There are several high density sections bounding the Tasman Sea, crossing the Southern Ocean to Antarctica, Tasman Sea and Southern Indian Ocean. The lower resolution observations are collected at lower density but higher frequency – xbt’s are dropped every 4 hours instead of 1 or 2 but the lines are run either weekly or monthly instead of 4 times a year.  

High density lines

High density transects, used to observe smaller scale ocean features, are designed to resolve fronts and mesoscale eddies that are prevalent in much of the ocean, and boundary currents that transport large quantities of heat in regions where thermal gradients are large (and where sampling intervals ma, therefore, be as fine as 10-15km).

The Tasman (Sydney to Wellington) and Coral Sea (Brisbane to Fiji) IMOS sections along with a further transect (Auckland to Fiji) fully enclose the Tasman Sea and have been maintained since 1991.  This has been given the name 'The Tasman Box’.  The location of typical transects are shown on the right; the 3 sections have designations of PX34, PX30 and PX06, respectively.   The sections are occupied 4 times a year and are sampled at fine spatial densities, between 10km within boundary current regions to 50km in mid-basin portions.  To ensure that high quality data are collected, a scientific observer operates an automatic launcher around the clock on each voyage.  There are now more than 60 realisations for each section.

The final high density line that we maintain is Durmont d’Urville to Hobart (IX28).  This is run with volunteers on Astrolabe, the French Antarctic resupply vessel. We occupy this line 3 times during the Austral summer, resulting in a total of 6 sections (XBTs are dropped during both the north and southward legs). 

Low density lines

There are two lower resolution lines that originate in Fremantle – one to Indonesia and the other to the Red Sea. The IX12 line (Fremantle to Red Sea) has been maintained since 1986 and is occupied 12 times a year.  The IX1 (Fremantle to Sunda Strait) has been in operation since 1983. The line is occupied 26 times a year and xbts are dropped every 4 hours to allow us to calculate currents and water transports along this section.

Additional repeat lines include…Port Headland-Japan (IX22- PX11), Flores Sea to Torres Strait (PX2)

We acknowledge the contribution of XBTs by the United States of America's National Oceanic and Atmospheric Administration (NOAA) to partly maintain the XBT lines IX01 and IX28.

RAN samples opportunistically based on operational requirements. This provides data from areas that are otherwise data sparse. RAN also provides the majority of the XBTs used in these transects by both BOM and CSIRO.