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

Abstract

A Mean Dynamic Topography (MDT) is required to estimate mean transport in the ocean, to combine with altimetry to derive instantaneous geostrophic surface velocities, and to estimate transport from shipboard hydrography. A number of MDTs are now available globally but differ most markedly in boundary currents and the Antarctic Circumpolar Current. We evaluate several MDTs in two boundary currents off New Zealand (in the subtropical western boundary current system east of the country and in the Subantarctic Front to the south) using satellite, hydrographic, and Argo observations near two altimeter tracks. Argo float trajectories are combined with estimates of shear to produce new MDTs along both altimeter tracks: sufficiently high numbers of Argo floats travel in both boundary currents to allow a useful estimate of the mean flow at 1000 m depth and conservation of potential vorticity is used to account for more realistic flow paths. In finding a MDT, we show the uncertainties in the estimates of velocity differences between 1000 m and the surface from density climatologies, while often not estimated, need to be considered. The MDT computed from the Argo trajectories is generally consistent with the CLS09 MDT in both boundary currents and, in some locations, distinctly different from the MDT using a ''level of no motion'' assumption. The comparison suggests velocities from Argo trajectories can be usefully combined with other observations to improve estimates of flows and MDT in boundary currents.

Moana Ocean Hindcast – a > 25-year simulation for New Zealand waters using the Regional Ocean Modeling System (ROMS) v3.9 model

“…Their results show values of 12.4 ± 4.5 and 12.6 ± 2.6 Sv derived from 21 years of altimetry and 28 years of XBT measurements, respectively, and 8.4 ± 6.2 Sv from CTD casts along same the altimeter track. These values are also consistent with a volume transport of 8-15 Sv derived from Argo float trajectories in the same region (Bowen et al, 2014).…”

Characterizing the Variability of Boundary Currents and Ocean Heat Content Around New Zealand Using a Multi‐Decadal High‐Resolution Regional Ocean Model

“…The depth of the mean EAUC core is 750 m (to the +0.05 ms −1 velocity contour), consistent with observations presented in Santana et al (2021). The modeled mean alongshore transport across the section is 10.2 ± 5.71 Sv to the southeast, consistent with the estimate of 9 Sv from mean hydrographic climatology by Roemmich and Sutton (1998), 9.5 ± 5.5 Sv from altimetry and CTD data by Stanton and Sutton (2003), 8-15 Sv derived from Argo float trajectories in the same region (Bowen et al, 2014), and 8.4-12.5 Sv from long-term data from altimetry, XBT measurements, and CTD casts (Fernandez et al, 2018; Table 1). EAUC transport is of comparable magnitude to the total mean transport exiting the EAC system to the east, which is estimated to be 8-12 Sv and with the current extending to about 500-800 m (Kerry & Roughan, 2020; Sutton & Bowen, 2014; Recirculation in the NCE feeds back into the EAUC and results in a deepening of the current (Figure 7).…”

“…High-modeled SSH variability occurs off North Cape and in the WE region, with SSH variability the lowest off the west coast of the North Island and over the Campbell Plateau and Chatham Rise (Figures 2c and 2d). The SSH (Figures 2c and 2d) and SST (Figures 2g and 2h) variability are greater in the model (5 km resolution) than in AVISO and OISST, respectively, but show similar spatial structure with the exception of the region off North Altimetry and hydrography, transport to 2,000 m 17 ± 5.3 (including NCE recirculation to 33°S), 9.5 ± 5.5 (excluding recirculation to 31°S) (Stanton & Sutton, 2003) Current meter arrays 1995-1998, altimetry andCTD data 15-23 (to 33°S), 8-21 (to 31.75°S) (Bowen et al, 2014) MDT and Argo trajectories 10.5 ± 4.4, 12.4 ± 4.5 (Fernandez et al, 2018) Altimetry, CTD and XBT data 2. EAUC (Sth) 28.0 ± 8.40 151/2,300(FD) 14.8 ± 3.2 (Fernandez et al, 2018) Altimetry, CTD and XBT data 3.…”

Planktonic protist diversity across contrasting Subtropical and Subantarctic waters of the southwest Pacific

“…The SC advects this mix of STW and SAW northwards off the east coast and reaches south of the Chatham Rise through the Mernoo Gap and the Bounty Through. Further east and along the flanks of Campbell Plateau, the flows associated with the Subantarctic Front (SAF) carry the largest portion of SAW, about 50 Sv into the region south and east of the Chatham Rise (Bowen et al 2014;Stanton and Morris 2004). Access of SAW onto the plateau from the east occurs through the bathymetric gaps, saddles and ridges where waters then become isolated from the neighbouring circulation and significantly contribute to the development of oceanographic and climatic processes such as subantarctic mode water formation (Forcén-Vázquez et al 2021).…”