Variability of the ACC transport across the Kerguelen plateau from current meter data and altimetry: relation with the wind forcing.

Frederic Vivier (CNRS/ LOCEAN-IPSL, France)

CoAuthors

Young-Hyang Park (MNHN / LOCEAN-IPSL, France); Wilbert Weijer (Los Alamos National Laboratory, USA); Julien Le Sommer (CNRS / MEOM-LGGE, France); Hela Sekma (LEGOS, France)

Event: 2014 Ocean Surface Topography Science Team Meeting

Session: Science Results from Satellite Altimetry: Regional and basin-scale processes and sea level rise

Presentation type: Type Poster

The Antarctic Circumpolar Current (ACC) plays a major climatic role. Changes in its strength or a meridional shift of its pathway associated with an increase of the westerlies driven by the Southern Annular Mode (SAM) may greatly impact water mass transformations in the Southern Ocean, and thus the meridional overturning circulation.
There is therefore a great motivation to improve our knowledge of the variability of this current which requires to enhance our capacity to monitor it. While the monitoring of the ACC has been undertaken for many years at different choke points on the ACC pathway (SR3 WOCE section south of Tasmania, south of Africa or Drake Passage), there are vast areas where even the ACC circulation pathway and mean transport of associated branches is still poorly known.
This is in particular the case for the Indian sector of the Southern Ocean, where the Kerguelen Plateau stands out as a major obstacle on the pathway of the ACC. About two thirds (~90 Sv) of the total ACC transport are diverted to the north of the plateau, with the remaining transport (~50 Sv) constrained to cross the large region between Kerguelen Islands and Antarctica, which spans ~2000km. The main candidate routes is the Fawn Trough, a deep passage reaching 2800 m depth, cutting the Kerguelen Plateau at 56° S. Satellite and hydrographic observations, in particular from instrumented elephant seals, have provided a bundle of evidence that waters from the Enderby Basin are primarily funneled into the Fawn Trough . The existence of a major branch of the ACC at this location was confirmed and quantified from the dedicated TRACK program (TRansport ACross the Kerguelen plateau; Park et al 2009) which included in particular the deployment of 3 current meter moorings across the passage underneath groundtrack #94 of the Jason-2 satellite altimeter for nearly one year.

We have estimated the variability of the ACC transport across the Kerguelen Plateau from this set of current meter data, but also derived an estimate of the transport based on altimetry. Our analyses show that interannual transport fluctuations of this major branch of the ACC can be confidently estimated from the continuing altimeter time series suggesting that this Fawn Trough section might be a relevant addition to the Southern Ocean Observing System.
Based on 20 year long (1992-2012) series of transport of the Fawn Trough Current reconstructed from the altimeter archive, we analyze mechanisms driving the variability of the current at different time scales. These include the barotropic variability at infra seasonal scales investigated in particular from a shallow water model, while the relationship with dominant climate modes of the southern hemisphere is examined for interannual time scales.
 
Frederic Vivier
CNRS/ LOCEAN-IPSL
France
frederic.vivier@locean-ipsl.upmc.fr