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PHANTOM II

PHANTOM II
(upper) Three major choke points where the southern limb of the ACC is strongly guided by submarine topography to pass through deep troughs cut across prominent ridge systems, such as in the regions of Drake Passage, the Kerguelen Plateau and the Udintsev Fracture Zone (UFZ)/Eltanin Fracture Zone (EFZ). During the previous PHANTOM I phase, the field observations combined with altimetry have been exploited in terms of the ACC transport monitoring and the poleward heat transport in the former two sites. The present PHANTOM II project concerns mainly the Udintsev Fracture Zone in terms of field observations, although its eventual implications concern the peri-Antarctic evaluation of the net poleward heat flux via high-resolution numerical model outputs from MERCATOR (G. Garric) and the assessment of the inter-choke point linkage of the ACC transport variability. (bottom) A detailed altimeter-derived time-mean surface velocity filed reveals the Udintsev choke point as an extraordinary site where the ACC is most narrowly concentrated (< 2° in latitude at the entrance of the UFZ) in its whole circumpolar path. We have conducted in February 2016 an oceangraphic cruise in the latter site onboard the Korean Icebreaker ARAON, with a series of CTD casts made along the Jason ground track 219 and three lines of current meter moorings installed near the Subantarctic Front (SAF), Polar Front (PF), and Southern ACC Front (SACCF), in collaboration with a Korean team from KIOST (J.-H. Lee). The recovery cruise is planned in January/February 2018 on the ARAON.

Author:

Christine Provost - (LOCEAN

Co-Principal Investigator(s):

Young-Hyang Park (LOCEAN)

Co-Investigator(s):

Jae-Hak Lee (KIOST)
Gilles Garric (MERCATOR)
Isabelle Pujol (CLS)
Isabelle Durand (LOCEAN)

Abstract:

A precise assessment of the poleward oceanic heat flux across the near-zonal strong jet of the Antarctic Circumpolar Current (ACC), which is indispensable for the closure of air-sea heat flux balance south of the ACC, forms a great challenge both in theory and observations. A long-standing and still most influential paradigm of the subject claims mesoscale eddies as the predominant contributor to the poleward heat flux, much the same as in the atmosphere. However, this paradigm has been seriously challenged by our recent works based on direct observations of currents across the Fawn Trough of the Kerguelen Plateau (Sekma et al., 2013) and in the Drake Passage (Ferrari et al., 2014). These have demonstrated clearly that the major contributor of the poleward heat flux at the southern limb of the ACC is not mesoscale eddies but the time-mean flow. This is a fresh challenge to the long-standing paradigm of eddy-driven poleward heat flux, although in situ observations are very limited and mostly concentrated close to the northern flank of the ACC, except for just-mentioned our two recent works. The outstanding objective of the present project is thus to contribute to an increasing effort for deploying current meter moorings together with CTD observations at critical choke points of the ACC. Under this prospective, we have recently conducted a current meter mooring cruise in the Udintsev Fracture Zone, South Pacific, on board the KOPRI-owned Korean Icebreaker ARAON in February 2016.

After the mooring recovery planned in January/February 2018, the current meter data will be analyzed to document the relative contribution of eddies vs. time-mean flow to the poleward heat flux in the Udintsev region. Although these one-point observations should be invaluable in face of the absolute paucity in actual current meter mooring situation in the Southern Ocean, they are far from sufficient for accessing the circumpolar-net poleward heat flux. For this purpose, the use of a high-resolution, realistic numerical model appears indispensable, as clearly demonstrated in Ferrari et al. (2014) who showed encouraging results in the Drake Passage region via outputs of the ORCA12 model developed by MERCATOR. We will extend a similar exercise over the entire circumpolar path.

As by-products, the present project aims also to address two more subjects using in situ observations and altimetry. The one is a precise determination of the local ACC structure by using the combined data of altimetry and hydrography, which should lay a foundation of a revised up-to-date map of the ACC fronts over the entire Southern Ocean. A preliminary result of this can be noticed in the here-joined figure. The other is the construction of a time series of the ACC transport at the Udintsev site by combination of the moored current meter data and altimetry, similarly to Koenig et al. (2016) in Drake Passage and Vivier et al. (2015) at Kerguelen. The resulting transport time series over the past 25-yr altimetric period at our three choke points will be inter-compared in order to document any linkage among them and to address any underlying mechanisms.