposite sign to the bottom vertical velocities. This suggests that the rotation of the mean velocity vectors with depth is mainly due to bottom topography. The rough hilly topography in DP likely promotes the small-scale vertical velocities and temperature fluxes. Eddy heat fluxes and cross-stream temperature fluxes are integrated over mass-balanced regions defined by the model transport streamlines. The contribution of the mean flow to the ocean heat fluxes across the Southern ACC Front in DP (covering about 4% of the circumpolar longitudes) is about four times as large as the eddy heat flux contribution and the sum of the two represent on the order of 10% of the heat loss to the atmosphere south of 60°S