Physico-Chemical Description at the West of the Gulf of ... · The Gulf of Tehuantepec is exposed to strong winds from the north during the winter, called ''Tehuantepecanos" or "Tehuantepequeros"

Physico-Chemical Description at the West of the Gulf of Tehuantepec, Mexico, at the End of

the 1982-1983 ENSO Episode

Guadalupe de la Lanza-Espino" Julieta Luna-Martínez'

Ángel Francisco Verdi-Lara'

Resumen

La distribución de las características fisicoquímicas locales del Golfo de Tehuantepec es afectada por la dinámica del Pacífico Este y los vientos denominados Tehuantepecanos. Situación estudiada por diversos autores, pero sin considerar el efecto de fenómenos eventuales como El Niño, motivo del presente estudio. La capa de mezcla estuvo comprendida en los 30 m de profundidad en donde se perdió la identidad de la masa de agua en la época de lluvias y huracanes. La termoclina y la capa anóxica hacia el oeste del área de estudio se ubicaron a 100 m de profundidad, interrumpidas por dos giros anticlónicos costeros detectados por la temperatura y la salinidad. Debido a que el estudio fue realizado en la fase final de El Niño 1982-83, perceptible por el incremento de 1 ºC, es posible que haya influido en la alternancia de los giros.

Abstract

Distribution of the local physico-chemical characteristics of the Gulf of Tehuantepec is affected by oceanic dynamics of the Eastern Pacific and by winds called ''Tehuantepecanos". This distribution has been studied by several authors without considering the effect of eventual phenomena such as El Niño. These spatial condi tions were the purpose of this study. The mixing layer was found from the surface to 30 m of depth, after which the water mass could no longer be identified during the rainy and hurricane seasons. The thermocline was deepest to the west of the study

• Dirección General de Oceanografía Naval, Secretaría de Marina. Edificio B ler. Nivel, Calle Capi tán de Fragata Pedro Sainz de Baranda Borreyro s/n. Col. los Cipreses, Coyoacán, México, D.F.

•• Instituto de Biología, Apdo. Postal 70-153, Univ. Nal. Autón. de México, C.P. 04510, México, D.F. e-mail: [email protected]

Derechos Reservados Citar fuente - Instituto Panamericano de Geografía e Historias

36 Guadalupe de la Lanza-Espino et al. Revista Geoflsica 54

area and an anoxie layer was found at 100 m, interrupted by two anticyclonic coastal gyres detected through measurements of temperature and salinity. Since the study was performed during the final phase of the 1982-83 El Niño, perceived through a 1 ºC increase in water temperature, this phenomenon could have influenced the al ternation of gyres.

Introduction

The environment of the Gulf of Tehuantepec has been well studied by several au thors. Since 1929, Hurd gave a detailed description of the pattern of dominant winds. The U.S. Hydrographie Office (1951; vide in Secretaría de Marina, 1974) studied surface currents during a "Tehuantepecanos'', Blackburn (1962) detected constant winds from the north during the winter in the Gulf of Tehuantepec. These winds originate a circulation pattern that results in a wavy pycnocline and a hypothetical eutrophication, apparently responsible for the high biological productivity of this area. The same author found the lowest temperature to the south of the Gulf, due to the upwelling generated by the wind.

Roden ( 1961) detected winter upwellings based on lower surface temperatures than in other coastal areas to the south of Mexico, and pointed out the wind as the main cause of upwellings.

With respect to surface circulation, Blackbum (1962) described three gyres go ing NW, SE, and NE, mainly from October to January and from February to April. Flow was anticyclonic near the coast and cyclonic out to the sea, with velocities from 3 to 90 cm/seg depending on the strength of the wind, latitude, and the season.

Wyrtki (1966) detected less than 1 ml/I of dissolved oxygen in the minimum oxygen layer within the Gulf.

The Gulf of Tehuantepec is strongly affected by land runoff to the west during the autumn, which decreases salinity significantly. This salinity decrease in diverse water units has been widely reported: Secretaría de Marina (1982), Moulin (1988), Luna Martínez (1991), Vázquez Gutiérrez et al. (1998), and Secretaría de Marina (2000). Moulin (1988) defined an equatorial type, north Pacific sub-type water mass in the Gulf of Tehuantepec, based on changes in salinity from 30 to 35.4 psu and temperatures from 27 to 30ºC.

Trasviña and Barton (1997) refer to an upwelling event on the east side of the Gulf ofTehuantepec in January and February of 1989, which was not intense enough for the thermocline to breach the surface. Besides, they reported that the intense surface cooling was a result of vertical turbulent mixing induced by the wind. On the western side of the Gulf, a depression of the thermocline resulted from the advection of warm surface waters on their way to the Gulf's interior as they form an anticy clonic gyre.


enero-junio 2001 Physico-Chemical Description at the West of the Gulf... 37

Periodic climatic changes, such as hurricanes and cyclones, may alter differently the physico-chemical structure of coastal and oceanic waters. The Gulf ofTehuantepec exemplifies this, through changes in the thermocline depth, mixing layer and anoxie layer depths, and in other properties. The El Niño phenomenon can also modify the physico-chemical structure (Gallegos García et al., 1984).

Gallegos García et al. (1984) detected increases in temperature (6ºC) and a low ering of the anoxie layer to depths below 100 m in front of the State of Guerrero, in the Mexican Pacific during the 1982-83 El Niño.

Studies on the effect of the winds ("Tehuantepecanos") will enrich oceanographic information, allowing to distinguish high energy conditions from low energy condi tions as well as natural conditions, created by tropical storms and eventual phenom ena, such as the ENSO (El Niño Southern Oscillation).

The present study was aimed at increasing the knowledge on the physico-chemi cal structure (temperature, salinity, dissolved oxygen and orthophosphates) of the water column in an area of oceanic gyres in the Gulf of Tehuantepec during the last phase of 1982-1983 El Niño.

Description of the Study Area

The Gulf of Tehuantepec (Figure 1) is located to the south of the Mexican Republic in the region of the Intertropical Convergence (Roden, 1961) at 14°33 '00" - 92º13' 15" Wand 15º38'00" - 96º30' OO"N. The coast arches inland between Puerto Angel, Oaxaca, and the mouth of the Suchiate river in Chiapas, 463 km to the SE. The Gulf has a triangular shape, its base is an imaginary line joining Puerto Angel to the mouth of the Suchiate river, 87 km to the east. This area includes an 18 to 22 km shoal with minimum depths of 9 to 11 m within the 180 m depth line.

The Gulf of Tehuantepec is exposed to strong winds from the north during the winter, called ''Tehuantepecanos" or "Tehuantepequeros" (Blackburn, 1962). These winds mix the surface layer of the Equatorial Water Mass that originates in the tropi cal region of the Central Pacific at a depth of 50 to 500 m (Roden and Groves, 1959).

During a "nortes" season with high winds proceeding from the Gulf of Mexico, the water moves to the SW and causes a rising current that moves to the north to fill the space left by the water mass that moved away initially. The resulting circulation patterns define a wavy pycnocline that reaches a maximum depth of 12.3 m from the surface and enriches the surface layers with cold and nutrient water (Blackbum, 1962, 1963; Secretaría de Marina, 1974). Besides, the system of currents is gener ally very irregular due to changes in wind direction and frequent lapses of calm weather. Currents are found to the east and southeast along the coast between Manzanillo (Colima) and Sacrificios (Oaxaca) during the winter, and to the west and northwest during the summer when the dominant winds come from the southeast.


38 Guadalupe de la Lanza-Espino et al. Revista Geojîsica 54

Further out to the sea and with respect to an imaginary line from Acapulco, Guerrero, to Isla Motuosa in California, the current is localized to the east-northeast, whereas near the coast of Guatemala it is to the E or SE, events that affect strongly the local currents in the Gulf of Tehuantepec (Secretaría de Marina, 1974).

Material and Method

A total of 28 oceanographic stations along four north-south transects (Figure 1) were sampled from June 15 to June 30, 1983.

Temperature and salinity were measured in situ at depths of O, 5, 10, 20, and 30 m with a calibrated Hydrolab type Kahlsico multianalizer; a calibrated bathyther mograph was also used at depths ofO, 30, 61, 91, 122, 153,183,213, and 244 m, to determine the thermocline depth.

Dissolved oxygen concentration was determined at depths of2.5, 100,500,900, and 1200 m following the Micro-Winkler method described by Strickland and Par sons (1972). The analyses were completed on board the Research Vessel Mariano Matamoros of the Mexican Navy.

Orthophosphates (µM), were analyzed at depths of 2.5, 100, 500, 900, 1200, 1600, and 2000 m with a Technicon MT-II Autoanalyzer as proposed by Murphy and Ryler (1962) and described by Strickland and Parsons (1972).

Data were organized for groups of stations based in Tables (1, 2, 3) and figures with a profile representation for each transect.

Results

Temperature varied from 29 to 32ºC at the surface and from 28 to 31 ºC at a depth of 30 m, at which the thermocline was localized in all transects (Figures 2 A, B, C, D). The thermal profile in the first 30 m indicated a possible sinking with its center at a distance of 154 km off the coast (Stations 4, 11, 18, 25) (Figures 3 A, B, C, D).

Salinity varied from 30.l to 35.6 psu at the surface and from 31.1 to 34.7 psu at 30 m depth. Two sinkings were noted, one at 154 and the other at 286 km off the coastal line (Stations 4,11,18,25) (Figure 4A,B,C,D). Based on the figures, it can be implied that the west area is barely more saline than the east.

Dissolved oxygen varied greatly from the surface down to 100 m depth. At the surface, values were 3.76 to 4.04 ml/I, and at a 2000 m depth (Figures 5 A, B, C, D) it varied from 0.17 to 1.85 ml/I. The vertical profile by transect revealed scarce dif ferences in depth at the begining of the oxycline, the oxygen concentration was relatively more superficial on the east side (F'igures 6 A, B, C, D).

A great mass of anoxie water with 0.5 ml/I was found at a 286 km distance at depths of 350 m, and at a 2000 m depth at a distance of 100 km.


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Table 1 Profile of the temperature at the west of the Gulf of Tehuantepec

Stations Depth (mJ Temperature (ºCJ Stations Depth (mJ Temperature (ºCJ

I o 30.6 6 122 18.06 30 31.66 163 16.94 61 30.00 183 16.39 91 22.50 213 16.11

2 o 30.70 244 15.83 30 31.94 274 15.28 61 29.17 7 o 29.80 91 22.78 30 31.67

122 18.89 61 24.44 163 16.39 91 19.44 183 15.28 122 16.94 213 14.72 163 16.11 244 14.44 183 15.56 274 13.89 213 15.00

3 o 30.30 244 14.44 30 31.39 274 14.17 61 29.44 8 o 30.20 91 21.11 30 31.67

4 o 30.80 61 23.89 30 31.67 91 20.28 61 27.50 122 18.33 91 19.72 163 16.67

122 16.39 183 15.83 163 15.00 213 15.00 183 14.44 244 14.44 213 14.17 274 13.89 244 13.61 9 o 30.90 274 13.06 30 31.67

5 o 30.20 61 26.67 30 31.39 91 19.44 61 26.67 122 16.67 91 21.11 163 15.83

122 18.33 183 15.28 163 16.67 213 15.00 183 15.83 244 14.44 213 15.28 274 14.17 244 14.72 10 o 29.80 274 14.17 30 31.39

6 o 30.00 61 26.67 30 31.94 91 21.11 61 26.94 122 18.06 91 20.00 163 16.89



Continue Table I

Stations Depth (m) Temperature (ºC) Stations Depth (m) Temperature (ºC)

10 183 15.56 15 o 30.65 213 15.00 30 31.67 244 14.44 61 31.67 274 14.17 91 25.56

Il o 30.90 122 18.33 30 30.83 163 16.11 61 26.11 183 15.56 91 20.28 213 15.00

122 16.94 244 14.72 163 15.56 274 14.17 183 15.00 16 o 30.60 213 14.44 30 31.39 244 14.44 61 31.11 274 13.89 91 24.44

12 o 30.70 122 19.17 30 31.67 163 16.39 61 24.72 183 15.56 91 20.56 213 15.00

122 18.89 244 14.44 163 16.94 274 14.17 183 15.83 17 o 30.55 213 15.28 30 31.67 244 14.72 61 28.33 274 14.44 91 23.33

13 o 30.50 122 18.33 30 31.94 163 16.39 61 30.00 183 15.56 91 23.89 213 15.28

122 18.33 244 15.00 163 16.39 274 14.44 183 15.56 18 o 31.45 213 14.72 30 30.56 244 14.44 61 26.67 274 14.17 91 20.56

14 o 30.55 122 17.50 30 31.94 163 15.56 61 31.11 183 14.72 91 25.56 213 14.17

122 19.44 244 13.89 163 16.94 274 13.61 183 15.83 20 o 30.09 213 15.28 30 31.67 244 14.72 61 25.56 274 14.44 91 19.44



Continue Table I

Stations Depth (m) Temperature (ºC) Stations Depth (m) Temperature (ºC)

20 122 17.22 24 213 15.00 163 16.11 244 14.44 183 15.56 274 14.17 213 15.00 25 o 31.00 244 14.72 30 31.11 274 14.17 61 25.00

21 o 30.50 91 18.33 30 31.94 122 16.39 61 27.78 163 15.28 91 21.94 183 14.72

122 19.72 213 14.44 163 16.94 244 13.89 183 16.39 274 13.61 213 15.56 26 o 30.40 244 15.28 30 31.67 274 14.72 61 28.89

22 o 30.00 91 21.94 30 31.94 122 18.06 61 25.00 163 16.94 91 20.56 183 15.83

122 18.89 213 15.28 163 17.22 244 15.00 183 16.11 274 14.44 213 15.28 27 o 30.40 244 14.72 30 31.94 274 14.44 61 31.67

23 o 30.00 91 23.33 30 31.67 122 18.33 61 26.67 163 16.94 91 20.00 183 15.83

122 17.22 213 15.28 163 15.83 244 15.00 183 15.28 274 14.72 213 15.00 28 o 30.60 244 14.44 30 31.67 274 14.17 61 31.11

24 o 30.08 91 23.06 30 31.67 122 16.94 61 26.39 163 15.56 91 21.11 183 14.44

122 17.22 213 13.89 163 16.1 I 244 13.61 183 15.56 274 13.06



Table 2 Profile of dissolved oxygen and phosphorus at the west of the Gulf of Tehuantepec

Dissolved Dissolved Depth Oxygen Phosphorus Depth Oxygen Phosphorus

Stations (m) (mVI) (µM) Stations (m) (ml!/) (µM)

I 2.5 4.05 0.76 8 2.5 4.00 0.69 100 2.25 1.12 IOO 1.53 1.46 300 0.24 - 500 0.60 1.80 500 - 2.04 900 0.54 2.02

2 2.5 4.00 2.38 1200 1.80 2.76 100 4.IO 0.72 9 2.5 4.00 0.60 300 - 1.68 IOO 1.05 1.84 500 0.42 - 500 0.40 0.50 900 0.33 2.80 900 0.40 0.48

1200 0.30 2.60 1200 - - 3 2.5 4.00 0.76 IO 2.5 4.00 1.74

100 4.11 0.60 100 1.02 2.44 500 4.IO 0.78 500 0.27 0.70 900 0.24 2.48 900 0.70 1.52

1500 1.12 4.04 1200 0.57 1.88 2000 2.00 1.06 li 2.5 4.00 2.20

4 2.5 4.00 100 2.25 2.18 100 1.14 1.44 500 0.27 0.64 500 0.30 2.24 900 0.80 1.12 900 0.30 1.36 1200 0.70 2.20

1500 0.45 1.76 12 2.5 4.00 3.18 2000 0.30 1.96 100 1.80 2.78

5 2.5 4.00 0.74 500 0.46 0.78 100 0.40 1.96 900 0.28 1.32 500 0.72 2.40 1200 0.45 2.34 900 0.24 2.58 13 2.5 4.00

1200 0.70 2.66 100 1.60 2.54 6 2.5 3.93 1.04 500 0.81 2.52

100 0.70 1.36 900 0.70 1.20 500 0.40 1.98 14 2.5 4.00 2.12 900 0.66 2.38 100 2.31 2.44

1200 0.57 2.44 500 0.30 0.56 7 2.5 4.00 0.82 15 2.5 4.00 1.10

100 1.80 1.76 100 2.10 2.28 500 6.00 2.30 500 0.33 0.60 900 0.18 2.40 16 2.5 4.00 0.61

1200 1.35 2.20 100 4.00 1.98



Continue Table 2

Dissolved Dissolved Depth Oxygen Phosphorus Depth Oxygen Phosphorus

Stations (m) (mlll) (µM) Stations ( m) (mlll) (µM)

16 500 0.80 0.78 22 900 OJO 2J2 900 0.87 0.40 1200 0.70 2.64

17 2.5 4.00 2.02 23 2.5 4.00 0.62 100 3.00 2.14 100 1.35 1.48 500 OJ3 0.60 500 0.48 1.66 900 0.20 0.92 900 0.45 2J2

1200 0.40 2.17 1200 0.50 0.90 18 2.5 4.00 2.48 24 2.5 4.00 0.54

100 1.00 2.52 100 0.90 1.52 500 OJO 0.64 500 2Jl 1.90 900 0.74 2.00 900 0.40 2J6

1200 0.22 2.64 1200 1.02 2.48 19 2.5 4.00 2.45 25 2.5 4.00 0.70

100 1.41 1.84 100 2.00 2J4 500 OJO 0.62 500 0.50 2.58 900 0.20 1.36 900 0.60 2.64

1200 1.10 2.28 1200 0.55 3.80 20 2.5 4.00 2.18 26 2.5 4.02 0.90

100 4.05 2.42 100 1.80 1.26

500 0.72 0.50 500 0.54 2.06

900 1.15 1.32 900 OJ3 2.46

1200 0.66 1.84 1200 OJ6 2.94 27 2.5 4.00 0.76

21 2.5 4.00 2.49 100 2.00 2.24 100 1.71 1.08 500 0.52 2.50 500 OJ3 1.92 900 0.54 2.50 900 0.72 - 1200 0.56 2.40

1200 0.72 2.40 28 2.5 4.00 0.78 22 2.5 4.00 0.76 100 2.22 1.40

100 1.80 1.04 500 OJ3 2.54 500 0.60 1.86



Table 3 Profile of the salinity at the west of the Gulf of Tehuantepec

Stations Depth (m) Salinity UPS Stations Depth (m) Salinity UPS

I o 35.6 10 s 32.I s 34.2 10 32.8

10 34.7 20 33.8 20 34.7 30 33.8 30 34.7 Il o 31.45

2 o 32.7 s 31.45 s 32.7 10 33.1

10 32.7 20 33.S 20 33.4 12 o 32.S

3 o 32.7 s 32.S s 32.7 10 32.S

10 32.7 20 33.0 20 33.0 13 o 32.09

4 o 30.1 s 32.76 s 30.1 10 33.0

10 30.1 20 33.4 20 30.I 14 o 32.S 30 31.1 s 32.S

s o 31.1 10 32.S s 31.7 20 32.S

10 32.S IS o 32.S 20 33.2 s 32.S

6 o 32.8 10 32.S 5 33.S 20 32.S

10 33.5 16 o 32.09 20 33.S 5 32.09

1 o 31.8 10 32.3 5 32.1 20 33.4

10 32.5 17 o 32.09 20 32.8 s 32.09

8 o 32.1 10 32.76 5 33.1 20 32.76

10 33.5 18 o 31.26 20 33.5 5 31.46

9 o 31.46 10 32.09 5 32.76 20 32.09

10 33.1 30 32.76 20 32.8 19 o 32.76

10 o 32.1 s 33.I


62 Guadalupe de la Lanza-Espino et al. Revista Geof(sica 54

Figure 8. Temperature-Salinity diagram at the first 30 min the western Gulf of Tehuantepec (Trasviña personal communication).


56 Guadalupe de la Lanza-Espino et al. Revista Geof(sica 54

Continue Table 3

Stations Depth (m) Salinity UPS Stations Depth (m) Salinity UPS

19 10 33.5 24 o 32.7 20 33.5 5 32.7 30 33.5 10 33.7

20 o 31.7 20 33.4 5 31.7 25 o 30.7

10 33.2 5 32.08 20 33.2 10 32.10

21 o 30.4 20 32.76 5 33.2 26 o 32.08

10 33.2 5 32.0 20 33.2 10 32.76

22 o 31.8 20 32.76 5 32.1 27 o 32.09

10 32.8 5 32.76 20 32.8 10 32.76 30 33.4 20 32.76

23 o 32.08 28 o 33.0 5 32.08 5 33.0

10 32.76 10 33.0 20 33.1 20 33.0 30 33.5

Surface orthophosphate concentrations in the study area oscillated between 0.6 to 3.18 µMat the surface and 0.54 to 3.80 µMat a depth of 1200 m. Isolines were concentric and formed a nucleus at 70.5 km from the coastal line at a depth of 1500 min the west side (Figures 7 A, B, C, D). Concentration differences were noticeable, being relatively higher in the west than in the east, as well as a decreasing gradient from the coast to open sea.

Discussion

Surface temperature recorded in June varied from 29 to 31 ºC differing from those recorded by Roden and Groves ( 1959) in 1 ºC. These authors registered surface tem peratures of 28 to 30ºC, similar to those recorded by the U.S. Hydrographie Office (1947). However, Blackbum (1962) and Reid (1969) recorded temperatures of 28 to 31 ºC and 29 to 30ºC, respectively.

Ritter and Guzman (1983) recorded surface temperatures ranging from 29 to 31 ºC in front of Salina Cruz, Oaxaca, in June 1962, which is the same as the one recorded in this study for June 1983. However the Secretaría de Marina (l 978) re-


enero-junio 200 I Physico-Chemical Description at the West of the Gulf... 57

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enero-junio 2001 Physico-Chenúcal Description at the West of the Gulf... 59

corded a range of 29.3 to 30.SºC, and Trasviña and Barton (1997) referred surface temperatures of 29-29.SºC for the summer of 1959.

The slight difference of 1 ºC that was observed, in general, for surface waters, might be related to the 1982-1983 El Niño, which had been reported by the Tropical Ocean Global Atmosphere, of which we studied the advanced stage.

Thermal profiles define a well-developed thermocline that starts at 30 m and ends at 125 m (Figures 2A, 2B, 2C, 2D). Taking into consideration the information provided by other authors, without details of depth, the thermocline in the Gulf of Tehuantepec is shallower to the east and deeper to the west of the fraction area studied in this paper.

An analysis performed by Trasviña and Barton (1997) of data obtained during the 1958 EASTROPIC cruise, shows a thermocline elevation (to between 10 and 20 m), especially to the east of the Gulf of Tehuantepec. Beyond the Gulf, this eleva tion is reduced to betwen 30 and 40 m to the east. This distribution is associated to the circulation dynamics generated by the wind as referred by Blackburn (1962).

The mixing layer to the west of the Gulf of Tehuantepec was detected from a depth of 30 m, where the thermocline began. Moulin (1988) found a mixing layer at depths of 20 to 40 m to the east, of 15 to 25 min the central section, and of 10 to 70 m to the west, with a thermocline approximately from 50 to 80 m. According to the quarterly charts of thermocline topography of the eastern tropical Pacific by Cromwell (1958), the mixing layer in the Gui ofTehguantepec has an average depth of between 10 to 30 m, except in the October-December quarter, when it was less than 10 m. In this sense, Alvarez et al. (1958) found no vertical thermal structure throughout a 30 m water column.

Longhurst and Pauly ( 1987) pointed out that the mixing layer in the Pacific Ocean lies betwen depths of 20 to 40 m and has a narrow thermocline. They also stated that the topography of the thermocline and the depth of the mixing layer are important factors that contribute to the formation of anticyclonic gyres in the Northern Pacific Ocean. These gyres cover thousands of kilometers as they are formed by the Trade Winds and the influence of the continental slope. At this point, the U.S. Navy Hy drographie Office (1951) referred: transitory eddies, clockwise in the west of the Gulf and anticlockwise in the east, which seems to occur rather regularly in associa tion with northerly winds, as judged from the following remarks on inshore currents.

An anticyclonic gyre with high temperatures between the surface to 30 m depth was detected in this study in the western region of the Gulf of Tehuantepec, and towards the center of the study area. A surface diameter of aproxima tel y 88 nautical miles (163 km) was calculated for the gyre, together with another one of 122 nauti cal miles (226 km) at a depth of 30 m (Figures 3A, 3B, 3C, 3D) in the same line of latitude. This coincides with the presence of gyre alternation calculated, based on dynamic topography, by Trasviña and Barton (1997) in three different cruises (No vember, September, and March), although at different elevations each one, allowing


60 Guadalupe de la Lanza-Espino et al. Revista Geojîsica 54

them to state that the circulation in the Western Gulf is dominated by a slow flow to the south, away from the coastal line. Minimal elevations coincided with the maxi mal elevation of the thermocline and cyclonic circulation; whereas, to the west end, the curve of the dynamic elevations indicated an anti-cyclonic circulation, which was the main finding of this study, especially at latitude 14º19' (stations 4, 11, 18, and 25) and between 25º22' and 96º16'W, which was the most important gyre found in the most western part of the Gulf.

McCreary et al. (1989) consider that throughout the wind event, cyclonic and anticyclonic gyres spin up offshore on either side of the jet axis to Ekman pumping. Entrainment cools SST offshore, on and to the right (looking onshore) of the jet axis, and virtually eliminates the cyclonic gyre. The advection terms intensify the anticy clonic gyre and give it a more circular shape. After a wind event, the anticyclonic gyre propagates westward due to ~-

Stumpf (1975) detected a 200 km wide circular anticyclonic gyre near the coast to the west of the Gulf ofTehuantepec in December 1973, although smaller and less developed towards the end of the same month. The same author detected another gyre in this area at the beginning of 1974 after a "Tehuantepecano". Another 280 km wide anticyclonic gyre was observed through satellite images in February, 1976, 40 km from the coastal line (Stumpf and Legeckis, 1977).

Roden ( 1961) also detected an anticyclonic gyre in the same area as Stumpf and Legeckis (1977), that is, in the northern region of the Gulf of Tehuantepec (Salina Cruz), when making observations onboard a ship. These authors have pointed out that during the winter the two areas with greatest upwellings in the Pacific are those of the Gulf of Tehuantepec and the Gulf of Papagayo, Costa Rica. Moulin (1988) registered an anticyclonic gyre at 95°30' in October 1979, which is well within the present study area.

The small difference of 1 ºC, during a maximum grade-4 El Niño phenomenon as the one registered for 1982-83, as compared with the values obtained by the above mentioned authors might be explained by the degree of advancement of the 1982-83 El Niño. To this regard, Gallegos-Garcia et al. (1984) and Ramos-Duron et al. (1986) reported that, in the mid-phase of the grade-4 El Niño 1982-83, the temperature increased between 4-7ºC.

Surface salinity varied from 30.1 to 33 psu and increased with depth. This range of values is different from Blackbum 's ( 1962) of 34 to 34.1 psu. The Secretaría de Marina (1978) obtained values of 33.4 to 33.8 psu that also increased with depth. Moulin's (1988) values for the Gulf of Tehuantepec varied from 31 to 33 psu. Salin ity was generally low probably due to the heavy rains generated by hurricane Bar bara that hit during the sampling season and to the great volume of epicontinental waters discharged by the Copalita, Tehuantepec, and Hondo rivers. It is important to point out the changes in climatic and oceanographic conditions caused by the 1982- 83 El Niño. These were an increase in sea level, surface thermal anomalies, strong



rains and salinity lower than 33 psu. The latter might have been caused by an advec tion of water from the north and a dilution due to local rains, as Cucalón ( 1987) pointed out for Ecuador.

The low salinities and high temperatures, when plotted on the T-S curve, re vealed density jumps (Figure 8) that, according to Trasviña (personal communica tion), are unbelievable since they do not characterize any water mass; and the salinity cannot decrease so much by just coastal rain discharges and strong rains resulting from hurricanes, However, this salinity decrease, to 30 psu or even less, was not only recorded in this study (June 1983), but also by other authors (Moulin, June 1988; Rivero-Belatran and Benitez Torres in September and "Nortes" season of 1990; V ázquez Gutiérrez et al., November 1998 and Secretaría de Marina, October inedited). These latter findings support the notion that these low salinity recordings were not due to technical or methodological problems but rather, that in the first 30 m, (coin ciding with the depth of the mixing layer based on temperature) the water mass looses its identity during the season of strong rains, "nortes", and very especially in the presence of hurricanes.

A gradual decrease in salinity was registered from the coast at a distance of 83 nautical miles (154 km) followed by an increase in salinity. These changes indicate an anticyclonic gyre with a surface diameter of73 nautical miles (148 km) (Figures 4A, 4B, 4C, 4D), similar to that defined by temperature, for which the greatest diam eter of 130 nautical miles (241 km) was detected at a depth of 30 m. A cyclonic gyre was registered in more oceanic waters with a 73 nautical miles (148 km) diameter which increased to 109 nautical miles (292 km). Moulin (1988) reported an anticy clonic gyre to the west of the Gulf and a cyclonic gyre to the center of the area, whereas Longhurst and Pauly (1987) state that the Gulf of Tehuantepec is character ized by an anticyclonic gyre. This alternation of gyres is considered a generalized phenomenon in the Gulf of Tehuantepec.

The concentration of dissolved oxygen, calculated for the surface layer in June, varied from 3.71 to 4.04 ml/I and is similar to the values reported by Blackburn (1962) of 3.5 to 4.5 ml/I and Moulin (1988) of 2.5 to 5 ml/I. However, values re ported by the Secretaría de Marina (1978) were different and varied from 0.5 to 5 mUI, as well as those recorded by Rivero Beltrán and Benitez-Torres ( 1990) of 5 to 6 ml/I between Salinas del Marquéz and Bahía Ventosa, Oaxaca.

A range of0.5 to 4.1 ml/I was detected at depths from 100 to 200 m. The lower concentration coincides with that recorded by Blackburn (1962). Apparently, these values are subjected to changes, since the Secretaría de Marina (1978) measured from 0.09 to 3 mut and Moulin ( 1988) obtained a value of 0.25 ml/I.

The minimum dissolved oxygen layer was found above 100 m, whereas Blackbum (1962) placed it betwen 50 and 100 m and Gallegos et al. (1984) betwen 35 and 55 m, in the coastal waters off Guerrero during February and April 1982. The upper limit of the minimum oxygen layer is generally reported at the same depth (Figures



SA, SB, SC, SD); although Gallegos et al. (1984) found it below 100 min November 1983 because of the 1982-83 El Niño.

It is important to point out that the minimum oxygen layer is interrumpted by two sinkings, where it was found below 100 m, from 168 to 318 km off the coast. Blackburn (1962) placed the anoxie layer betwen 75 and 125 m. However, Sverdrup et al. (1970) mentioned it from 100 to 800 m in the Gulf of Tehuantepec and the Secretaría de Marina (1978) registered it from 50 to 1000 m.

Surface orthophosphates for June 1983 varied from 0.54 to 3.18 µM and were quite different from those registered by Blackburn (1962) of 0.25 to 0.5 µM, 20 years earlier.

It is important to mention that orthophosphate distribution depends on advective circulation (Bigelow, 1931; Wyrtki,1961, 1962), which is responsible for the trans portation of nutrients to the euphotic zone. The great variation in values can be confirmed by Moulin's (1988) maximum of 1 µMIi. Ramos et al. (1986) also found a range of 1 to 3 µMil in January 1983 in front of the state of Guerrero, which is similar to the value recorded in this study.

Information on orthophosphates below 1000 m is scarce, therefore, data used in this discussion refers only to depths of 100 to 200 m, and varies from 0.6 to 2.5 µM. Most reports fall within this range: Blackburn's (1962) of 2.25 to 2.5 µM and Secretaría de Marina ofO.l to 2 µM for 1978, and of0.5 to 1.5 µM for 1988. Ramos et al. (1986) obtained a range of0.5 to 6 µMas a result of rains that caused a meses cale increase in the contribution of this nutrient and the topographic effects caused by the 1982-83 El Niño.

It must be mentioned that this study was performed towards the end of the 1982- 83 ENSO, which might be the reason for the sligth changes observed in the studied physico-chemical variables.

Conclusions

The mixing layer did not extend below 30 min June 1983, which confirms that its position in the Gulf ofîehuantepec is generally shallow. It is also closely related to the thermocline, which reaches a depth of 125 m.

Salinity concentration, which was generally low, changed due to dilution caused by the strong local rains (hurricane Barbara) that hit during the sampling season and to the contribution of epicontinental water to the ocean.

Oxygen distribution varied greatly. The minimum layer was detected above 100 m with an extention of 800 m. The latter was interrupted twice by sinkings betwen 138 and 168 km off the coast.

Orthophosphate distribution was closely associated to the minimum oxygen layer: orthophosphate concentration increased with a decrease in oxygen concentration.


64 Guadalupe de la Lanza-Espino et al. Revista Geofîsica 54

The analyzed variables allowed to detect alternate anticyclonic and cyclonic gyres to the west of the study area, possibly generated by the wind. The anticyclonic gyre was defined mainly by temperature and salinity and was localized to the northwest of the Gulf. The alternation of gyres is considered normal for this region of the Pacific Ocean. The dynamics of the anticyclonic and cyclonic gyres together with the geomorphology and bathymetry of the Gulf of Tehuantepec were influenced by the effects of the 1982-1983 El Niño in its final stage.

Acknowledgments

We thank Salvador Hernández for assistance in the preparation of the manuscript and figures.

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Physico-Chemical Description at the West of the Gulf of ... · The Gulf of Tehuantepec is exposed to strong winds from the north during the winter, called ''Tehuantepecanos" or "Tehuantepequeros"

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