On the relationship between the large‐scale property variations and fine structure in the Circumpolar Deep Water

Volumetric analysis is used to quantify the observed changes in the Circumpolar Deep Water (CDW) potential temperature/salinity relationship along the axis of the Antarctic Circumpolar Current. A marked increase occurs in the CDW potential temperatures and salinities in the South Atlantic. However, the vertical density distribution is not significantly affected: The volume of water in the deep water density class intervals remains virtually unchanged. Lateral mixing with the warmer and saltier North Atlantic Deep Waters (NADW) and the colder and fresher Antarctic deep waters appears responsible for this change. Temperature and salinity inversions have been observed on continuous salinity‐temperature‐depth soundings in much of the South Atlantic. This fine structure, presumably the signature of lateral mixing and not of internal wave origin, is particularly evident in the southwestern Argentine Basin, where 0.5°C temperature inversions with 50–200 m vertical scales are found at the depth of the NADW/CDW confluence, between 1500 and 2500 m. To quantify the intensity of the fine structure the temperature‐gradient variance in the 32–256 m vertical wavelength band is calculated for data from the southwest Argentine Basin the Drake Passage, Scotia Sea, the central and eastern Atlantic, and the southwest Indian Ocean. The largest variance levels occur in the Argentine Basin, the smallest in the Drake Passage, while those in the central Atlantic are intermediate. The temperature‐gradient variance data from the Argentine Basin are then used to estimate the lateral temperature flux in the NADW/CDW confluence with a statistical model developed by Joyce (1977). The calculated quantities are small; the lateral temperature flux is of the order of 4 × 10 −4 °C m s −1 and the lateral diffusivity of order 40 m 2 s −1 . The data are also used to calculate heat flux divergences. The close agreement between the heat flux divergence calculated from the temperature changes of the CDW core and the divergence calculated with the fine structure data suggest that lateral mixing accounts for the large‐scale property changes observed in the CDW core.