Meridional volume transport in the South Pacific: Mean and SAM-related variability

Abstract

The large increase in upper-ocean sampling during the past decade enables improved estimation of the mean meridional volume transport in the midlatitude South Pacific, and hence of the climatically important Meridional Overturning Circulation. Transport is computed using Argo float profile data for geostrophic shear and trajectory data for reference velocities at 1000 m. For the period 2004-2012, the mean geostrophic transport across 32 S is 20.6 6 6.0 Sv in the top 2000 m of the ocean. From west to east, this includes the southward East Australian Current (23.3 6 2.9 Sv), its northward recirculation (16.3 6 3.6 Sv), the broad interior northward flow (18.4 6 4.1 Sv), and the net northward flow (9.2 6 2.2 Sv) in opposing currents in the eastern Pacific. The basin-integrated geostrophic transport includes 7.3 6 0.9 Sv of surface and thermocline waters, 4.9 6 1.0 Sv of Subantarctic Mode Water, and 4.9 6 1.4 Sv of Antarctic Intermediate Water. Interannual variability in volume transport across 32 S in the South Pacific shows a Southern Annual Mode signature characterized by an increase during the positive phase of the Southern Annular Mode and a decrease during the negative phase. Maximum amplitudes in geostrophic transport anomalies, seen in the East Australian Current and East Australian Current recirculation, are consistent with wind stress curl anomalies near the western boundary.

Estimates of Surface and Subsurface Boundary Current Transport Around Australia

“…Analysis of the mooring array at 27°S revealed that occasionally the EAC recirculation extended offshore of the mooring array (155.30°E) and was stronger/similar magnitude to that of the poleward EAC transport (Sloyan et al, ). The ozROMS transport estimate of 13 ± 4.9 Sv (Table ) for the EAC recirculation at 32°S was similar (16 ± 3.6 Sv) to that by Zilberman et al (). The EAC recirculation was also present offshore of the EAC extension at 36°S (0–2,000 m) with EAC transport extended from the coast to 152.5°E and the EAC recirculation between ∼152.5°E and 154.5°E (x4; Figure d).…”

“…Cumulative transport across the EAC and EAC extension indicated a reversing flow pattern with both southward and northward circulation with the latter associated with the EAC recirculation (Zilberman et al, ). At 18°S EAC transport extended from the coast to 148°E with the EAC recirculation (northward) between 148°E and 152°E (x2; Figure d).…”

“…The estimated mean EAC transport between 18° to 35°S ranged between 12.6 Sv (excluding EAC recirculation) and 23.4 Sv (including EAC recirculation). Previous estimates of the mean transport between 18°S and 35°S ranged between 25 and 37 Sv (Mata et al, ; Ridgway & Dunn, ; Zilberman et al, ). The estimated TO, was 2.9 ± 2.4 Sv with estimates of TO encompassing a large range in the literature and van Sebille et al () reported values of ∼4.2 ± 4.5 Sv.…”

“…The northern branches of the SEC (North Vanuatu Jet and North Caledonia Jet, Figure ) bifurcate at ∼15°S to initiate the HC, a clockwise circulation that flows into the Gulf of Papua (Figure ) and the southward flowing EAC along the Australian coast (Godfrey et al, ). The EAC recirculation is defined as the northwards flow located further offshore of the main southward EAC flow (Zilberman et al, ). The main EAC flow separates from the Australian coast at latitudes 30°S–34°S and flows eastward as the Tasman Front (TF) (Bowen et al, ; Cetina‐Heredia et al, ).…”

Low‐mode internal tides and balanced dynamics disentanglement in altimetric observations: Synergy with surface density observations

“…This vision of the impact of mesoscale turbulence on ocean properties was well confirmed by Ocean Global Circulation Models and climate studies [ Griffies et al ., ]. SSH observations led to estimates of the near‐surface ocean horizontal circulation and, once combined with Argo data, to estimates of the horizontal transport of heat and tracers [ Zilberman et al ., ]. The upcoming launch of a new generation of wide‐swath altimeters (SWOT, 2020) is expected to be the next stride forward: the high resolution (down to 15 km) maps of SSH that will be collected should provide access to the oceanic 3‐D circulation and associated vertical transports of biochemical tracers [ Fu et al ., ].…”