FINE THERMOHALINE STRUCTURE OF THE COLOMBIAN PACIFIC OCEAN

Nancy Villegas

Description

ABSTRACT
The present document shows strata classification of the Colombian Pacific Ocean – COLUMBIAN PACIFIC OCEAN, done by first time according its fine thermohaline structure, based on temperature and salinity fields analysis. Layers, where different mechanisms of fine structure predominate, were determined and everywhere in the area a stable stratification was observed, although conditions for not stability as a result of the double diffusion were present.
RESUMEN
El presente documento muestra la clasificación de las capas de la Cuenca del Pacífico Colombiano (CPC), hecha por primera vez de acuerdo con su estructura termohalina fina, basada en el análisis de los campos de temperatura y salinidad. Las capas fueron determinadas con predominancia de diferentes mecanismos de estructura fina y se observó que en toda la región existe una estratificación estable, aunque se encontraron condiciones de inestabilidad como resultado de una doble difusión.

Sujets

• Estructura

Informations

• Publié par: erevistas
• Publié le: 01 janvier 2004
• Nombre de lectures: 38

Extrait

EARTH SCIENCES
RESEARCH JOURNAL

Earth Sci. Res. J. Vol 8, Nº 1 (Dec. 2004): 45 - 51

FINE THERMOHALINE STRUCTURE OF THE COLOMBIAN
PACIFIC OCEAN

Nancy Villegas
Department of Earth Sciences, Metheorology, Universidad Nacional de Colombia
Bogotá, Colombia E-mail: [e-mail]




ABSTRACT

The present document shows strata classification of the Colombian Pacific Ocean – COLUMBIAN PACIFIC
OCEAN, done by first time according its fine thermohaline structure, based on temperature and salinity fields
analysis. Layers, where different mechanisms of fine structure predominate, were determined and everywhere
in the area a stable stratification was observed, although conditions for not stability as a result of the double
diffusion were present.

Key Words: fine thermohaline structure, small-scale thermohaline structure, stability of the ocean, stratified
ocean, salt finger stratification, differential-diffusion convection, and step structure


RESUMEN

El presente documento muestra la clasificación de las capas de la Cuenca del Pacífico Colombiano (CPC),
hecha por primera vez de acuerdo con su estructura termohalina fina, basada en el análisis de los campos de
temperatura y salinidad. Las capas fueron determinadas con predominancia de diferentes mecanismos de
estructura fina y se observó que en toda la región existe una estratificación estable, aunque se encontraron
condiciones de inestabilidad como resultado de una doble difusión.

Palabras clave: estructura termohalina fina, estructura termohalina de baja escala, estabilidad en el océano,
estratificación oceánica, estratificación de salinidad, convención de difusión diferencial, paso de estructura.

© 2004 ESRJ - Unibiblos.








Manuscript received April 2004 45
Paper accepted August 2004
Nancy Villegas


INTRODUCTION

On a fine structure scale, the profiles of Turbulent mixing regions are outlined on the
hydrophysical fields show alternated sections with vertical profiles of hydrophysical fields as the fine
small and high gradient properties, the first one structure elements. The mixture process in the
called quasi-homogeneous layers and the second turbulent volume decreases temperature and
one by layers. Inside temperature (T) and salinity salinity gradients inside it, being critical on its
(S) profiles, sections with inverted properties boundaries.
distributions are frequently encountered. Spots formation due developed turbulence may
The fine structure is understood as a model of also occur because instability or breaking of
physical fields represented by a set of layers internal waves (Fedorov, 1976, Fedorov, 1991,
instead of homogeneous properties in strata Konyaev and Sabinin, 1992). This mechanism is
ranging from 0.1 to 1 m, divided by even more observed when the Richardson´s number achieve
thin layers with sharp gradients of temperature a value below some critical value, being the
and salinity. The vertical gradients are 10 to 100 probability of formation of spots of turbulence at
times higher and more, exceeding corresponding instability higher than at breaking of internal
average gradients values (Karlin, 1988). waves (Ohotnikov and Panteleis, 1985). The sizes
The fine stratification of water layer is testified by of vertical instable areas of internal waves vary
numerous measurements, done in different regions from decimeters to meters.
of the ocean with low-inertia sounding equipment, The differential-diffusion instability is due to
in particular CTD- probes. Contrasting profiles diffusion of two or more components of
obtained by standard hydrological instruments, hydrodynamic system at different speeds. Such
these profiles contain many structural details, instability forms a fine structure of temperature
reproduced by repeated soundings and, therefore, and salinity fields in water with a wide circulation
which live long. The frequently curves of vertical at the ocean. Because differences in the molecular
distribution T and S take the form of correct steps thermal conductivity coefficients and salt
or variables on the sign of deviations from the diffusion in the system, the convective instability
average profiles. There are fine-structure appears, producing intensive mixing and forming
heterogeneities in the seasonal and main quasi-homogeneous layer divided by layers with
thermoclines on big depths and even in the upper sharp gradients properties (Turner, 1985).
quasi-homogeneous layer of the ocean (Karlin et A special attention is paid to the mechanism of
al., 1988). Fedorov showed (Fedorov, 1976) that differential-diffusion instability, it allows
below the upper quasi-homogeneous layer, a fine accelerated heat and salts transfers without energy
structure is formed in the majority of cases against from external sources (Karlin et. al, 1988,
the hydrostatic stability background on the Fedorov et. al, 1986, Karlin, 1988). This
vertical distribution density. mechanism is observed, when temperature and
On these conditions, two mechanism of fine salinity gradients produce opposite contributions
structure formation are possible. The first one to the vertical density gradient. Here, the potential
requires kinetic energy from an external source, energy associated to vertical stratification is used,
used to increase the potential energy in the liquid contributing to destabilize the density gradient.
layer. The mechanisms present are shift instability Liberation of instability energy is due to
of currents, internal waves, etc. The second differences in coefficients of molecular exchange
mechanisms do not require external energy of heat and salts. Differential-diffusion instability
sources. There, the structurization occurs by is manifested in the regime of salt fingers and
liberation of accessible potential energy, layered or diffuse convection.
dissipated later as heat. The mechanism presents
instability due to differential-diffusion convection METHODOLOGY
or dual diffusion (Zhubas and Ozmidov, 1984,
Ozmidov, 1983). Initial natural data
In shift instability of currents and internal waves,
turbulence and elements of fine structure of The calculation of stability of layers in waters of
hydrophysical fields arise under action of strong Colombian Pacific Ocean (Villegas, 2001) was
hydrodynamical instability in zones with high made according to the fields of temperature and
vertical gradients of speed current, like located salinity, obtained in the scientific expedition
areas observed as separate turbulent spots during May 2000 (Otero y Pineda, 2000). The
(Benilov, 1985, Maderich and Nikishev, 1986). vertical distribution of water stability and the
46
Fine Thermohaline Structure Of The Colombian Pacific Ocean


Vaisala-Brunt’s frequency (Villegas, 2002b, possible fine-structure activity in Colombian
Karlin and Villegas, 2003) were made based on Pacific Ocean is represented below.
the example of 3 hydrometeorological stations According to data obtained on Colombian Pacific
obtained in this expedition: 14, 49 and 111 (Figure ocean during May 2000 (Otero and Pineda, 2000),
1). and the help of the Hesselberg Sverdrup’s
criterion (1) ([Malinin, 1998; Valerianova and
9 Zhukov, 1974; Kamenkovich and Monin, 1978),
Panama stability is calculated and contributions of
8 temperature and salinity are located into general
stability Colombian Pacific ocean (Villegas,
7 8 2001).
17 9 1
25 18 10 2 ∂ρ dS ∂ρ dT ∂ρ dT6 A (1) E = + +
34 26 19 11 3 ∂S dz ∂T dz ∂T dz
5 107 99 91 83 75 67 59 51 43 35 27 20 12 4
∂ρ108 100 92 84 76 68 60 52 44 36 28 21 13 5 With: as density variation with a change in
∂S4 109 101 93 85 77 69 61 53 45 37 29 22 14 6
3the salinity, kg/m ‰; 110 102 94 86 78 70 62 54 46 38 30 23 15 7
dS as vertical gradient of salinity, ‰/m; 111 103 95 87 79 71 63 55 47 39 31 24 163
dz
112 104 96 88 80 72 64 56 48 40 32
∂ρC olombia as density gradient with change in
113 105 97 89 81 73 65 57 49 41 332
∂T
114 106 98 90 82 74 66 58 50 42 3temperature, kg/m °Cm; Ecuador
1
dT84 8 3 82 81 80 79 78 77 Is the vertical gradient of temperature, °C/m;
Western longitude dz
dTFigure 1. Hydrometeorological stations on the Colombian A as vertical gradient of adiabatic temperature,
Pacific Ocean. dz
°C/m;
These stations, covering the 3 more interesting
zones: coastal zone-station 14 (at 4°N/78°W), The temperature and salinity stability components,
open sea-station 111 (at 3°N/84°W) and zone of and the density relationship R (2) (Karlin et al., p
mixing waters-station 49 (at 2°N/80°W), were 1988, Malinin, 1998, Fedorov, 1991) have been
selected on basis to the quasi-homogeneous zones used for carrying out of diagnostics of possible
on this study region (Villegas, 2002a). fine structure activity at ocean on background T-S
profiles.
Diagnostic of possible fine structure activity in