Salinity variability in the Indian Ocean

The new global array of Argo profiling floats, which measure temperature and salinity from 0 to 2000 metres every 10 days, provides real-time monitoring of the full density structure of the upper ocean for the first time. Using Argo data from the southeast Indian Ocean during 1999 to 2005, combined with CTD data from hydrographic cruises from 1989 to 2000, I have documented the interannual variability in upper ocean salinity in the Indonesian Throughflow region. The dominant influence on the salinity of the region is the El Niño–Southern Oscillation. During El Niño times, when the Indonesian region suffers drought conditions, the southeast Indian Ocean is unusually salty. During La Niña the region is unusually fresh. The relationship between observed upper ocean salinity and the freshwater flux (precipitation - evaporation) over the region suggests that it is the freshwater flux over the Indonesian seas being advected into the southeast Indian Ocean that dominates the salinity budget, rather than the freshwater flux local to the southeast Indian Ocean . I am now using a high-resolution (tenth-degree) global ocean model, OFAM developed by CSIRO Marine and Atmospheric Research, to calculate a full salinity budget for the region. This will provide some insight into the relative roles of advection and mixing on the evolution and decay of these massive and widespread salinity changes.

During the second phase of this work, I will extend the analysis of the Argo data to the entire Indian Ocean . I will adapt a technique for salinity calibration, applied to the southeast Indian floats, to the rest of the Indian Ocean floats. Once the data are quality controlled and calibrated they will be mapped onto a regular grid in space and time to provide a 4-dimensional view of temperature and salinity variability. The 4D maps will provide insight into air-sea interaction processes near the sea surface. At deeper levels we will be able to compare the maps with historical CTD sections to explore lower frequency variability and climate change signals. One avenue for research using the maps and the high resolution model is to explore whether the southeast Indian Ocean salinity anomalies extend far into the interior of the Indian Ocean along the path of the subtropical gyre, or whether they are mixed into the background salinity field before then.