Authors

Citation

H. Peters et al., DETAIL AND SCALING OF TURBULENT OVERTURNS IN THE PACIFIC EQUATORIAL UNDERCURRENT, J GEO RES-O, 100(C9), 1995, pp. 18349-18368

Citations number

61

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

18349 - 18368

Database

ISI

SICI code

2169-9275(1995)100:C9<18349:DASOTO>2.0.ZU;2-V

Abstract

We analyze turbulent overturns in the high-shear, low Richardson numbe
r flow of the upper 350 m at 0 degrees/140 degrees W. Profiles of shea
r and stratification combined from fine- and microscale sensors resolv
e the vertical wavenumber spectrum from large-scale to dissipation sca
les. We compute a turbulent length scale l from Thorpe-sorting potenti
al temperature, while using potential density to avoid thermohaline in
trusions. Pragmatically, we consider scales smaller than I turbulent a
nd scales large than l nonturbulent. From local fine-scale velocity sp
ectra, we extract the horizontal turbulent kinetic energy at scales sm
aller than I and thus estimate the turbulent velocity q, a parameter c
haracteristic of the energetic eddies. The independently observed visc
ous dissipation rate epsilon, q, and I follow Taylor scaling, epsilon
= C-q q(3)/l, with C-q approximate to 4. Similarly, the measured therm
al dissipation rate chi arid the turbulent temperature fluctuation T',
also estimated by spectral extraction at scales smaller than l, follo
w similarity scaling, chi = c(t) T'(2)q/l, with c(t) approximate to 7.
From q, l, buoyancy frequency N, and kinematic viscosity nu, we estim
ate turbulent Reynolds numbers Re-t = ql/nu and turbulent Froude numbe
rs Fr-t = q/(Nl). The more energetic overturns of vertical thickness e
xceeding 1 m have 0.1 less than or similar to Fr-t less than or simila
r to 3 and 250 less than or similar to Re-t less than or similar to 10
(5). Ozmidov scales follow overturning scales as described by Dillon (
1982) only on average, but not in individual overturns. Richardson num
bers Ri of overturns show large scatter around a median of Ri = 0.23 a
nd virtually no correlation with epsilon. The mixing efficiency shows
a weak increase with increasing Ri and a weak decrease with increasing
Fr-t. Turbulence parameters are briefly compared with formulations in
turbulence closure models. A few individual mixing events are analyze
d in detail, with focus on possible forcing mechanisms. In such identi
fiable events, enhanced turbulence is paralleled by enhanced fine-scal
e variance of velocity, shear, and temperature. One mixing event shows
signatures of critical layer absorption. A very large overturn in the
nighttime turbulent layer near the Surface surprisingly shows dissipa
tion rates indistinguishable from the surroundings.