Decadal changes in the south Indian Ocean thermocline

A significant change in properties of the thermocline is observed across the whole Indian Ocean 32 degrees S section between 1987 and 2002. This change represents a reversal of the pre-1987 freshening and decreasing oxygen concentrations of the upper thermocline that had been interpreted as a fingerprint of anthropogenic climate change. The thermocline at the western end of the section (40 degrees-70 degrees E) is occupied by a single variety of mode water with a potential temperature of around 13 degrees C. The thermocline at the eastern end of the 32 degrees S section is occupied by mode waters with a range of properties cooling from similar to 11 degrees C at 80 degrees E to similar to 9 degrees C near the Australian coast. The change in theta-S properties between 1987 and 2002 is zonally coherent east of 80 degrees E, with a maximum change on isopycnals at 11.6 degrees C. Ages derived from helium-tritium data imply that the mode waters at all longitudes take about the same time to reach 32 degrees S from their respective ventilation sites. Dissolved oxygen concentration changes imply that all of the mode water reached the section similar to 20% faster in 2002 than in 1987.