and on the other hand, along the whole continental slope of South America as quickly propagating coastal trapped waves-like features.

The intraseasonal BT variability in the Yaghan Basin is not generated and advected from the exterior but is developed within the Yaghan Basin where it eventually dissipates without propagating outside of the basin. It is a basin mode (Koenig et al., 2015). More work is needed to get a deeper insight of the generating mechanism of the 50-day BT signal.

Aside from moderate annual to biannual signals, the interannual transport variability at DP is completely overwhelmed by the prevalent intraseasonal variability, at least during the last two decades examined here. We did not find any systematic relationship between climatic indices such as SAM or ENSO and the DP transport variability, but their co-variability depends on a particular epoch and periodicity in question. The relatively short data length (20 years) prevents documenting any decadal-scale variability of the DP transport and its response to concomitant atmospheric forcing. Much longer transport data are needed to address these questions.

## Meridional heat flux across the ACC: mean flow and eddy contributions

Point measurements from current meter moorings provide only unit-dependent mean temperature flux estimates and not meaningful estimates of the heat flux due to the mean flow because the zero on the temperature scale is arbitrary. For a proper estimate of heat flux due to the mean flow, the individual mean temperature fluxes need to be summed over a mass-balanced region. To handle this major constraint of integrating estimates over a mass-balanced region, we use both mooring data and outputs of a high-resolution three-dimensional numerical model of the ocean circulation with realistic topography (Fig. 3).

Thus, the contributions of the eddy flow and mean flow to the across-stream heat fluxes in DP were calculated using two in situ mooring data sets in DP and MERCATOR model outputs. The two in situ mooring data sets, the year-long historical DRAKE 1979 data and the 3 year-long DRAKE 2006–2009 data, span the entire DP from north to south and are located in each side of a major submarine ridge, the SFZ (Fig. 3).

<figure><img src="image_71.png" /></figure>

**Figure 3:** 3 year (2006-2009) mean transport streamfunction from Mercator model outputs. Streamlines between -85 and +40 m^4 s^(-1) are plotted in black solid lines with a 5m^4 s^(-1) step. Major branches of the SAF, PF and SACCF are identified with a green, red, and blue solid line, respectively. Black dots east of the SFZ correspond to the Drake 79 moorings, those west of SFZ to Drake 2006-2009. Note the mean cyclonic recirculation between the SAF and PF.
Model eddy heat fluxes compared satisfactorily with estimates from in situ moorings, both in amplitude and direction. They were mostly poleward except for the central Yaghan Basin, in the cyclonic recirculation region. Model eddy heat fluxes are of the same sign and larger in the upper layer than below 500m. The model vertically integrated eddy heat fluxes exhibit scales compatible with the size of mesoscale eddy activity. The model provides reasonable eddy heat fluxes at the mooring locations, showing a maximum between the SAF and PF and then decreasing southward. To make a large-scale budget, we relied on the model estimates. The order of magnitude of the poleward eddy heat flux in DP at 60°W is in the order of 2% of the heat loss to the atmosphere south of 60°S, whereas DP (between 48°W and 64°W) covers about 4% of the circumpolar longitudes and is a region of the ACC with a large EKE.

Cross-stream temperature fluxes due to the mean flow show both positive and negative significant values at the mooring sites. Downstream of the SFZ, all the significant fluxes are positive, i.e., equatorward, in agreement with the general downwelling observed in the Yaghan and Ona Basins [Ferrari et al., 2012, 2013]. However, in the area upstream of the SFZ, the distribution of equatorward and poleward fluxes is more complex. Model-derived temperature fluxes are of the same order of magnitude and when significant have the same sign as estimates from in situ observations with an 86% agreement. Model-derived depth-integrated temperature fluxes by the mean flow have values between +300 and -300 kW m⁻² and exhibit small spatial scales with alternating signs. These small-scale patterns are similar to those of bottom vertical velocity estimates having opposite signs. The rotation of the mean velocity vectors with depth is mainly due to non-zero currents over sloping topography. The rough hilly topography in DP leads to the small-scale vertical velocities and temperature fluxes. In contrast to the eddy heat flux, the contribution of the upper layer (0–500 m) is not significant for the heat flux due to the mean flow.

The integrated values along the mass-balanced regions delimited by different fronts in DP are all found to be poleward, with a maximum in the region between the SAF and PF for both eddy and mean flow contributions to the meridional heat flux. The contribution of the mean flow is larger than the eddy contribution for any of the mass-balanced regions, the ratio between the two varying between 8.6 (in the SAF region) and 2.3 (in the SACCF-PF region). The model-estimated poleward heat flux across the SACCF in DP (covering about 4% of the circumpolar longitudes between 48°W and 64°W) is 20.055 PW (eddy contribution: 20.01, mean flow contribution 20.045 PW), i.e., in the order of 10% of the heat lost to the atmosphere south of 60°S. In other words, the across-stream heat flux in DP has a contribution from the mean flow of about 80% and the eddy flow for the remaining 20%. It plays a significant role for the circumpolar poleward heat flux despite its limited longitudinal extent.

References:

Provost C., A. Renault, N. Barré, N. Sennéchael, V. Garçon, J. Sudre, and O. Huhn, 2011. Two repeat crossings of Drake Passage in austral summer 2006: short term variations and evidence for considerable ventilation of intermediate and deep waters. *Deep-Sea Res. II*, issue 25-26, pp. 2555-2571, doi:10.1016/j.dsr2.2011.06.009.

Renault A., C. Provost, N. Sennéchael, N. Barré, and A. Kartavtseff, 2011. Two full-depth velocity sections in the Drake Passage in 2006-Transport estimates. *Deep-Sea Res. II*, 58, issue 25-26, pp. 2572-2591, doi:10.1016/j.dsr2.2011.01.004.

Barré N., C. Provost, A. Renault, and N. Sennéchael, 2011. Fronts, meanders and eddies in Drake Passage during the ANT-XXIII/3 cruise in January-February 2006: A satellite perspective. *Deep-Sea Res. II*, 58, issue 25-26, pp. 2533-2554, doi:10.1016/j.dsr2.2011.01.003.

Barré N., C. Provost, A. Renault, and N. Sennéchael, 2011. Fronts, meanders and eddies in Drake Passage during the ANT-XXIII/3 cruise in January-February 2006: A satellite perspective. *Deep-Sea Res. II*, 58, issue 25-26, pp. 2533-2554, doi:10.1016/j.dsr2.2011.01.003.

Ferrari R., C. Provost, A. Renault, N. Sennéchael, N. Barré, Y-H. Park and J.H. Lee, 2012: Circulation
in Drake Passage revisited using new current time-series and satellite altimetry. Part I: The
Yaghan Basin. J. Geophys. Res. doi:10.1029/2012JC008264.

Ferrari R., C. Provost, A. Renault, N. Barré, N. Sennéchael, Y.H Park and J-H Lee, 2013: Circulation in Drake Passage revisited using new current time-series and satellite altimetry. Part II: The Ona Basin J. Geophys. Res. doi:10.1029/2012JC008193.

Ferrari R., C. Provost, Y-H Park, N. Sennéchael, Z. Koenig, H. Sekma, G. Garric and R. Bourdallé-Badie 2014: Heat fluxes across the Antarctic Circumpolar Current in Drake Passage: Mean flow and eddy contributions. J. Geophys. Res., 119, DOI: 10.1002/2014JC010201

Koenig Z., C. Provost, R. Ferrari, N. Sennéchael and M-H Rio 2014: Volume transport of the Antarctic Circumpolar Current: Production and validation of a 20 year long time series obtained from in situ and satellite observations, J. Geophys. Res. 119, DOI: 10.1002/2014JC009966

Koenig Z., C. Provost, Y.H. Park, R. Ferrari, N. Sennéchael, Anatomy of the Antarctic Circumpolar Current volume transports through Drake Passage J. Geophys., Res. (sub judice).
# Direct observations of the ACC transport across the Kerguelen Plateau

Young-Hyang Park and Frédéric Vivier

LOCEAN, UMR 7159, CNRS/UPMC/MNHN/IRD, Université Pierre Marie Curie, Paris, France

The Kerguelen Plateau is a major topographic obstacle to the eastward flowing Antarctic Circumpolar Current (ACC). Whilst approximately two thirds of the ACC transport is diverted to the north of the Kerguelen Islands, most of the remaining flow engulfs in the Fawn Trough, the only deep passage across the plateau. As part of the TRACK (TRansport ACross the Kerguelen plateau) project, three mooring lines of current meters were deployed in the Fawn Trough for one year in February 2009 and recovered in January 2010, underneath ground-track 94 of the Jason-2 satellite altimeter. Full depth CTD-LADCP casts across and in the vicinity of the trough were carried out during the deployment and recovery cruises (Fig. 1b).

The TRACK constituted the first direct observations of the ACC transport crossing the Kerguelen Plateau (Park et al., 2009). The 2009 TRACK1 cruise data showed that the net eastward transport to the south of the Heard/McDonald Islands is 56 Sv (1 Sv = $10^6 m^3 s^{-1}$), most of which (43 Sv) being tightly channeled into the Fawn Trough, which appears as a predominant cross-plateau gateway of circumpolar flow associated with the Southern ACC Front (SACCF), as shown by a schematic of the major branches of the ACC transport transiting through the Kerguelen Plateau region (Fig. 1a). There are also two secondary passages, with 6 Sv being attached to the nearshore slope just south of the Heard/McDonald Islands and 7 Sv passing through the northern Princess Elizabeth Trough. With an additional 2 Sv inferred just south of the Kerguelen Islands, the transport across the entire plateau amounts to 58 Sv, accounting for about 40% of the total ACC transport (147-152 Sv) passing across the Kerguelen longitudes. Here, the dominant branch of ACC (~90 Sv) flows along the Subantarctic Front (SAF) north of the Kerguelen Islands.

<figure><img src="image_72.png" /><figcaption>**Figure 1.** (a) Schematic of major pathways and transports (in Sv) of the ACC system (bold solid lines) and the deep western boundary current of the Australian-Antarctic Gyre (bold dashed lines). (b) The 2009 TRACK cruise plan, with three moorings (M1, M2, M3) in the Fawn Trough and an array of CTD stations (black dots). Adapted from Park et al. (2009).</figcaption></figure>

Based on the one-year-long current meter mooring data, a time series of the transport in the Fawn Trough has been estimated (Vivier et al., 2014), featuring a mean eastward transport of 34 Sv (possibly biased low by at most 5 Sv) and a root mean squared variability of 6 Sv, consistent with
LADCP estimates (43 Sv in February 2009 and 38 Sv in January 2010). In addition, we have analyzed to what extent the transport can be directly monitored from along-track satellite altimeter data, which would enable study of the variability of the Fawn Trough Current centered at the SACCF from a now 20-year long archive. While a canonical method to derive transport from altimetry gives here unsatisfactory comparisons with in situ estimates, an ad-hoc approach using only the two northernmost mooring lines yields an estimate well correlated ($r \approx 0.8$) with in situ transport at subseasonal time scales during the one year period of observations. At interannual time scales, however, both methods provide significantly correlated ($r \approx 0.7$) transport estimates, suggesting that long-term transport fluctuations across the Kerguelen Plateau can be confidently estimated from altimetry (Fig. 2). These consistently indicate a measurable impact of the outstanding 1997-98 El Niño-Southern Oscillation (ENSO) event, yielding an increase of the annual mean transport of $\approx 3$ Sv, possibly with a one year lag. The transport estimate based on the ad-hoc approach is significantly correlated ($r \approx 0.6$) with the Southern Annular Mode (SAM) index at interannual time scales, suggesting that an intensification of the circumpolar winds drives an increase in the transport across the Kerguelen Plateau.

<figure><img src="image_73.png" /><figcaption>**Figure 2.** Normalized yearly averaged volume transport time series at Fawn Trough estimated from two different methods based on altimetry: canonical approach using all three moorings (black solid) vs ad hoc approach using the two northernmost moorings (back dashed). Also superimposed are the ENSO index (red) and SAM index (blue). From Vivier et al. (2014).</figcaption></figure>

Finally, the 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 ACC is mainly due to mesoscale transient eddies and the cross-stream heat flux by time-mean flow is insignificant. This long standing paradigm has been challenged by the TRACK observations. In fact, the moored current meter data in the Fawn Trough permitted us to estimate the cross-stream heat flux caused not only by transient eddies but also, for the first time, by the time-mean flow (Sekma et al., 2013). It has been 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 (Table 1). This is related to the existence of the nonequivalent barotropic structure of the mean flow, which arises from the unusual anticlockwise turning of currents with decreasing depth. The latter feature is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the Fawn Trough. The circumpolar implications of these local observations are
discussed in terms of the depth-integrated linear vorticity budget from the FRAM model, 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.

<table><caption>**Table 1.** Estimates of heat flux (kW m⁻²) on the three moorings in the Fawn Trough. Significant estimates at the 95% confidence level are shown boldfaced. Error bars for the mean heat flux are given as ± the standard error. From Sekma et al. (2013).</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Pressure<br>(db)</th><th colspan="3">Eddy Heat Flux</th><th>Mean Heat Flux</th></tr><tr><th>Geographic</th><th>90d-<br>LP</th><th>Shear</th><th>Shear</th></tr></thead><tbody><tr><td rowspan="4">M1</td><td>281</td><td>0.10</td><td>0.21</td><td>0.24</td><td><strong>-84.8 ±24.7</strong></td></tr><tr><td>981</td><td>-0.23</td><td>-0.25</td><td><strong>1.61</strong></td><td><strong>-489.0 ±18.5</strong></td></tr><tr><td>1454</td><td>-0.35</td><td><strong>-0.39</strong></td><td>-0.71</td><td><strong>164.9 ±31.4</strong></td></tr><tr><td>1950</td><td>-0.22</td><td>-0.20</td><td></td><td></td></tr><tr><td rowspan="4">M2</td><td>362</td><td><strong>-2.35</strong></td><td><strong>-2.38</strong></td><td>1.42</td><td>-248.4 ±138.5</td></tr><tr><td>1026</td><td>1.03</td><td>0.82</td><td>-3.50</td><td>-211.4 ±178.9</td></tr><tr><td>1510</td><td>0.41</td><td>0.38</td><td>0.32</td><td><strong>-611.7 ±12.2</strong></td></tr><tr><td>2311</td><td>-0.68</td><td>-0.69</td><td></td><td></td></tr><tr><td rowspan="4">M3</td><td>532</td><td>-0.58</td><td>-0.65</td><td><strong>-2.40</strong></td><td><strong>-368.7 ±27.1</strong></td></tr><tr><td>1228</td><td>-1.80</td><td>-1.81</td><td>2.28</td><td><strong>-141.6 ±56.9</strong></td></tr><tr><td>1732</td><td>1.52</td><td>1.55</td><td><strong>-3.72</strong></td><td><strong>-260.1 ±15.9</strong></td></tr><tr><td>2372</td><td><strong>1.33</strong></td><td><strong>1.31</strong></td><td></td><td></td></tr><tr><td>Depth mean</td><td></td><td>-0.09</td><td>-0.12</td><td>-0.52</td><td><strong>-218.6 ±22.8</strong></td></tr></tbody></table>

References

Park Y.-H, F. Vivier, F. Roquet, and E. Kestenare (2009): Direct observation of the ACC transport across the Kerguelen Plateau. *Geophys. Res. Lett.*, 36, doi:10.1029/2009GL039617.

Sekma H., Y.-H. Park, and F. Vivier (2013): 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. J. Phys. Oceanogr.*, 583-601.

Vivier, F., Y.-H. Park, H. Sekma, J. Le Sommer (2014): Variability of the ACC transport across the Kerguelen Plateau, *Deep-Sea Res. II*, http://dx.doi.org/10.1016/j.dsr2.2014.01.017.