ALBERT

Description: We present a new method based on a combination of optimum multiparameter analysis and CFC/oxygen mixing analysis to determine the ages of water masses in regions of mixing. It enables us to follow water mass movements in greater detail than with other methods, which give only the combined pseudoage of a water mass mixture. We define the age of a water mass as the time a water parcel needs to spread from its source region, where it received its individual tracer characteristics, to the point of observation. The age distribution allows us to determine pathways of water masses, which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes. We apply the method to hydrographic data along World Ocean Circulation Experiment section I5 in the south east Indian Ocean. In the thermocline, Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) meet and mix. These distinct water masses have different formation mechanisms but similar temperature/salinity characteristics. It is shown that the convective formation of SAMW is a major ventilation mechanism for the lower Indian thermocline. In the eastern part of the south Indian Ocean, SAMW dominates the oceanic thermocline and is found to be about 5 years old. Pure ICW is present only in the thermocline of the region 48 degrees-55 degrees E, with increasing age with depth, confirming the subduction theory. While most SAMW joins the equatorward gyre movement of the southeastern Indian Ocean, some of it propagates westward through turbulent diffusive mixing, reaching 55 degrees E after 15-20 years. It takes ICW some 25-30 years to reach 110 degrees E.

Description: Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) are the two major sources for the ventilation of the permanent thermocline in the Indian Ocean. ICW is formed by subduction in the region of negative wind stress curl, while SAMW is formed by convective overturning at the Subantarctic Front. SAMW contributes to the depth range of ICW, but is not easily identified, because most hydrographic properties (temperature, salinity and nutrients) of SAMW do not differ much from those of ICW. This study identifies ICW and SAMW in a zonal section near 32°S and evaluates the relative importance of convection vs. subduction for the ventilation process. Oxygen and nutrient data from the eastern part (50–114°E) of WOCE section 15 are used with temperature and salinity to determine water mass fractions of subducted ICW and of SAMW from water mass mixing analysis. The individual age fields of the two water mass components are then derived from a combination of the fractions obtained with a linear oxygen/CFC mixing model. Unlike earlier studies, which derive an uncalibrated apparent age, our results express water mass age in true units of time (years). The core of the SAMW near 114°E is about 5 years old, while the core of the subducted ICW (at 60–80°E) shows an increase of age with depth, in agreement with the subduction process. ICW moves eastward with the South Indian Current, reaching an age of 35 years at 114°E. SAMW spreads westward against the mean flow through turbulent diffusion, reaching an age of 25 years at 50°E.