ON THE PARAMETERIZATION OF EQUATORIAL TURBULENCE - EFFECT OF FINE-SCALE VARIATIONS BELOW THE RANGE OF THE DIURNAL CYCLE

Citation
H. Peters et al., ON THE PARAMETERIZATION OF EQUATORIAL TURBULENCE - EFFECT OF FINE-SCALE VARIATIONS BELOW THE RANGE OF THE DIURNAL CYCLE, J GEO RES-O, 100(C9), 1995, pp. 18333-18348
Citations number
49
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Oceanografhy
Journal title
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS
ISSN journal
2169-9275 → ACNP
Volume
100
Issue
C9
Year of publication
1995
Pages
18333 - 18348
Database
ISI
SICI code
2169-9275(1995)100:C9<18333:OTPOET>2.0.ZU;2-C
Abstract
We seek relationships among turbulence and fine-scale and large-scale flow by ensemble averaging the observations taken in the Pacific Equat orial Undercurrent (EUC) at 0 degrees/140 degrees W in April 1987. A c ombination of fine- and microscale sensors resolves vertical wavenumbe r spectra of shear and temperature from scales of 300 m to the viscous and thermal diffusive cutoffs. We study the depth range 50-350 m, whi ch encompasses high-shear layers below and above the core of the EUC a nd the thermostad, but not the diurnal cycle near the surface. Fine-sc ale shear dominates over large-scale shear (related to the slowly vary ing EUC) at 50-170 m and below 270 m, where large-scale gradient Froud e numbers (Fr) drop below I, while large-scale shear dominates in the weakly stratified thermostad at 170-270 m, where ($) over bar ($) over bar Fr > 1. We analyze shear fluctuations in different vertical waven umber bands and pragmatically separate nonturbulent and turbulent fluc tuations, the latter being associated with vertical overturning and vi scous dissipation. In part of the fine-scale range, shear spectra fall off approximately in inverse proportion to vertical wavenumber. The s hear variance in this wavenumber band stays close to the squared buoya ncy frequency independent of large-scale shear. At a vertical resoluti on that resolves turbulent overturning, the Kunze et al. (1990) model of shear instability well predicts average dissipation rates below 100 m. Yet instantaneous high-resolution gradient Froude numbers show vir tually no correlation with turbulent dissipation rates. At 20-m vertic al resolution, mean dissipation rates from below 50 m and total rms sh ear S-tot are well correlated as ($) over bar epsilon similar to S-tot (-3.5). Fine-scale shear is essential in this relationship. Large-scal e gradient Froude numbers and large-scale shear are comparatively poor ly correlated with mixing parameters.