The expendable bathythermograph (XBT) sub-facility collects temperature profiles of the oceans surrounding Australia using high-resolution repeat lines samples collected by merchant ships equipped with a XBT. High-resolution temperature profiles are regularly collected along the 6 IMOS sections. The XBT probes sample to approximately 800m and have a nominal accuracy of 0.15°C. The depth of the temperature profile for an individual probe is determined by a fall rate equation. These data are used in many studies to characterise the mean and time-varying properties of the major ocean currents around Australia.
For more details on the XBT instruments see here.
The two Tasman and Coral Sea 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.
Brisbane – Fiji
On each section up to 100 XBT casts are made which enables the fine temperature structure to be fully resolved down to 800m. In the section below we see the East Australian Current (EAC) at the western (Brisbane) end of the section. This is represented by the isotherms (contours of temperature) sharply rising towards the surface.
Sydney – Wellington
A typical example of a section along the Sydney to Wellington track is shown below. The black dots show the locations of the XBT casts - they are more closely spaced at the Sydney end to capture the full structure of the EAC and eddies. The upper panel shows the sea surface temperature (SST) and the middle panel shows the sea surface height – both obtained from satellite measurements. The most notable features are the 2 eddies that the ship track cuts through. The first one, near Sydney, forms part of the East Australian Current. The lower panel gives the temperature as obtained from the XBT casts. We are able to see the depth structure of the 2 eddies – they reach down to at least 800m, the maximum depth of the XBT profiles.
The 4 temperature sections collected along PX30 and PX34 in 1993. The EAC and eddies are nearly always present at the western end of the sections. An unusual occurrence was in May 1993 when the EAC was very weak.
After collecting many measurements on each section we are able to calculate the average temperature pattern for each ocean region. The figure below shows the mean (or average) temperature (the contour lines) and the variability (standard deviation) along each section of the Tasman Box. The presence of the EAC is clearly shown by the high variability off Sydney. These temperature patterns show that the current breaks up into large eddies by the time it reaches the Sydney region.
Fremantle to Indonesia
Another very important set of measurements are made between Fremantle and Indonesia. Here we are able to estimate the flow of water coming from the Pacific Ocean into the Indian Ocean through the Indonesian Islands. The strength of this flow has a major influence on the climate of Western Australia and is a central factor on the distribution of ocean properties around the global ocean.
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.