Direct velocity measurements in the Subantarctic Front and over Campbell Plateau, southeast of New Zealand

Abstract

[1] Flow variability in the Subantarctic Front (SAF) and over the Campbell Plateau, southeast of New Zealand, is examined using data from a current meter mooring array along with conductivity-temperature-depth (CTD) and satellite altimetry data. These data are part of an ongoing study of ocean climate variability in the region. Contrasting flow regimes were found with very energetic and highly variable flows in the SAF flowing along the Subantarctic Slope and much weaker flows over the Campbell Plateau. Currents up to 75 cm s À1 were observed in the core of the SAF, while flows over the plateau were typically less than 10 cm s À1 . Estimates of the total bottom referenced transport from three CTD sections across the SAF varied between 48 and 68 Sv. Transports were also estimated using TOPEX/Poseidon altimetry sea truthed with the CTD data. The resulting 8-year time series exhibited high variability resulting from current meanders and eddy activity. One event in the record appeared to be the result of a cyclonic eddy passing through the mooring array. This event was modeled using an idealized geostrophic eddy. The simulation reproduces the main features observed in the current meter records and the surface topography from the TOPEX/Poseidon altimeter. Inspection of the TOPEX/ Poseidon data suggests that large cyclonic and anticyclonic eddies occur in the frontal region with a frequency of around 9 yr À1 .

Physical oceanography of the deep seas around New Zealand: a review

“…This compares with about 14% in the depth-independent component both south of Tasmania (Phillips & Rintoul 2000) and on the Subantarctic Slope (Stanton & Morris 2004). Transport in the SAF through the gaps in Macquarie Ridge is 52 Sv (Rintoul et al 2014), similar to the 48 to 68 Sv found on the Subantarctic Slope (Stanton & Morris 2004).…”

Estimating mean dynamic topography in boundary currents and the use of Argo trajectories

“…The mean velocities from the Argo trajectories are similar to the mean velocities at 1000 m from the dynamic height of the three hydrographic sections using the bottom as the LNM. Current meters deployed on a mooring at 52:9 S for 7 months show mean velocities of 25 and 23 cm s 21 at 1500 and 500 m depth, respectively [Stanton and Morris, 2004; values from their Table 1 with speeds projected into the cross track direction], which bracket the mean values from the Argo trajectories at 1000 m.…”

“…The mean is shown in subsequent figures for a comparison of a mean derived from the LNM. Transport has been estimated using these hydrographic sections and altimeter data by Stanton and Morris [2004]. They use a ''canonical'' velocity section from a subset of the hydrography and weight the section by the altimeter surface flow to find a transport more representative of 8 years than the three hydrographic sections alone.…”

“…An MDT was created from the three repeat sections along the subantarctic altimeter track using the bottom as a level of no motion, as has been assumed previously [Stanton and Morris, 2004] (Figure 10a). Subtracting the altimeter heights from the dynamic heights reduces the height difference across the current in all three sections (Figure 10b) indicating that all three sections were taken during times of higher than average flow.…”

“…Dynamic heights in the Subantarctic Front along the Campbell Plateau were calculated from temperatures and salinities collected on three hydrographic surveys along the satellite altimeter track in 1997 and 1998 with the bottom used as a level of no motion [Stanton and Morris, 2004]. Stations with depth of 1500 m or greater were used to find the dynamic height.…”