Web address of this page: http://www.es.flinders.edu.au/~mattom/STF/fr0298.html Last update of this page: 18/2/99
This preliminary report is superseeded by the final publication:
James, C., M. Tomczak, I. Helmond and L. Pender (2002) Summer and winter surveys of the Subtropical Front of the south eastern Indian Ocean. J. Mar. Systems 37, 129-149.
Density Compensation in the Subtropical Front of the Indian Ocean south of Australia
Part 2: Summer
A preliminary data report by Matthias Tomczak and Lindsay Pender
This report is not to be cited without prior permission by all authors. © M. Tomczak and L. Pender
This is a preliminary report on the Seasoar data collected during the research voyage FR02/98 of R/V Franklin. The voyage took place during the summer of 1997/98 and is the first in a series of two voyages.
The second voyage took place during the following winter. Readers not familiar with the Subtropical Front will find the hydrographic situation during winter much easier to understand and may want to study the winter data first. The commentary to the sections shown here assumes that the reader is familiar with the winter data. A brief summary of the main features of the winter front can be found at the end of this introduction.
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 extends from the coast of Argentina, where it is found near 30°S, through the Atlantic, shifting gradually southward to 40°S south of Africa and continuing along that latitude through the Indian and central Pacific Oceans, where it shifts northward again to arrive at the coast of Chile as far north as 25°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.
The existence of the front raises several questions which still await answers. To begin with, the existence of a frontal zone in a region of general Ekman layer convergence indicates the presence of a process of frontogenesis other than the effect of the large scale negative wind stress curl. Large seasonal excursions of the STF particularly in the Indian Ocean point towards the local wind field as a decisive factor, but other processes, such as convergences of the heat and salt transports produced through interaction between the surface heat and freshwater fluxes and the converging Ekman transport, cannot be ruled out.
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. To our knowledge, the consequences of this highly intermittent process have not been explored systematically, but it is feasible to imagine that the resulting modification of the large scale temperature and salinity field can lead to enhanced SST and SSS gradients.
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 1997/98. The cruise was designed to contribute to the answers to some of the questions just discussed and to focus in particular on the role of the synoptic atmospheric frontal systems in the dynamics of the STF.
The data for this description of the STF were collected by R/V Franklin between 30 January and 17 February, 1998, during voyage FR02/98 and cover the region south of Australia from 118°E to Tasmania (see the cruise track). The cruise was designed to achieve as many crossings of the STF as possible within the available time span. The locations of individual crossings were based on satellite SST information available before the cruise.
The front was mapped using 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 10 - 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 between front crossings (legs 1 - 14 in the cruise track) and calibrated by placing the dual CTD system into a seawater bath. Additional calibration checks were made against CTD data. CTD stations to 1500 m depth were performed at the end points of front crossings.
The raw Seasoar data were averaged to 2 m depth increments and converted into vertical CTD-type casts with approximately 1 km horizontal distance through interpolation. Data from CTD stations were likewise subsampled to 2 m depth increments.
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) and salinities.
Another aspect of the STF south of Australia is its high degree of density compensation. The front is always present as a temperature and salinity front, but the cross-frontal gradients compensate each other nearly completely with respect to density, and the STF shows very little density contrast across the frontal zone, particularly in the east (towards Bass Strait). For instrumental reasons explained in the data comments this feature is not as clearly seen in the summer data as it comes out in the winter data, but its effects - such as interleaving and intrusions - are present here as well.
Web address of this page: http://www.es.flinders.edu.au/~mattom/STF/fr0298.html Last update of this page: 18/2/99