Tracer stirring and variability in the Antarctic Circumpolar Current
near the Southwest Indian Ridge
Abstract
Oceanic macroturbulence is efficient at stirring and transporting
tracers. The dynamical properties of this stirring can be characterized
by statistically quantifying tracer structures. Here, we characterize
the macroscale (1-100 km) tracer structures observed by two Seagliders
downstream of the Southwest Indian Ridge (SWIR) in the Antarctic
Circumpolar Current (ACC). These are some of the first glider
observations in an energetic standing meander of the ACC, regions
associated with enhanced ventilation. The small-scale density variance
in the mixed layer (ML) was relatively enhanced near the surface and
base of the ML, while being muted in the middle, suggesting the
formation mechanism to be associated to ML instabilities and eddies. In
addition, ML density fronts were formed by comparable contributions from
temperature and salinity gradients, suggesting the dominant role of
stirring, over air-sea interactions, in their formation and
sustainability. In the interior, along-isopycnal spectra and structure
functions of spice indicated that there is relatively lower variance at
smaller scales than would be expected based on non-local stirring,
suggesting that flows smaller than the deformation radius play a role in
the cascade of tracers to small scales. These interior spice anomalies
spanned across isopycnals, and were found to be about 3-5 times flatter
than the aspect ratio that would be expected for O(1) Burger number
flows like interior QG dynamics, suggesting the ratio of vertical shear
to horizontal strain is greater than $N/f$. This further supports that
small-scale flows, with high-mode vertical structures, stir tracers and
impact tracer distributions.