XBT Analysis - Antarctic Circumpolar Current
XBT observations, combined with Argo temperature-salinity profiles, and with satellite products, are used along the AX25 repeat XBT transect to study the link between local wind forcing mechanisms and the variability of the upper-ocean temperature and the dynamics of two different fronts in the Antarctic Circumpolar Current (ACC) region south of South Africa: the Subantarctic Front (SAF) and the Antarctic Polar Front (APF).
Using these observations, it is found that meridional changes in the westerlies drive temperature anomalies in the Ekman layer and changes in the Subantarctic Front (SAF) and Antarctic Polar Front (APF) transports through dynamic height adjustments. The link between the changes in the wind stress and the SAF and APF transport variations occurs through a combination of thermosteric sea level changes due to temperature anomalies in the Ekman layer, and of Ekman pumping driven sea level change. Observations also indicate that the SAF and APF locations in this region are not linked to the local wind forcing.
Below are shown results in support of these conclusions.
AX25 XBT transect
Eddy Kinetic Energy between 1993-2010. Overlaid on the fields are the locations of sixteen AX25 XBT realizations (gray lines), the position of the reference transect used in this work (thick magenta line) and the 1000 m and 3000 m isobaths (thin black lines).
Baroclinic transport - XBT-derived cumulative transport is computed along the AX25 repeat XBT transect using a previously reported method [Baringer and Garzoli, 2007].
Frontal location - The SAF and APF location is defined for each XBT transect using a previously reported method [Orsi et al. 1995]:
- SAF location is defined by the temperature change (θ400m >4-5°C) associated with the northern boundary of the transitional zone between the Subantartic Surface Water and the Antarctic Surface Water (AASW)
- APF location is defined temperature maximum (θ0-200m<2°C), from south to north, associated with the sinking AASW
Frontal transport - The SAF and APF transports are defined as the transport within the region defined by geostrophic velocity minimums to the north and south of the frontal location.
Satellite altimetry is used extend back the time-series of frontal locations to the beginning of the altimetry period.
XBT-derived cumulative transport across AX25
Total transport of sixteen AX25 realization (gray lines) and their average (black line). The total transport per degree of latitude is shown with positive (black) and negative (gray) shading. The mean locations of the Subtropical Front (STF), SAF, APF, southern ACC Front (sACCf), and of the Southern ACC Boundary (SBdy) is shown.
SAF and APF location
Time-series for the location of the SAF and APF based on the altimetry (black lines) and on XBT data (black squares) overlaid on the altimetry-derived geostrophic velocities (in m s-1). The magenta and white lines delimit the SAF and APF regions, respectively.
SAF and APF transports
(top) Transport time-series for the SAF (blue line), showing the one-year running mean (black line) and standard deviation (gray shaded area), and the annual means (black diamonds). (bottom) Same, but for the APF.
Wavelet Spectrum Density of: (a) SST within 45°S-55°S along AX25, (b) SAF latitude, (c) SAF transport, (d) APF latitude, (e) APF transport, (f) SAM, (g) wind stress within 35°S-45°S along AX25, and (h) wind stress within 50°S-60°S. The solid white contours represent the peak-based significance levels, computed at 95%. The dash curve indicates the cone of influence. Power and units are the square of the parameter unit (e.g temperature - °C2).
These products are updated on a yearly basis.
The XBT data along AX25 can be obtained here>>
Domingues, R. M., G. Goni, S. Swart, S. Dong: Wind forced variability of the Antarctic Circumpolar Current south of Africa between 1993-2010, submitted to J. Geophys. Res.-Oceans.
Swart, S., S. Speich, I.J. Ansorge, G.J. Goni, S. Gladyshev, and J.R.E. Lutjeharms, 2008: Transport and variability of the Antarctic Circumpolar Current south of Africa. J. Geophys. Res., 113(C9):C09014, doi:10.1029/2007JC004223. [PDF]
Ansorge I., S. Speich, J. Lutjeharms, G. Goni, C. Rautenback, P. Froneman and S. Garzoli, 2005: Monitoring the oceanic flow between Africa and Antarctica: Report of the first Good Hope cruise. South African. J. Sci., 101(3-4):29-35.