Dianeutral mixing and transformation of Antarctic Intermediate Water in the Indian Ocean


  • Yuzhu You


Transformation of Antarctic Intermediate Water (AAIW) is achieved through one or more processes of epineutral advection, epineutral diffusion, dianeutral advection, and dianeutral diffusion. This paper points to the importance of dianeutral mixing in achieving the AAIW water-mass transformation in the Indian Ocean. Six neutral surfaces were mapped to span the intermediate water of the Indian Ocean between 580 and 1500 m at the reference cast. South of the Antarctic frontal zone, AAIW shows a diffusive tongue in the meridional Turner angle (after J. Stewart Turner) sections. On its equatorward transition, the transformed AAIW is characterized by a tongue of doubly stable conditions extending a great distance to as far as about 5°S. A maximum downwelling dianeutral velocity of −2×10−7 m s−1 due to cabbeling is found in the Antarctic frontal zone, which is 3 orders larger than that in the subtropical gyre. Thermobaricity acts similarly to cabbeling in the Antarctic frontal zone and contributes a maximum downwelling dianeutral velocity of −1×10−7 m s−1 but mainly arises in upwelling north of the frontal zone. A strong downwelling dianeutral velocity of −2×10−7 m s−1 contributed by vertical turbulent mixing occurs on the upper two neutral surfaces south of the frontal zone. With assumed constant epineutral diffusivity K of 103 m2 s−1 and dianeutral diffusivity D of 10−5 m2 s−1, an area-mean net dianeutral upwelling velocity of 0.11×10−7 m s−1 is found north of 32°S across the lowermost neutral surface σθ=27.66. It means a net upward volume transport of 0.6 Sv (1 Sv=106 m3 s−1). This weak but net upwelling transport roughly corresponds to a net 0.5 Sv transported downward, with a downwelling dianeutral velocity of −0.25×10−7 m s−1 across the same neutral surface south of 45°S. Toward the core of AAIW, the net dianeutral velocity increases to 0.15×10−7 m s−1 across the“27.37” neutral surface. The corresponding net dianeutral transport increases to 0.8 Sv. You [1996] has estimated a net dianeutral upwelling transport of 1.4 Sv across the lower thermocline north of 32°S, which suggests about the same amount of volume transport upward across the upper intermediate layer. However, south of 45°S a much stronger area-mean net downwelling dianeutral velocity of −2.63×10−7 m s−1 is found across the uppermost neutral surface σθ=27.1, indicative of a net 5.4 Sv transported downward. That the downwelling transport across the uppermost surface is 10 times larger than that across the lowermost surface south of 45°S has a strong implication for the equatorward advection of AAIW between the uppermost and lowermost neutral surfaces. When both the epineutral and dianeutral diffusivities are increased by 1 order of magnitude to K=104 m2 s−1 and D=10−4 m2 s−1, the above estimated integrated total dianeutral velocity and transport would increase by almost 10 times. However, when the diffusivities are decreased by 1 order of magnitude to K=102 m2 s−1 and D=10−6 m2 s−1, the integrated total dianeutral velocity and transport would decrease by only less than one time.