This material is based on work supported by the National Science Foundation under Grants OCE-93204941 and OCE-9912320.
Visit the main SAFDE
image (9915 bytes) of 600 dbar field on December 1, 1995
GIF animation (137721 bytes)
Quicktime movie (2412054 bytes)
These movies show the temperature and velocity fields mapped by the IESs. In the smaller GIF animation, the fields at 300 dbar are mapped at 4 day intervals for a 2-month period during December 1995 and January 1996. The larger Quicktime movie shows the 600 dbar fields at daily intervals through to February 8, 1996. During this time period large meanders of the SAF propagate from west to east through the array region.
The Subantarctic Fluxes and Dynamics Experiment (SAFDE) has been a collaboration amongst US PIs (D. Luther, A. Chave, J. Richman, and R. Watts) and Australian PIs (S. Rintoul and J. Church). The objective was to observe the structure and variability of the absolute (barotropic and baroclinic) currents and temperature field within the Subantarctic Front (SAF) south of Tasmania using long-duration, spatially-coherent current (u,v) and temperature T records. Horizontal Electric Field (HEF) recorders and Inverted Echo Sounders (IES) were moored along WOCE line SR3 from March 1995 to March 1997, spanning 450 km near 143E. These seafloor instruments observed the vertically-averaged barotropic and gravest baroclinic mode structure of the (u,v) and T field throughout full water column. In addition a seven-element current meter mooring array, supporting 1-5 current meters from 300m to as deep as 3200m, was centered on the SAF for local dynamics studies.
The University of Rhode Island was responsible for the IES component of this experiment and for the calibration-CTDs from our deloyment and recovery cruises aboard R/V Melville. These efforts were highly successful. All 18 IESs were recovered, with >97% good data return on these 2-year-long records.
IESs measure the vertical acoustic travel time (VATT) from the bottom to the surface. The T and specific volume anomaly (SVAN) fields slope steeply across the SAF throughout most of the water column. As geopotential height (PHI) and VATT both change systematically across the SAF, they exhibit an empirical relationship to each other and to the fundamental vertical structure of T and SVAN. Combining eight high-quality CTD sections (ours plus six from Rintoul and Bindoff), the IES data have been interpreted using a ``gravest empirical mode'' (GEM) representation of the vertical structure. The GEM structure also incorporates the average annual cycle in the near-surface layers. In the depth range 150-3000 dbar, more than 96% of the variance in the T and SVAN fields are captured by the GEM representation. IES measurements of VATT thus determine the T(p) and PHI(p) thickness profile above each IES. Comparisons with moored T records confirm agreement within 0.3, 0.3, 0.1, and 0.05C at depths 300, 600, 1000, and 2000dbar. Horizontally-separated IESs determine lateral gradients in PHI from which geostrophic velocity profiles are calculated. Velocity shear comparisons with moored current meters agree within 5 cm/s at 300 and 600 dbar relative to 3000 dbar; sampling differences from point measurements account for the small residuals. The T and (u,v) fields have been mapped daily on several levels. The HEF data will be included to determine the absolute currents and fluxes.
We also conducted a shipboard CTD and ADCP synoptic survey of a steep meander of the SAF. The vertical velocity w and cross-frontal motions were diagnosed using (a) the density equation, (b) the omega-equation in Q-vector form, and (c) by tracing salinity anomalies. Waters entering the meander crest exhibited significant upwelling of O(0.05 cm/s) and diameter O(100 km); the flux exceeded 1 Sv across the vertical plane of the front. Much of the global meridional property fluxes in intermediate waters could be associated with mesoscale-driven cross-frontal exchanges all along the meandering subantarctic front. Watts et al. will prepare a journal article on these observed meander-driven fluxes.
Combining IES and Hydrography
Observe the Time-varying Baroclinic Structure of the Subantarctic Front
D. Randolph Watts, Che Sun, and Steve Rintoul
Poster presented at WOCE Conference Halifax, Canada, 24-29 May 1998
Watts, D.R., Sun, Luther, Richman, Chave, and Rintoul. 1998. Cross-frontal
Motion in a Meander in the Subantarctic Front. WOCE
Conference---Halifax, May 1998, FLX-03x, p.141.
Sun, C. and D.R. Watts. 2001. A circumpolar gravest empirical mode for the Southern Ocean Hydrography. J. Geophys. Res., 106, 2833-2855. PDF
Watts, D.R., C. Sun, and S Rintoul, 2001. A two-dimesnional gravest empirical mode determined from hydrographic observations in the Subantarctic Front. J. Phys. Oceanogr., . PDF
Disclaimer: Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).
Return to the Dynamics of Ocean Currents and Fronts Homepage
Last Revised: July 21, 2008