This report is not to be cited without prior permission by all authors. © M. Tomczak and L. Pender
NOTE: The text in this report has been superseeded by the final publication: Tomczak, M., L. Pender and S. Liefrink (2004) Variability of the Subtropical Front in the Indian Ocean south of Australia. Ocean Dynamics 54, 506-519.
This is a preliminary report on the Seasoar data collected during the research voyage FR02/01 of R/V Franklin. The voyage took place during the summer of 2000/2001.
The following text describes the aim of the voyages and the observational strategy. It can be reached by clicking on the "introduction" link in the contents list on the left. The other links in the contents list give access to graphic display of various data.
The region between the core of the Trade Winds and the maximum Westerlies which stretches around the globe between approximately 20° and 45° on either side of the equator is characterised by negative wind stress curl in the atmosphere and by associated Ekman transport convergence in the ocean. In oceanography this region has therefore become known as the Subtropical Convergence. Its hydrography shows a decrease of sea surface temperature (SST) from the tropics to the temperate zones, accompanied by a decrease of sea surface salinity (SSS). As a general rule, the poleward increase of surface density produced by the decrease in SST is larger than the decrease of surface density produced by the decrease in SSS, and as a consequence surface density increases with distance from the equator.
Embedded in the Subtropical Convergence and somewhat on its poleward side is a zonal band of enhanced meridional SST and SSS gradients known as the Subtropical Front (STF). In the southern hemisphere this front can be described as beginning off the coast of Argentina as the southern boundray of the Brazil Current, stretching across the Atlantic Ocean at about 40°S, then passing south of Africa and continuing along that latitude through the Indian and central Pacific Oceans, where it eventually shifts northward to arrive at the coast of Chile as far north as 30°S (Tomczak and Godfrey, 1994). It defines the southern limit of the subtropical gyres and separates them from the broad westward flow of the Circumpolar Current further south (Stramma and Peterson, 1990; Stramma, 1992; Stramma et al., 1995).
Compared with other frontal systems of the world ocean, the Subtropical Front is relatively weak, displaying a temperature contrast of about 2°C and a salinity contrast of about 0.5 over a distance of 200 km. This combines with large short term and seasonal variability of its location and a high incidence of eddy formation and eddy shedding and explains why the STF is rarely seen in atlas data of ocean climate properties based on long term mean distributions. The STF is, however, a very distinct feature in any meridional crossing of the subtropics and can be seen in synoptic data to at least 250 m depth.
A particular characteristic of the STF south of Australia is the degree of density compensation across the front. Stramma (1992) shows that east of South Africa the STF is associated with a geostrophic transport of some 30 Sv (1 Sv = 106 m3 s-1) and that this transport is reduced to 10 Sv as Australia is approached. South of Australia it decreases further, reaching negligible magnitude east of 130°E (Schodlok et al., 1997; Schodlok and Tomczak, 1997). This indicates that the effects of the temperature and salinity changes across the STF on density compensate each other more and more from west to east.
This preliminary data report documents observations of the Subtropical Front in the south east Indian Ocean made during a research cruise in the summer of 2000/2001.
The data were collected by R/V Franklin between 16 February and 6 March, 2001, during voyage FR02/01. The study was designed to investigate mixing processes in an oceanic front on space scales of several kilometres and time scales typical of the passage of synoptic weather systems under conditions of partial or complete density compensation. The location of the study area was determined from knowledge of the regional position of the STF gained during two previous research voyages (Tomczak and Pender, 1998a, 1998b; Tomczak et al., submitted) and from satellite SST information available before the cruise.
A buoy drogued at 70 m depth was placed in the front and used as reference for a repeated survey pattern across the front. The survey covered a square of 36.8 nautical mile side length and followed two patterns (pattern 1 and pattern 2 in the figure) in alternating fashion. Each pattern took about one day to complete. As a result, each of the two diagonals, 52 nautical miles in length, was repeated once every day, while the side transects were completed once every 2 days.
The original orientation of the STF in the region was roughly parallel to the sides AB and CD of the patterns. As a result, the sides AC and BD give a nearly normal transect across the front, and the diagonals AD and BC cross the front at an angle of about 45°.
At one stage during the experiment it became apparent that the buoy had moved somewhat north from the STF. The survey was therefore adjusted to the modified pattern shown below. This modified pattern was only followed for one day.
The instruments used for the survey were a Seasoar and an acoustic doppler current profiler (ADCP). Continuous SST and SSS information was availabe from a thermosalinograph. The Seasoar is a remotely controlled device fitted with a dual CTD system which is towed behind the ship at a speed of 8 knots (1 knot = 1.852 km/h) and undulates over a depth range of up to 300 m. For the present investigation the flight path was set to cover the depth range 2 - 200 m, which gives it a repetition rate between successive dives of less than 4 minutes or a distance between dives of 1 km.
The Seasoar was recovered every 24 - 36 hours and calibrated by placing the dual CTD system into a seawater bath. Additional calibration checks were made against CTD data. CTD stations were performed to 1500 m depth.
The raw Seasoar data were averaged to 1 m depth increments and converted into vertical CTD-type casts with 1 minute time separation through interpolation. Data from CTD stations were likewise subsampled to 1 m depth increments.
The location of the STF on the poleward side of the Subtropical Convergence places it on the southern edge of the atmospheric high pressure belt of the subtropics. This is the region where atmospheric frontal systems travel eastward, exposing the oceanic surface layer to strong winds with systematic changes in wind direction. These atmospheric fronts move the Ekman layer back and forth, creating eddies and shearing the upper mixed layer off from the underlying oceanic structure.
The effect of this shearing movement depends on the direction of the movement itself. If the Ekman layer is displaced poleward, water of low density is moved over denser water. This enhances the stability of the water column and produces a strong thermocline at the bottom of the mixed layer. The process can be reversed by moving the surface layer back to its original position. If, on the other hand, the Ekman layer is displaced equatorward, the surface layer is pushed into a region where the underlying water is less dense. This produces convection and makes the process irreversible: If the surface layer is returned to its original position, its SST and SSS properties have changed, and a volume of mixed water remains below the mixed layer at the location where the convection occurred during the period of equatorward displacement of the surface layer. The data set presented here illustrates this very clearly.
The STF is commonly identified by the location where the 12°C isotherm crosses the 150 m or 200 m depth level. During winter the 12°C isotherm reaches the sea surface, usually not far from where it rises through the 150 - 200 m depth level. The STF is then also easily identifed in surface temperature observations.
The presence of a warm surface mixed layer during summer makes the situation more complex. The STF is still identifiable through the position of the 12°C isotherm relative to 150 - 200 m depth. At the surface it is also associated with a temperature and salinity front but at much higher temperatures (14 - 18°C depending on location), while the salinity front reaches right through from below the mixed layer to the surface.
A brief summary of the situation can help to interpret the data presented in this report. The STF is identified by a steep rise of the 12°C isotherm and the 35.2 isohaline from below 200 m depth to at least 100 m depth. It is a feature of the permanent thermocline and thus embedded in the Indian Central Water. During winter the 12°C isotherm and the 35.2 isohaline both reach the surface, and the STF is readily seen in SST data. A meridional TS-diagram across the front shows the well known TS-relationship of Central Water, and the two sides of the front are associated with two TS-points at the upper end of the TS-relationship (Figure A).
During summer the STF is capped by a warm mixed layer but the salinity structure is barely changed. The front still has a surface expression in the salinity field and is also seen as a surface temperature front but at higher temperatures. The salinity front reaches the surface virtually unchanged (Figure B).
 |
 |
| Figure A: The Subtropical Front in winter. The temperature front reaches through the mixed layer to the surface. The green TS-point gives TS-properties on the cold side of the front, the red TS-point represents the warm side. | Figure B: The Subtropical Front in summer. Warming of the surface mixed layer raises the temperature on either side of the front by the same amount. The result is a surface temperature front at a higher temperature above the subsurface expression of the STF, which remains unchanged. |
Below the mixed layer the STF changes position only slowly. The mixed layer, on the other hand, is exposed to movement by the synoptic wind systems and is only loosely coupled to the STF position underneath. Equatorward movement of the mixed layer produces the situation sketched in Figure C, poleward movement results in the situation shown in Figure D. Both situations and various variants of them can be seen in the data.
The vertical stability of the water column in Figures C and D is fundamentally different. Moving the mixed layer poleward increases vertical stability, moving it equatorward can lead to instability and produce convection events that reach below the mixed layer. Double diffusion will also act to stir the water column. The data show plenty of evidence for remnants of convective and/or double diffusive events.
 |
 |
| Figure C: The STF during summer when the mixed layer is shifted poleward. Only water from the cold side of the front is found in the frontal region below the mixed layer; it is capped by water from the warm side of the mixed layer. The TS-diagram reflects this transition from cold to warm. | Figure D: The STF during summer when the mixed layer is shifted equatorward. Water from the warm side of the front is found below the mixed layer in the frontal region; it is capped by water from the cold side of the mixed layer. The TS-diagram shows a strong salinity and temperature inversion. |
Last update of this page: 14/3/2001