In November 2008, our Argo technician Alan Poole boarded the MV Wellington Express, a 188 metre long, 23000 tonne container ship plying the shipping route from Melbourne to Singapore, via the Great Australian Bight and the Indian Ocean. During the course of the 5 day voyage, Alan deployed a total of 13 Argo floats. Two of these are the first Iridium floats in the Australian program. The floats were deployed along a south-to-north route in the Indian Ocean starting at the southerly latitude of 32 ˚S near Perth and finishing at 10 ˚S close to Christmas Island.
The floats are programmed to drift with the ocean currents at a depth of 1000 m before sinking to 2000 m depth and sampling temperature and salinity as they slowly ascend to the sea surface. The floats repeat this cycle every 10 days, transferring their data via satellite during their surface interval. Float deployment locations are pre-determined and specifically designed to fill gaps in the Argo array (which has a nominal 3 x 3 degree spacing) that arise when floats reach the end of their life, are grounded, or fail prematurely due to hardware or software problems. The largest gaps in the global array are usually in remote areas far from normal shipping routes and finding deployment opportunities can often be a challenge. The assistance received from merchant vessels like the MV Wellington Express through the SOOP (Ship-of-Opportunity Program) is invaluable in enabling us to deploy floats around Australia.
To avoid floats grounding on the sea floor - it is important that they are deployed in water deeper than their maximum profiling depth of 2000 m. Most commercial vessels (including the MV Wellington Express) do not have sounders that range deeper than 2000 m. This is because commercial vessels are not interested in the detailed abyssal topography of the sea floor but more importantly the shallower parts of the ocean that require more careful navigation.
Before deployment, a float must be activated with a magnet that starts the pump. A special Argos detector unit is then placed next to the antennae to verify transmissions and ensure that the float is operating correctly. The timing interval of the transmissions indicates whether the float is ready to be deployed or if there is a problem requiring technical intervention. Once it has been confirmed that the float is operating correctly, it is placed into a cardboard box that protects the sensors from damage while it is lowered over the side. Once the box is released into the water the cardboard disintegrates quickly releasing the float to start its first profile.
The floats deployed on this voyage have been drifting for 5 months and the data that has been sent back via satellite is shown in the temperature-salinity plot below. The water properties vary dramatically from south to north in the Indian Ocean. Starting with the most southerly float (5901761, dark blue profiles), there is a distinct low salinity minimum in the colder, deep waters (bottom of the plot) corresponding to Antarctic Intermediate Water (AAIW). This water mass is formed in the Southern Ocean and spreads northwards to fill the ocean basins at depths of 500 to 800m. At the surface we see warm, high salinity water due to the high evaporation in the subtropics.
Moving north to float 5901676 (cyan profiles) we see that the AAIW is weaker and in the upper ocean a distinct salinity maximum feature which becomes weaker further to the north. Float 5901678 (green profiles) exhibits a weaker salinity minimum which is substantially eroded and the upper water column shows high variability and the low surface salinity characteristic of the high rainfall tropical latitudes. The profiles of float 5901680 (yellow) show very large variability in the upper water column associated with the front between salty south Indian waters and Indonesian throughflow waters from the Banda Sea. The profiles for the most northerly float (5901683, red) are largely isohaline through the water column. One profile exhibits extremely fresh surface water indicating that this float has surfaced in a localised freshwater lens after a tropical thunderstorm.
Data from Argo floats like these give us an in-depth understanding of the structure of the ocean beneath the surface. They have also increased our knowledge of the circulation and variability of these remote oceanic regions.