Upper ocean thermohaline structure and its temporal variability in the southeast Indian Ocean

Abstract

We examine the upper ocean thermohaline structure in the southeast Indian Ocean and its temporal variability based on XBT/XCTD observations from four cruises across the Southern Ocean from Fremantle, Australia to Prydz Bay, Antarctica. The transects were occupied in March 1998, November 1998, March 2000 and March 2002. Three major fronts—the subtropical front, subantarctic front and polar front—are clearly identified from our surveys and compared with earlier studies. Particularly, two polar fronts, separated by a few degrees of latitude, appear southeast the Kerguelen Plateau. The primary polar front is characterized by a strong horizontal thermal gradient extending deep in the water column, while the secondary polar front is identified by the 2°C isotherm at T min depth and has a relatively shallow frontal expression. Dynamic height across the Antarctic Circumpolar Current (ACC) was calculated for the transects in 2000 and 2002. With a negligible yearly variation in the total transport across the ACC, the higher sample density in 2002 reveals more detailed structure of the ACC: two jets associated with the SAF and primary polar front are embedded within the broad ACC. We find a strong temporal variation in the upper ocean thermal structure in the polar ocean southeast the Kerguelen Plateau. The depths of the mixed layer and T min layer increase over time. The mixed layer temperature decreases while the T min temperature increases during the same period. In addition, we see an increase from 1998 to 2002 in the length of the ice-free period prior to each XBT/XCTD sampling and surface wind forcing, calculated as the friction velocity cubed during the ice-free period. Our analysis suggests that the longer the ocean is exposed to the atmosphere and the stronger the wind stirring, the more enhanced is the turbulent mixing. This results in a greater mixed layer depth and more entrainment of colder water from the T min layer to the mixed layer. This surface forcing also enhances internal diffusive processes that mix the T min water with the warmer waters above and below the T min layer. The surface forcing is apparently dominant in determining the upper ocean thermal structure in this polar region.