Upper Ocean Thermohaline Structure and its Temporal Variability in the Southeast Indian Ocean

XBT/XCTD observations from four cruises across the Southern Ocean from Fremantle, Australia to Prydz Bay, Antarctica were analyzed to examine the upper ocean thermohaline structure in the southeast Indian Ocean and its temporal variability. The transects were occupied in March 1998, November 1998, March 2000, and March 2002. A strong temporal variability is found in the upper ocean thermal structure in the polar gyre southeast of the Kerguelen Plateau. The depths of the mixed layer and T_min layer deepen with time. The mixed layer temperature decreases while the T_min temperature increases in the same time. In addition, the ice-free period prior to XBT/XCTD sampling and surface wind stirring measured by friction velocity cube during the ice-free period increase from 1998 to 2002. The analysis suggests that the longer period that ocean is exposed to the atmosphere and the stronger wind stirring will cause enhanced turbulent mixing, which result a deeper mixed layer depth and entrainment of more cold water from the T_min layer to the mixed layer. These surface forcing also enhance internal diffusive processes that mix the T_min water with the warmer waters above and below the T_min layer. The surface forcing apparently dominate in determining the upper ocean thermal structure in this polar region.