Low-frequency variability of Antarctic Circumpolar Current transport in the Pacific sector centered at the Udintsev/Eltanin Fracture Zones and concurrent atmospheric forcing

Young-Hyang Park (LOCEAN UMR 7159 SU/CNRS/IRD/MNHN, France)


Christine Provost (LOCEAN UMR 7159 SU/CNRS/IRD/MNHN, France); Isabelle Durand (LOCEAN UMR7159 , France)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Science II: Large Scale Ocean Circulation Variability and Change

Presentation type: Type Forum

Increasingly realistic high-resolution operational models assimilating satellite and in situ data tend to provide a mean volume transport of the Antarctic Circumpolar Current (ACC) through Drake Passage around 155 Sv, a value about 15-20 Sv larger than in situ observations or combined in situ/altimetric solutions (135-140 Sv).
We use here one of such a 1/12°-resolution altimetry-assimilating model from Mercator Ocean, GLORYS12 reanalysis, to estimate the ACC transport transiting through the Udintsev and Eltanin Fracture Zones where the ACC is the narrowest in the Southern Ocean. Comparison with recent current meter and hydrographic data along a meridional section across the Udintsev Fracture Zone at 144°W (Park et al., 2019) shows that GLORYS12 reproduces well the vertical structure of the ACC, with a significant temporal correlation in velocity between the model and observations, consistent with a recent validation of GLORYS12 at Drake Passage by Artana et al. (2019). Time series of monthly transport stream function were computed at each grid point in the central South Pacific region (45°-65°S, 110°-160°W) for 1993-2018 corresponding to the contemporaneous altimeter period. The climatological mean ACC transport in our Pacific region from GLORYS12 is 155 Sv, unbelievably exactly same as that at Drake Passage (Artana et al., 2019), suggesting that our computation of the 3D (x, y, t) transport stream function has no gross error, although the coincidence of the exactly same value might be fortuitous.
Empirical Orthogonal Function (EOF) analysis of the transport stream function reveals that the gravest mode (EOF1) is significantly anticorrelated with El Nino-Southern Oscillation (ENSO), meaning that the ACC transport in the Pacific sector increases (decreases) during the La Nina (El Nino) phase. EOF2 is best correlated with the Southern Annular Mode (SAM), the principal climate mode in the Southern Hemisphere extratropics. In the eastern South Pacific just upstream of Drake Passage eastward surface currents strengthens (weakens) during La Nina (El Nino) especially along the northern boundary of the ACC, while the SAM mode affects preferentially the central branches of ACC. The combination of these two EOFs of transport stream function, in contrast to individual mode giving insignificant correlation, reproduces reasonably well the interannual variability of the ACC in the South Pacific sector locating just upstream of Drake Passage; so, much the same is expected to occur at Drake Passage (Park et al., in prep).

Artana C., R. Ferrari, C. Bricaud, J.-M., Lellouche, G. Garric, N. Sennéchael, J.-H. Lee, Y.-H. Park, and C. Provost (2019). Twenty-five years of Mercator ocean reanalysis GLORYS12 at Drake Passage: velocity assessment and total volume transport, Advances in Space Research, doi:10.1016/j.asr.2019.11.033.
Park, Y.‐H., Park, T., Kim, T.‐W., Lee, S.‐H., Hong, C.‐S., Lee, J.‐H., M.‐H. Rio, M.‐I. Pujol, M. Ballarotta, I. Durand, and C. Provost (2019). Observations of the Antarctic Circumpolar Current over the Udintsev Fracture Zone, the narrowest choke point in the Southern Ocean. Journal of Geophysical Research: Oceans, doi:10.1029/2019JC015024.
Young-Hyang Park