Voyage Plans and Summaries

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Franklin Voyage Plan No. FR02/2001

Title

Mixing processes in the Subtropical Front.

Itinerary

Depart: Adelaide 1000 hrs, Friday 16 February 2001
Arrive: Hobart 1000 hrs, Tuesday 6 March 2001

Principal Investigator

Matthias Tomczak
Flinders University of South Australia
School of Chemistry, Physics and Earth Science
GPO Box 2100 Adelaide S.A. 5001, Australia
Telephone: (08) 8201 2298
Fax: (08) 8201 2676
E-mail: Matthias.Tomczak@flinders.edu.au 

Scientific Objectives

During research voyage FR02/2001 R/V Franklin will use its SeaSoar in a study of small scale mixing processes in the Subtropical Front south of Australia. There are four principal objectives:

1. To determine the relationship between thermohaline structures and density compensation in the Subtropical Front.
2. To determine the influence of density compensation on vertical mixing.
3. To determine the influence of changing surface wind stress on horizontal advection and vertical mixing.
4. To determine the temporal scales of interleaving events in the Subtropical Front.

Cruise Objectives

During a summer survey of the Subtropical Front south of Australia (FR02/98) a rich variety of thermohaline structures were observed below the surface mixed layer. Because of the relatively high degree of horizontal density compensation present in the region, vertically sheared horizontal advection can lead to interleaving of temperature and salinity fields. This process is enhanced by periodic reversals of the local wind stress due to passing low pressure systems. Horizontal temperature gradients in the atmosphere can also perturb the horizontal density compensation and lead to small scale vertical mixing and convection.

The main objective of this cruise is to identify a single thermohaline structure in the Subtropical Front similar to those observed during FR02/98. These structures appear to be due to vertical mixing and horizontal interleaving of water across the front. Once identified, we will study its evolution and reaction to atmospheric forcing by performing repeat surveys of the feature relative to a drifting buoy. The object of this is to study the time evolution of this feature and to identify which specific mechanism is responsible for its formation. The duration of the cruise should be sufficient to observe at least one reversal of the local wind field and we should be able to evaluate the relative importance of these reversals on the observed interleaving.

Cruise Track

The precise cruise track will be determined shortly before the cruise and will be based on satellite images of the Subtropical Front. However, based on observations from FR02/98 and FR10/98 we anticipate locating a suitable feature somewhere between 140 - 142.5 degrees E and 40.0 - 42.5 degrees S (see Figure 1).

The initial estimate for the position of the feature will be derived from satellite imagery. After departing Adelaide we will approach the identified feature with the SeaSoar deployed from about 200km before reaching its estimated location. The SeaSoar will be set to profile the top 200-300m.

An initial survey of the area will be performed to determine the precise positioning of a reference buoy, which will be placed near the centre of the feature.

Once the buoy is deployed the ship will commence with a series of repeat "butterfly" sections with the drifting buoy at the centre, in the following two alternating patterns:

Pattern A:

56.6 km due east
80 km due south west
56.6 km due east
80 km due north west

Pattern B:

56.6 km due south
80 km due north east
56.6 km due south
80 km due north west

The patterns will be located in such a way that the buoy is at the cross over point of the two long (80 km) tows. Adjustments for buoy drift will be made at every passing of the buoy.

Each section should take approximately 24 hours to complete with occasional CTD stations for along-front transport estimates. As many sections will be completed as time permits. At the conclusion of the study R/V Franklin will collect the buoy and continue on to Hobart.

Time Estimates

The number of sections proposed is for optimal conditions, this schedule provides approximately one day of flexibility for unanticipated delays.

Franklin Equipment

SeaSoar, CTD, thermosalinograph, meteorology.

User Equipment

One drifting buoy equipped with radio transmitter, radar reflector, sea anchor and three Seacats.

Personnel List

Matthias Tomczak, Chief Scientist , Flinders University
Charles James, Flinders University
Aneurin Henry-Edwards, Flinders University
Roger Matthews, Flinders University
Lindsay Pender, Cruise Manager, Computing, CSIRO Marine and Atmospheric Research
Phil Adams, Electronics, CSIRO Marine and Atmospheric Research
Gary Critchley, Hydrochemistry, CSIRO Marine and Atmospheric Research
Kevin Miller, Ocean Engineering, CSIRO Marine and Atmospheric Research

Figure 1. The region proposed for the study of the Subtropical Front during FR02/2001.

The figure shows a satellite image of sea surface temperature, indicating a series of eddies and filaments along the frontal region. The white line is the cruise track of R/V Franklin during its research voyage FR10/98. The rectangle indicates the most likely area for research voyage FR02/2001. The final location will depend on satellite information obtained before the voyage.

Updated: 31/01/03