Decadal variability in overturn at 32S

In the subtropical gyre of the south Indian Ocean, at 32°S, salinity and oxygen in the thermocline changed markedly from 1987 to 2002 (McDonagh et al., 2005). Both salinity and oxygen increased, opposite to the trends farther south closer to the Antarctic Circumpolar Current (Bindoff et al., 2007; Aoki et al., 2005). Associated subtropical changes also occurred in the South Atlantic and South Pacific (Talley et al., 2008), most likely due to increased subduction rates due to intensified Ekman pumping associated with strengthening of the Southern Annular Mode (SAM) (Roemmich et al., 2007). These mid-gyre regions are sandwiched between the Southern Ocean and tropical oxygen declines (Stramma et al., 2008), which experienced reduced ventilation.  The large-scale changes indicate the usefulness of oxygen budgets as well as heat, freshwater and carbon. Using state estimation, POP, along with hydrographic analysis of both the previous and the new 2009 32°S section in the Indian Ocean we look to investigate a number of questions related to these changes.

1. What caused the apparent increase in ventilation of the upper ocean in 2002 compared with 1987? Although the relation with SAM is likely robust given the large-scale changes in oxygen and salinity, it is important to study and understand the continuous change in circulation, which is possible with ECCO and POP. The latter has CFCs and an age tracer, which makes it even more attractive for this problem.

2. How widespread is the circulation change inferred from the oxygen and salinity changes at 32°S from 1987 to 2002?  We might expect it to extend through all of the Indian Ocean’s subtropical gyre if changes in the wind forcing are the source. Meridional repeat hydrographic sections (WOCE to CLIVAR) show the same changes and bracket the range of latitudes  - it is a subtropical change (Talley et al., 2008). Again, POP and ECCO will be useful to understand context for the circulation change.

3. Has there been any circulation change since 2002?  This is an observational question, to be answered from the March-May, 2009, 32° section.  The SAM reached a maximum in 2000 and has been decreasing since, so it is possible the conditions have reversed. We will analyze the new section, and also place it in context using ECCO, SOSE, and POP.