Is interleaving in the Agulhas Current driven by near-inertial velocity perturbations?

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Abstract

Recent observations taken at a number of latitudes in the Agulhas Current reveal that the water mass structure on either side of its dynamical core is distinctly different. Moreover, interleaving of these distinct water masses is observed at over 80% of the stations occupied in the current, particularly within the subsurface density layer between tropical surface water and subtropical surface water masses, and within the intermediate layer between the Antarctic Intermediate Water and Red Sea water masses. Direct velocity measurements allow for a comparison between the characteristic vertical length scales of the Agulhas intrusions and those of velocity perturbations found throughout the current. It is found that the interleaving scales match those of the velocity perturbations, which are manifest as high-wavenumber vertical shear layers and are identified as near-inertial oscillations. Furthermore, the properties of the intrusions indicate that double diffusion is not an important process in their development: they are generally not associated with a density anomaly, their slope and thickness fall outside the predicted maxima for instability, and a strong horizontal shear field acts to separate water parcels more quickly than intrusions would be able to grow by double-diffusive processes. Instead, the position, thickness, and slope of Agulhas intrusions relative to the background salinity and density field suggest that they are forced by rotating inertial velocities, with subsequent growth possibly driven by small-scale baroclinic instabilities. However, not all the evidence points conclusively toward advectively driven intrusions. For instance, there is a discrepancy between the observed salinity anomaly amplitude and the predicted inertial displacement given the background salinity gradient, which deserves further examination. Hence, there is a future need for more pointed observations and perhaps the development of an analytical or numerical model to understand the exact nature of Agulhas intrusions.

Original languageEnglish (US)
Pages (from-to)932-945
Number of pages14
JournalJ. PHYSICAL OCEANOGRAPHY
Volume37
Issue number4
DOIs
StatePublished - Apr 2007

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perturbation
water mass
salinity
double diffusion
Antarctic Intermediate Water
surface water
anomaly
baroclinic instability
oscillation
seawater
water

ASJC Scopus subject areas

  • Oceanography

Cite this

Is interleaving in the Agulhas Current driven by near-inertial velocity perturbations? / Beal, Lisa.

In: J. PHYSICAL OCEANOGRAPHY, Vol. 37, No. 4, 04.2007, p. 932-945.

Research output: Contribution to journalArticle

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