inter-american tropical tuna commission comision ......a study of the temporal and spatial...

INTER-AMERICAN TROPICAL TUNA COMMISSIONCOMISION INTERAMERICANA DEL ATUM TROPICAL

Bulletin - Bolet~n

Vol. 14. No. 3

ON THE PHYSICAL AND BIOLOGICAL OCEANOGRAPHYNEAR THE ENTRAN'CE OF THE GULF OF CALIFORNIA,

OCTOBER 1966 - AUGUST 1967

OCEANOGRAFIA FISICA Y BIOLOGICA CERCA A LAENTRADA DEL GOLFO DE CALIFORNIA,

OCTUBRE 1966-AGOSTO 1967

by- por

Merritt R. Stevenson

La Jolla. California

1970

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mnnn...n".nmmn ..mm.mmm393

CONTENTS-INDICEENGLISH VERSION-VERSION EN INGLES

Page

INTRODUCTIONn .muu.389Observations. m mm.humm.m.mU ...hm mmmu.mm ..390Acknowledgements ....n__ . __ n. m.mnh...... mmmm.n.m.391

DESCRIPTION OF THE AREAm.mnummmmmnm nmnm..m391Meteorologymn.um.. mmunuum 391Water charaoteristics.L.h.'m. muu', ...u.hn'm"nh Uh mmu .mm..u.uh 392Surface circulation uuumuu..uuumum..m m.u m.nmu.393

DISCUSSION.....nnu.m mu..u nmmmm n""'mnmm ..n..nm..n...uL"JOn unn u U m..nun m.mum.mA82

Observaciones.. .mm.m.483Reconocimiento ..m.u.. nm mum...484

DESCRIPClON DEL AREAu .mn mnnm.n ..484

Caracteristicas del agua .. m ....mmumm.485Circulacion '" n·H .."f;,,;... nn.nn ..U.nuu486

DISCUSION u n'n"n.nm ,mnn .. mn' .u mnu n.mn n..n mmn.u.m.487Corrientes superficiales n..m.m.mmmn487Corrientes a 100 111.............. ..n.mn . .489Caracteristlcas observadas del aguam ...m.mnn.nm.unm..489Caracteristicas de la superficie mn'n" ...n.nn..'n ..nn,"n ..n""'"'''''''' n.... um.·'''JVDistribucion vertical de temperatura y salinidad ..uu nn..mmmmn492Profundidad de la capa mixta, profundidad del disco Secchi,

concentraci6n de clorof'ila, fijaci6n de carbona yradiaci6n solaLu.n.mun.m. .nnn.nnu...nnn....... n..mn .um..m..m..n494

SUM ARlO Y CONCLUSIONES. m....n..498

LITERATURE CITED-BIBLIOGRAFIA CITADA .. ....u.n.502

ON THE PHYSICAL AND BIOLOGICAL OCEANOGRAPHY

NEAR THE ENTRANCE OF THE GULF OF CALIFORNIA,

OCTOBER 1966-AUGUST 1967

by

Merritt R. Stevenson

ABSTRACT

A study of the temporal and spatial distribution of larval tunas and the con-comitant oceanic conditions was made in cooperation with the Direccion Generalde Pesca e Industrias Conexas of Mexico. Field work consisted of eight hydro-graphic cruises made from October 1966 through August 1967 near the entranceof the Gulf of California.

From January through April, surface currents were southerly at velocities upto 20 em/sec; currents in June were variable in direction and mostly less than10 em/see; by August the surface current was northerly at 10-15 em/sec. Surfacewinds were usually secondary to the distribution of mass as an influence on thesurface circulation. Currents at 100 m were generally similar in direction tothose at the surface, but the water moved more slowly. Between the surface and100 m, southbound currents crossed the entrance of the Gulf at velocities of 5-10em/see during January and April, forming frontal boundaries with the CaliforniaCurrent water, which often occurred south of the entrance.

From April to August, the median concentration of surface chlorophyll aincreased from 0.65 to 0.97 mg/mr, while the median productivity increased from5.6 mgCz'ma/day in April to 17.8 mgCz'mv/day in June before returning to 2.6rngCzmVday in August. Primary productivity was closely correlated with theconcentration of surface chlorophyll a. Productivity was generally higher in thevicinity of the Gulf than that found for water in the open Pacific. Productivitywas highest near Islas Las Tres Marias and second highest near Cabo San Lucas,both locations of local upwelling. The standing crop of phytoplankton was shownto be subjected to progressively heavier grazing pressure in the spring and summerby zooplankton.

INTRODUC'TION

The waters off the coast of central Mexico are important to the fishingindustries of the United States and Mexico because they support a largepopulation of yellowfin tuna, T hunnus albaeares. Though adult fish occurhere throughout the year, they exhibit a great deal of mobility. Taggingstudies (Schaefer, Chatwin, and Broadhead 1961) and data on the distri-bution of the catches (Calkins and Chatwin 1967) show that the areasin which yellowfin occur in highest abundance appear to change seasonallyalong the Mexican coast.

The fish move from one oceanic regime to another during these mi-grations. Since the distribution and movements of the fish are believedto be influenced by the environment, it is necessary to understand the

389

390 STEVENSON

oceanography of the region before the movements of the fish can be fullyunderstood. Broadhead and Barrett (1964) and Blackburn (1963, 1965)have attempted to relate the distribution and abundance of tunas to variousocean properties such as wind, salinity, and surface temperature; theyshowed that temperature plays an important role in their distribution.

Though a great deal of work has been accomplished which attemptsto relate the distribution of adult yellowfin tuna to physical, chemical andbiological features of the ocean, little has been accomplished in relatingthe larval distribution of this species to such features. Such studies maymake it possible to predict anomalous conditions in the ocean and theireffect on the magnitude of a particular year class entering the fishery. Topredict such fluctuations in year-class strength is essential if managementagencies are to assure that maximum benefit is derived from the resourcesfor which they are responsible.

To learn more about the relationship of the early life history of theyellowfin tuna to its environment, the Inter-American Tropical Tuna Com-mission initiated a 2-year cooperative investigation termed the MazatlanProject, with the Direcci6n General de Pesca e Industrias Conexas (DGP)of Mexico in August 1966. The project involved monthly cruises of about5 days along a triangular track between Mazatlan, Cabo San Lucas, andIslas Las Tres Marias (Figure 1). The facilities for the field work wereprovided by the DGP at its laboratory in Mazatlan and aboard its researchvessels Yolanda and Tuxpan. In January 1967 the Mazatlan Project wasincorporated as a part of EASTROPAC, an international cooperative studyof the eastern tropical Pacific Ocean.

This report is concerned primarily with the temporal and spatialdistribution of temperature and salinity in the area of investigation. Therelations of phytoplankton production and abundance to the physical en-vironment and of the zooplankton abundance to the phytoplankton abun-dance are examined. An analysis attempting to relate the distribution ofthe larvae to the- environment is made in a separate publication (Klawe,Pella, and Leet, in press). A data report containing the details of the eightcruises, together with observations collected during each cruise (Leet andStevenson 1969), is available.

Obse-rvationsThe dates for the eight cruises completed during the investigation are:1st Cruise (MZ-1), 15-19 October 19662nd Cruise (MZ-2), 10-16 November 19663rd Cruise (MZ-3), 10-14 December 19664th Cruise (MZ-4), 10-14 January 19675th Cruise (MZ-5), 16-20 February 19676th Cruise (MZ-6), 14-18 Apri119677th Cruise (MZ-7), 22-29 June 19678th Cruise (MZ-8), 23-27 August 1967

OCEANOGRAPHY GULF OF CALIFORNIA ENTFiANCE 391

During each cruise 15 to 17 stations were occupied within 1 hour oflocal sunrise, noon, sunset, and midnight. Bathythermograph (BT) castswere made at each station to measure subsurface temperature, and Knud-sen-Morrison bottles were used to collect water samples for the determina-tion of salinity. No sub-surface salinity samples were collected on the firstthree cruises. Limited weather observations were made at each station atthe time of each BT cast. Water transparency was determined by Secchidisc readings. A solar radiation recorder was installed atop the laboratoryat Mazatlan just prior to the sixth cruise.

Zooplankton samples were collected by making surface and oblique nettows at each station. Samples of young fish were collected at night by dipnetting under a floodlight on the last three cruises. Estimates of thephytoplankton concentration at the surface were made by the measurementof the plant pigments, chlorophyll a and phaeophytin a. Carbon fixation byphytoplankton was measured by the uptake of C':',

AcknowledgementsPreparation of this report has been greatly facilitated with assistance

from the following persons: Mr. Christopher Psaropulos who assisted withthe computer processing of the data, and Messrs. Robert Wagner andGeorge Even, Miss Christine DaB, and Mrs. Judi Shepard who helped inthe preparation of the figures. Helpful suggestions were offered by Mr.Gunnar Roden, Dr. Maurice Blackburn, and Mr. Kimball Crocker.

DESCRIPTION OF THE AREAMeteorology

The region bounded by Mazatlan and Cabo San Lucas in the northand Cabo Corrientes in the south is a warm and relatively humid area. Themonthly mean air temperature is 28-30°C in July, and 17-22°C in January.The rainfall decreases from 100 ern/year near Cabo Corrientes to less than50 em/year at Topolobampo in the southern part of the gulf; 75-90% ofthe precipitation is received from May through October (Roden 1964).

West of Baja California the prevailing northerly winds are the drivingforce of the California Current. Due to seasonal change in the TradeWinds, the predominant winds in the area of investigation blow from thenorthwest at about 5 rrr/sec during winter and spring and from the south-east at about 3 m/sec during summer and fall (Hubbs and Roden 1964).These winds are generally responsible for the northwesterly seasonal trans-port of Eastern Tropical Pacific water into the vicinity of the Gulf. Inaddition, land and sea breezes near the coast help to move the surfacewater offshore or onshore.

From June through October strong local storms or gales calledchubascos may occur and last for several days. Hurricanes also sweep intothe area, especially in September,

392 STEVENSON

The predominantly clear skies and the latitudinal position of the arearesult in large amounts of insolation. The monthly amount of incomingsolar radiation for the latitudes 20° and 25° is shown in Figure 2. Theenergy curves are based on accumulated data for clear skies and aresmoothed (Roden 1959). The seasonal rate of evaporation from coastalstation estimates is fairly constant and amounts to 200-250 ern/year (Roden1964) .

Water characteristics

The area of the present study is considered to be a transition regionin which three main water masses interact to produce a complex localcirculation. These water masses, shown in Figure 3, are: 1) cool CaliforniaCurrent water of low salinity (S~34.60%0), which flows southward alongthe west coast of Baja California; 2) warm eastern tropical Pacific waterof intermediate salinity (34.65~S~34.85'%0),which flows into the area fromthe southeast; 3) and warm highly saline (S~34.90%0) Gulf of Californiawater (Roden and Groves 1959; Griffiths 1965) which appears intermit-tently.

In addition to the three water masses described above, Wyrtki (1967)has defined Subtropical Surface and Subtropical Subsurface water in thisarea. Subtropical surface water is formed by heating and evaporation ofCalifornia Current surface water. Subtropical Subsurface water is thesubsurface portion of the eastern tropical Pacific water referred to byRoden and Groves (1959). It originates at the sea surface south of theequator, sinks while flowing northward, and mixes with the EquatorialUndercurrent water with a consequent decrease ill salinity. It then under-goes further horizontal expansion and northward movement to the entranceof the Gulf of California, where it is found at about 200 m with a salinitymaximum greater than 34.7%0. Griffiths (1968) has included SubtropicalSurface and Subtropical Subsurface water in his comprehensive analysisof water masses near the entrance to the Gulf of California.

Frequently the meeting of California Current water with eastern trop-ical Pacific water or Gulf of California water near the entrance of theGulf produces abrupt changes in temperature and salinity which are calledfronts. Temperature and salinity values across a front may change by asmuch as 4°C and 0.5%0 in 60 miles, although regions possessing weakergradients are often denoted as frontal. In this paper, front will refer to ahorizontal area in the ocean across which there is an abrupt change intemperature, salinity, or some other property (Cromwell and Reid 1956;Clarke and Laevastu 1967). Frequently the gradients extend verticallyalso. Such boundaries separate adjacent water masses (Uda 1959; Griffiths1965), but may also represent the border where a portion of one watermass has upwelled near the coast. Dynamic instabilities frequently occuralong fronts where water of greater density overrides water of lesser den-

OCEANOGRAPHY - GULF OF CALIFORNIA ENTRANCE 393

sity. Such instability may occur in the upper 200 m, and is usuallydenced by inversions in temperature and salinity. The characteristics ofthe surface fronts near Cabo San Lucas were the basis of a study byGriffiths (1965). At times the fronts in this area may be submerged withlittle or no change detectable at the surface (Wooster and Jones 1966).

The oceanic boundary for the Gulf of California is delineated by tem-perature and salinity gradients; it is not confined to the geographic en-trance, but may extend from Cabo San Lucas to Cabo Corrientes (Rodenand Groves 1959).

Surface circulation

The circulation in the area is dependent primarily upon the movementsof the three water masses and the surface winds. The flow of the CaliforniaCurrent along the west coast of Baja California from January throughJune is southerly and parallel to the shore at speeds of about 12 em/sec;in July and August its direction changes to the northwest and it continuesas a weak offshore current (S5 em/sec) through December (Wyrtki1965). Detailed features of the surface circulation along the western BajaCalifornia coast, as inferred from computations of geostrophic flow, aregiven by Wyllie (1966). Estimates of the surface geostrophic currentsnear Cabo San Lucas based on 500- and 1000- decibar surfaces are givenby Griffiths (1968) for the period of April-May 1960. The average surfacecirculation in the area between Mazatlan, Cabo San Lucas and CaboCorrientes is similar to that along the west coast of Baja California; thatis, from November through April the surface water moves in a southerlydirection at speeds of 8-10 em/sec; in May and June the surface driftchanges direction, and by August it has a northwesterly set (5-7 em/sec)that continues through October. The coastal currents, however, may departconsiderably from and be in opposition to the direction of the flow fartheroffshore. Details of the mean monthly surface circulation based on ships'drift data have been outlined by Cromwell and Bennett (1959) and Wyrtki(1965) .

DISCUSSIONSurface currents

Surface currents (Fig. and those at a depth of 100 ill 5) weredetermined from the geostrophic approximation for the last five cruises.These currents are relative since the motion is referred to the 250- decibarsurface (approximated by the pressure level at 250 m) and assumes a zeroflow at 250 m, the greatest standard reference level for which data wereavailable. A comparison of the 250 m level of no-motion with currentsbased on the 500 m surface (Roden and Groves 1959), however, suaaeststhat currents at 250 m are probably less than 2 ern/sec. Briefly stated, with

394 STEVENSON

x directed to the east and y to the north, the horizontal pressure gradientis equated with Coriolis acceleration i.e.,

(1)

g

fu

-tv

p

where u and v are the east-west and north-south current velocities; f is theCoriolis acceleration and varies with the latitude denoted by ep (f =: 1.45 X10-4 Sin ep); f> is the water density; g is the acceleration of gravity; andthe remaining terms are the horizontal and vertical pressure gradients.Other accelerations and forces are assumed negligible in this approxima-tion. In practice geostrophic currents are determined from measurementsof temperature and salinity at different depths.

One complication that arises in the use of geostrophic currents is thatperiodic motions i.e., tides, are frequently superimposed on the geostrophicflow. Tides are known to become progressively larger with increased dis-tance into the Gulf from the entrance. Although records of sea level andsurface barometric pressure have been examined and their interactionestimated for the lower Gulf, the amount of correction necessary to removethe effects of tidal motion from geostrophic currents is uncertain.

Geostrophic surface currents based on the observed data are inreasonable agreement with the generalized circulation for the area. Thatis, from January-April surface water flowed in a southerly directionbetween Cabo San Lucas and Cabo Corrientes at speeds up to 20 em/sec.A portion of the California Current departed from this general pattern inJanuary, however, when it turned north and entered the entrance to theGulf. With the approach of June the circulation became progressivelymore non-uniform. Southeasterly flow was still present near 200N inJune but near Cabo San Lucas an eddy rotated clockwise at speeds ashigh as 20 em/sec; near the mainland a northerly coastal current beganto form. August the surface flow had a definite northwest set andmoved toward the vicinity of Cabo San Lucas; there water flowed south-westerly across the western side of the Gulf entrance.

Details of average surface currents entering and leaving the Gulf(Wyrtki 1965) differ somewhat in their time and location of occurrencefrom the circulation inferred from the present study. There is only fairagreement between the surface circulation during April and June of thepresent study and that during May 1960 (Griffiths 1968). The variationsmay be attributed to year-to-year differences in the surface currents, tothe use of different reference pressure levels and to a complicated localcirculation.

OCEANOGRAPHY GULF OF CALIFORNIA ENTRANCE 395

Surface currents are also produced by an applied wind stress on thesea surface. The frictional stress due to the wind is normally assumed todiminish with depth and become zero at some point referred to as the depthof zero frictional (wind) resistance. These wind-induced currents arereferred to as Ekman currents and are assumed to flow 45° to the right ofthe wind's direction in the northern hemisphere. The wind observationsfor the eight cruises were converted to stress values by the relation.

(2)

where W is the near-surface wind speed in em/sec and the constant em-bodies those factors necessary to convert wind speed to stress units ofdynes/em". The associated surface Ekman currents were then determinedfrom a relationship given by Sverdrup et al (1942);

(3)

where p is the water density; f is the Coriolis acceleration as defined abovein (1) and D is the maximum depth of frictional resistance, assumed to be30 m to correspond with the depth of the mixed layer. After the windstress values for the cruises were determined, the associated Ekman cur-rent values were computed. Emphasis was given to locate those observa-tions where the Ekman currents were 10% or more of the magnitude ofthe geostrophic currents. Only during the June 1967 cruise was the windstrength adequate to produce the specified Ekman currents.

The strongest wind-induced currents during the June 1967 cruisewere immediately south of Cabo San Lucas where speeds were estimatedto be up to 73 ern/sec (Fig. 6). Farther south an eddy-like Ekman currentwas evident with speeds of 10-16 em/sec. The eddy-like features in thewind-driven currents and the geostrophic circulation were due to a stormin the vicinity of stations 9-11. Winds in excess of 20 m zsec (40 kts)were observed but hydrographic observations were not made until stormconditions abated. A comparison of the wind-driven currents and geo-strophic currents from this cruise shows that the former were frequentlyoblique or even in opposition to the latter.

Currents at 100 m

The circulation at 100 m (Fig. 5) from January to April was, forthe most part, similar to the surface circulation. During January thesubsurface flow near Cabo San Lucas was southeasterly at 10 ern/sec butnear 10goW, water crossed into the Gulf, east of 10goW, saline Gulf waterflowed out across the entrance at 5-10 ern/sec, By the following month thesubsurface flow had reverted to a uniform southeasterly set at 15 em/sec.

sub-surface circulation from April through June was similar to therespective surface currents but at reduced speeds. Agreement between

396 STEVENSON

the horizontal circulation at 125 m in May 1960 (Griffiths 1968) and thecirculation at 100 m in April and June 1967, was rather poor; the discrep-ancy was due in part to the 25 m difference on the subsurface surfacesselected. Between June and August the currents at 100 m shifted to anorthwesterly set. Water from the southeast entered the Gulf entranceon the eastern side and left on the western side.

Observed water characteristics

Since water masses are characterized by specific temperature-salinity(T-S) relationships, comparison of individual T-S curves may provide in-formation about the extent of interaction and mixing of two or more watermasses. A T-S envelope based on observations discussed by Griffiths (1965)is shown in 3 for each of the three water masses. Although thetemperature and salinity of near-surface water are affected by precipita-tion, evaporation and solar radiation, local differences caused by thesefactors frequently may be assumed small for the same locality and timeperiods, in comparison with those caused by the interaction of the threewater masses present in the An example of the variations in tem-perature and salinity noticeable at each station and from station to sta-tion are shown for the MZ-4 cruise (Fig. 7).

The T-S curves for the fourth cruise (January) show a definite pro-from eastern tropical Pacific water on the eastern side of the

Gulf entrance (sta. 1, 2, 3 except for Gulf of California water at the surfaceof 1 and 2), to Gulf of California water about midway along the entrance(sta. 4), to California Current water near Cabo San Lucas (sta. 6, 7, 8). Tothe south and near the mainland (sta. 10-16), the T-S relationships implythe presence of water formed by both California Current and easterntropical Pacific waters. T-S curves for the stations 4 and 6 underwentsizeable salinity displacement near the 300 cl/T (thermosteric anomaly incentiliters/metric ton) density surface and resulted from the proximityof California Current and Gulf of California waters.

A month to month comparison of T-S curves with the reference water-mass envelopes reveals the occurrence of warm salty water in a thin surfacelayer along the Gulf entrance in January, April and June, indicating Gulfof California water leaves the Gulf intermittently and in limited supply.The same water was evident also at some more southerly locationsduring January and June.

California Current water with a salinity S34.69{)() was prevalent nearCabo San Lucas during this investigation and apparently occurs therethroughout the year. In the area between Cabo San Lucas and CaboCorrientes both California Current and eastern tropical Pacific water werefound to a or lesser extent, depending upon the general circulationat that time. While the California Current influences the water character-istics in the area beyond Cabo San most of the interaction occurs


above 100-150 m (based on available T-S curves). 'Tropical Pacific wateris characterized by a salinity maximum of 34.7-34.8%0 and was usuallyfound near 200 m. A detailed analysis of all the available T'-S curves fromthis study will not be presented here.

Surface features

One of the unusual features in the area of investigation is a largeannual change in surface temperature that frequently reaches 9°C (Rodenand Groves 1959; Wyrtki 1964). The salinity range is considered to beabout 0.4%0 and shows little seasonal influence (Roden 1964). Monthlysurface charts by Bennett (1966) show a range of 0.5%0 or more. Duringthe period of this investigation the annual range of temperature and salinitywas 19.7°C to 29.8°C and 34.14%0 to 35.42%0 respectively (Fig. 8). Themean" temperature observations show a strong seasonal cycle with thelowest value in January. The salinity observations indicate a small semi-annual cycle. Data for rainfall at Mazatlan (Fig. 8), compiled monthly bythe World Meteorological Organization (1968), show little if any relation-ship to the salinity changes in the offshore area. Surface salinities at thenearshore stations, however, drop by 0.4%0 in the July-August period, whenmost rainfall is received. The rainfall during August can readily accountfor the observed decrease in salinity. Seasonal changes in the thickness ofthe mixed layer can also be seen in Figure 8. The thickness of the mixedlayer varies annually with an annual mean of 30 m and a range of 5 m to100 m. The thickness of the mixed layer appears to be inversely relatedto fluctuations in the mean surface temperature. A comparison of themonth-to-month observations of surface temperature, salinity and densitymay be made from Figure 9 and for thickness of mixed layer from Figure11.

Sizeable variations in temperature, salinity and density occurred ona month-to-month basis and also within a single cruise period. AlthoughGulf water appeared infrequently on T-S diagrams, surface outflow acrossthe Gulf entrance apparently took place during most of the cruise periods(see salinity charts, Fig. 9).

From October through December, for example, the average surfacecirculation between Cabo San Lucas and Cabo Corrientes called for anorthwesterly set (Wyrtki 1965). The winds measured aboard the cruisevessel from October through December were so oriented as to produce asurface Ekman current across and out of the Gulf entrance. An outflowof warm, salty surface water present at the eastern end of the Gulf en-

*Weighted mean surface temperatures for each cruise were determined as follows:the areas contained between isotherm intervals were first planimetrically integratedand then multiplied by the mean temperature of the respective interval. Theweighted temperatures were summed and then divided by the area contained withinthe cruise track. The same method was used to determine values for the monthlymean salinity and depth of the mixed layer.

398 STEVENSON

trance in October was progressively advected westward along the entranceand by January was finally located south of Cabo San Lucas; this outflowdiminished appreciably in the period of several months. During the sametime interval, tropical Pacific water was evident from Cabo Corrientesnorth to the vicinity of the Gulf entrance. California Current water waspresent in limited quantity south and west of Cabo San Lucas as denotedby the area enclosed by the 34.6~JO isohaline. During January cold waterof intermediate salinity appeared the Gulf entrance at l09°W, repre-senting California Current water that was either advected intothe area by surface currents or representing a of the sub-surfaceCalifornia Current that surfaced (Fig. 9).

From February to April, low surface temperatures reflected theseasonal reduction in insolation. Low salinities were widespread during

indicating a minimal output of Gulf water beyond the Gulfentrance. Precipitation was not responsible for the reduced sincethere was no rainfall from February through April. The California Cur-rent was evident in general area but the surface layer had been modi-fied by a combination of wind mixing and evaporation. April incomingEastern Tropical Pacific water from southeast of Cabo Corrientes hadincreased the surface salinity in the eastern half of the region by aboutO.3'X)()1 California Current water was dominant west of lOgoW. A com-parison of the temperature and salinity distributions described by Griffiths(1968) with those of the present study shows that temperature and salinityconditions during May 1960 were similar to those made during April andJune 1967, except near Cabo San Lucas where the April and June salinitieswere lower than May 1960.

The period from June to August can be considered a transitional in-terval since the surface winds and currents shifted from a southerly flowto a northwesterly set. With the exception of the eddy feature south ofBaja California, surface currents were slow in June. A sizeable part ofthe surface area contained warm water with a salinity~35.0%o. Most ofthe increase in salinity from April to June is attributed to evaporation..Water with a salinity was limited to a northerly coastal flowwhere the temperature was 27°C or more. Surface salinities were lowerI"'Il'1'V''I'V'I(Y' August than June due to 40 em of rain (measured at Mazat-Ian) during August. In particular, salinities were reduced in an elliptical

from Islas Las Tres Marias to the entrance of the Gulf. Northwesterlysurface currents from the vicinity of Cabo Corrientes were responsiblefor advecting coastal water of lower toward the northwest.

Vertical distribution of temperature and salinityTwo vertical sections are presented for temperature and salinity for

each cruise 10). The northern section includes those stations thatbound the Gulf entrance; the southern section contains those stationswhose form a line from Cabo San l ...ucas to the southeasternmost

OCEANOGRAPI-IY GULF OF CALIFORNIA ENTRANCE 3£)9

station near Cabo Corrientes. Only temperature sections are available fromthe first three cruises.

The vertical temperature sections for the period October-Decembercontain numerous inversions between the surface and 250 m. Inversionsas large as 1°C were evident along the southern sections at depths of100-175 m. Thermal inversions usually result from advective processes inwhich cool water flows over warm resident water or when warm waterflows beneath cool resident water. The position, intensity and extent ofarea coverage, therefore, can be used to indicate the degree of penetrationof one water source into another. During October, for example, an inver-sion between the 15°C and 16nC isotherms was present in the southernsection and spread out horizontally for 96 km mi) , The rate of advec-tion in this instance was equal to or greater than the dissipative forcesof mixing.

Although thermal inversions than l()C occurred thesouthern sections in the last five cruises, the local interactions are bestdetailed by the two salinity sections (Fig. 10). A southerly subsurfacecurrent moving at about 13 ern-sec from the Gulf of California was rep-resented on both sections as a near-surface core of high salinity(S~34.g.%0). Between the two maximum salinity cores, a distance of 128km (71 mi) , the salinity decreased by and spread out horizontallydue to mixing processes. To the west a salinity minimum core representingthe California Current flowed at about the same speed beneath the salinecurrent leaving the Gulf. The most intense thermal inversions along thesouthern section occurred below the high and low salinity cores.

The only southbound current from the Gulf readily identified duringthe February-April period was evident along the eastern side of the en-trance. In this period, however, the salinity minimum core of the CaliforniaCurrent extended along the entire length of the southern section. The lowsalinity core was accompanied by a temperature inversion zone duringFebruary that extended horizontally for at least 208 km (115 mi). Theinversion zone was again located beneath the low salinity core. In Aprilthe California Current more effectively penetrated the resident waterbetween Cabo San Lucas and Mazatlan than during any other period ofthe investigation. The presence of low salinities (S

400 STEVENSON

ward while the eastern core moved in a northerly direction. It is uncertainwhether the two minima constituted a part of the eddy or if they werepart of the subsurface meandering flow. A comparison of appropriatesalinity profiles from May 1960 (Griffiths 1968) with those from Apriland June of the present study reveals that the near-surface flow of theCalifornia Current (evidenced by salinity minimum core) was strongerduring April and June than during May 1960.

Although the circulation assumed a northerly set during August,water entered the area from the vicinity of Cabo Corrientes in the southand from the west near Cabo San Lucas. To the north, subsurface waterfrom the California Current continued to flow into the region at depthsfrom 10 m to 150 m. The California Current also extended along the Gulfentrance, and at 100 m flowed into the Gulf. Temperature inversions ac-companied the low salinity core associated with the California Current;the inversions were again located beneath the low salinity core.

Mixed layer depth, Secchi disc depth, chlorophyll concentration, carbonfixation, and solar radiation

Distributions of mixed layer depths and their relationship to biologi-cal productivity are of interest to biologists and oceanographers alike andhave been the object of studies by Brandhorst (1958), Cromwell (1958),Forrester (1964), Wyrtki (1964) and others. The studies by Cromwelland Wyrtki are especially useful because monthly and seasonal maps ofthe mixed layer are included.

Mixed layer depth, as used in this paper, refers to the thickness of thelayer of isothermal water near the sea surface. Cromwell (1958) attestedto the difficulty of providing an objective definition of this term. Wyrtki(1964) stated that the temperature gradient must be greater than 0.2°C/10m for the thermocline, but did not state the extent of variability allow-able for the mixed layer. All determinations of the mixed layer depth forthis study were made from BT traces and posed no problem in the majorityof cases. Observations from adjacent stations were used as a guide inmaking decisions for the complicated traces.

Mean values of the mixed layer depth were determined for each cruise(Fig. 8) in the same manner as for the mean surface temperatures. Thegreatest mean mixed layer depth occurred during January, the smallestduring August. An inverse relation is indicated between the mean surfacetemperatures and mixed layer depths; the deepest and shallowest mixedlayer depths corresponded with the coldest and warmest water tempera-tures. Seasonal warming and cooling of the surface water by solar radia-tion (Fig. 2) would produce such an inverse relationship.

The depth of the mixed layer was found to vary both geographicallyand from month to month (Fig. 11). Changes in the mixed layer depthare brought about by the non-uniform action of wind mixing, by lateral


and vertical mixing processes associated with surface or near surfacecurrents and, especially, by the heating effects of insolation. The mixedlayer depth is biologically important since it affects the degree of replen-ishment of plant micronutrients in the depleted upper layers of the oceanby controlling the degree of vertical mixing with nutrient-rich deeperwaters.

A trough located roughly along 109°W was a persistent featurethroughout the investigation. From October through the following Augustthe trough was positioned between 108 0 and 109°30'W and varied inthickness from 25 m to 75 m, averaging about 45 m. The trough is at-tributed primarily to local mixing processes along the boundary of theCalifornia Current. The lack of temperature inversions within the troughsuggests that advection was less important than lateral and vertical mixing.The maximum mixed layer depth observed occurred on the western sideof the Gulf entrance during January. The trough was located betweenthe California Current and the outflowing current from the Gulf. Vigorousmixing is assumed in such a location. A frequent shoaling of the mixedlayer in the vicinity of Islas Las Tres Marias and Cabo Corrientes wasassociated with the movement of warm, low salinity water into the im-mediate area. The depth of the mixed layer in May 1960 (Griffiths 1968)and in April and June 1967, is quite similar.

When the necessary surface wind and current conditions are present,a small mixed layer depth can also be interpreted as implying the presenceof upwelling. The ascending motion of water brings the thermocline nearthe surface with a corresponding decrease in the surface isothermal layer.Upwelling is found to some extent around most islands, the Islas Las TresMarias being no exception. When surface wind and current conditionsare favorable, i.e., southbound in January and February, the mixed layerdepth becomes less near the islands than for other locations in the generalarea. Although cool surface temperatures were not found near the islands,the field operations did not include a detailed sampling plan around theislands and perhaps for this reason the observed surface water tempera-tures near the islands were not indicative of upwelling. Low salinitysurface water associated with the California Current was found to thesouth of the islands, however, and supports the idea of local upwellingsince water of this salinity is found at 30-50 m depth farther "vest.

Concentrations of phytoplankton productivity and standing crop wereestimated from several types of measurements. The Secchi disc was usedduring each cruise to provide an index of water transparency and, indirect-ly, to indicate the relative standing crop. Since water transparency isaffected by inorganic suspended material introduced by coastal runoff andby the abundance of planktonic forms, the method is best employed awayfrom areas influenced by coastal outfall. All Secchi disc observationsduring the investigation were made 20 km (11 mi) or more from landin the oceanic environment. The standing crop was estimated from sea

402 srrEVENSON

surface chlorophyll it (mgymx) and the productivity from uptake of radio-carbon (Cl4, mg/m-rday) during the last three cruises. Biological measure-ments have been previously published (Leet and Stevenson 1969).

Although limited numbers of biological observations were made foreach cruise period, correlation coefficients were determined for the severalmeasurements. For Secchi disc depth and mixed layer depth, the coeffi-cients were 0.84, 0.61, and 0.71 for the months of December, January, andApril respectively; and less for the other periods. Significance levels were

or larger, due to the small number of observations. A comparison ofcarbon fixation and plant pigments yield correlation coefficients of 0.96,0.91 and 0.75 for April, June and August respectively. The June coeffi-cient was significant at , the April coefficient probably significant at

and the August coefficient not significant at . For these cruiseperiods the high productivity was associated with large standing phyto-plankton crops. When Secchi disc measurements were matched to eitherchlorophyll or productivity measurements, negative coefficients as largeas 0.81 resulted at the level. As would be expected the transparencyof water in those instances was inversely related to the fertility of thewater.

Only a few productivity estimates (13) are available from the April,June and August cruises but the highest rates of carbon fixation usuallyoccurred near Islas Las Tres Marias and the second highest rate of fixa-tion near Cabo San Lucas (Table 1). The maximum rates during a singlecruise often differed from the smallest rates measured by a factor of 10.From Table 1 it is evident that productivity under incident light conditionswas usually less than for those samples incubated under the incom-

light level. For the area of the investigation, the median productivitywas estimated to be 4.1 and 6.9 mgC/m3/day for the incident light and

light level, respectively. If data from the three stations outside theusual cruise track are included, the medians are 3.3 and 6.1 mgC/m3/day,respectively suggesting that the productivity is higher near the Gulfentrance than farther offshore. The decrease in the measured productivityat the two light levels is considered to be primarily due to the effect oflight inhibition. In a review of the available primary productivity datafor the Gulf of California, Zeitzschel (1969) has found that rates withinthe Gulf are two to three times greater than for surface water at a similarlatitude in the Pacific.

Surface chlorophyll a concentration (Fig. 12) underwent a gradualincrease from April through August. Median concentrations of the dis-solved plant pigment were found to be 0.65, 0.78 and 0.97 mg zm: forApril, June and August, respectively. The range in the concentrationlevels decreased, however, suggesting that spatial inhomogeneities as-sociated with local "blooms" had decreased from spring through thesummer due perhaps to a gradual dispersion of surface phytoplankton.


Relatively high plankton concentrations were associated with frontalboundaries near Cabo San Lucas and near Islas Las Tres Marias.

The ratio of phaeophytin a to chlorophyll a (see Fig. 13) is consideredto be a measure of the grazing pressure of herbivorous zooplankton in thearea (Lorenzen 1967). Lorenzen also suggested that this ratio is a measureof nutrient replenishment and hence, is directly related to the productivityof the phytoplankton. Lorenzen's results were based on values integratedover the euphotic zone whereas the present investigation was limited tosurface measurements. During April for example, chlorophyll concentra-tion was 14 times more than phaeophytin. The correlation coefficient was0.97 and significant at the 0.5% level. By June the chlorophyll was only4 times greater and by August was reduced to 2.5 times the concentrationof phaeophytin.The correlation coefficients were 0.82 and 0.61 at theand 1 significance levels, respectively. The reduction in the ratio ofchlorophyll to phaeophytin from April through August suggests a phyto-plankton population that was gradually subjected to heavier and heaviergrazing pressure with the passage from spring to summer.

Zooplankton abundance was estimated from surface and obliquetows. Comparison of the two sets of measurements indicates that valuesfrom the oblique tows (representing zooplankton abundance integratedover the upper 75 m of water) are less variable than for surface tows.Changes in the zooplankton abundance from April to August, the periodfor which chlorophyll measurements are also available, suggest an overallincrease in the concentration (Fig. 14). Although detailed contouring ofsome biological properties is difficult to evaluate, two areas of high rela-tive zooplankton abundance occurred in April and August. One area issouth of Cabo San Lucas in a region where fronts often occur; the secondarea is in the vicinity of Islas Las Tres Marias. The dependence of thezooplankton abundance on the phytoplankton standing crop was also es-timated. A comparison of chlorophyll ajphaeophytin a and zooplanktonabundance gave coefficients of 0.17 and - 0.67 respectively for April andAugust. The coefficient for April was not significant at the 5% level butwas significant at the 0.5% level for August. The progressively lowerratios of chlorophyll a/paeophytin a coupled with the increasing zooplank-ton abundance noted above, provides strong support for the proposal ofincreased grazing pressure.

Incident solar radiation was continuously monitored atop the biologystation at Mazatlan from April 20 through July 2. After the daily traceswere planimetrically integrated, a calibration correction of wasapplied to each daily value. The daily amount of solar energy receivedis shown in the upper panel of Figure 2. Clear weather existed for mostof the time of record; 88% of the maximum solar energy available wasreceived during this period. The day-to-day uniformity of the input ofsolar energy is not to cause any noticeable difference in the level

404 STEVENSON

of productivity. The length of the trace, however, restricts any quantita-tive comparison with surface chlorophyll concentration or productivity.

SIJl\'IMARY AND CONCI.JIJSIONS

The circulation of the oceanic area between Mazatlan, Cabo SanLucas, and Islas Las Tres Marias is composed of currents resulting fromthe distribution of temperature and salinity (geostrophic currents) andnear-surface currents produced by the frictional effects of the wind onthe sea surface (Ekman currents). From January through April thesurface geostrophic currents had a southerly set at speeds up to 20 em/sec.In June the currents were variable and for the most part less than 10ern/sec. By late summer (August) the surface currents changed andmoved in a northerly direction at speeds of 10-15 em/sec. During theperiod of the investigation wind currents formed a small of the totalsurface circulation, only during June did the magnitude of the wind cur-rents amount to more than of the magnitude of the geostrophiccurrents. A comparison of the wind currents and geostrophic currentsshowed that the Ekman currents were frequently oblique or in oppositionto the geostrophic currents. Under such conditions contrasting currentscan exist in different water layers.

The circulation at 100 m was similar to that of the surface, but slower.Subsurface water exchange occurred across the Gulf entrance on severaloccasions. During January an appreciable outflow occurred at 5-10 em/secalong the eastern side of the entrance. In April, water flowed southwardfrom the Gulf at 10 em/sec and into the central part of the area of study.Water entered the Gulf on the eastern side of its entrance at 5 ern-secduring August and left at about 8 ern/sec on the western side of the en-trance. More study is needed along the Gulf entrance, however, to deter-mine the rate and direction of subsurface transport across the entrance.

The effect of the circulation is to bring different types of water intojuxtaposition and subsequently allow their interaction. A semi-permanenttrough in the mixed layer, located along 10goW, is considered an exampleof such interaction. Three water sources are found within the immediatevicinity of the investigation: 1) cool California Current water of low sa-linity (8::;34.60%u) which flows southward along the west coast of BajaCalifornia; 2) warm eastern tropical Pacific water of intermediate salinity(34.65::;S::;34.859{)()) which flows into the area from the southeast; and 3)warm highly saline water (S~34.909{)(») from the Gulf of California. Onthe basis of T-S curves, Gulf of California water was found to leave theGulf intermittently at or near the surface and in limited supply. CaliforniaCurrent water was consistently found near Cabo Sun Lucas. BetweenCabo San Lucas and Cabo Corrientes both California Current and easterntropical Pacific waters were found to a greater or lesser extent at about


150 m, depending upon the general circulation at that time. Eastern trop-ical water was usually found near 200 m depth.

In addition to variations in surface properties due to different watersources, seasonal changes also occurred. During the period of thisinvestigation the mean surface temperature was 25.1°C with a range of19.7°C to 24.8°C; the lowest mean temperature occurred during Januaryand the highest during August. The mean mixed layer depth was 30 m,with a range of 5 m to 100 m; it was deepest in January and shallowestin August. The fluctuations in both these properties are attributed toseasonal changes in insolation and wind mixing of the near-surface water.The mean salinity was 34.76'~60, with a range of 34.14'%0 to 35.42'%0' A smallsemi-annual cycle of salinity is apparent. Changes in salinity are attributedto seasonal differences in evaporation and precipitation. While precipita-tion was not shown to noticeably reduce offshore salinities, nearshoresurface values were reduced by an average of 0.4'%0 during the season ofheaviest rainfall. Some seasonal variability in salinity is also attributed toadvective movements of the three water masses in the area.

Vertical sections of temperature and salinity located between Mazatlanand Cabo San Lucas (northern section) and from Cabo San Lucas toIslas Las Tres Marias (southern section) revealed many details aboutthe advective and mixing processes in the area. Thermal inversions oc-curred along both sections during each cruise period, but were morefrequent, intense, and of greater horizontal extent along the southernsection. Inversions of 1°C or more were measured on several occasionsalong the southern section. Inversions were infrequent below 150 m, andrarely found as deep as 250 m. Thermal inversions were common in thevicinity of the subsurface salinity cores of the California Current and Gulfof California waters, where they were most frequently located beneathboth high and low salinity cores. The locations of the temperature inver-sions were apparently related to the interactions of the California Currentand Gulf of California waters, or in their absence, the resident easterntropical Pacific water. These inversions are attributed to advection ofone kind of water (mostly California Current) into another.

Variations in the salinity profiles indicated the presence of CaliforniaCurrent water and also of intermittent excursions of outflowing Gulf ofCalifornia water. Warm salty water at the surface often extended forshort distances across the entrance but these thin surface plumes werereadily dissipated by various mixing processes. In January, however, astrong current flowed from the GLLlf at the surface, and was still evidentalong the southern section at a depth of 50 m. The Isohalines associatedwith the core underwent spreading due to advection and lateral mixingand reduced the core salinity by 0.2'%0 in a distance of 128 km (71 mi) .The California Current as delineated by the salinity minimum core waspresent during January, February, April, June and August, months forwhlch salinity data were available. The values within the core were

406 STEVENSON

usually lower along the southern section than along the northern, implyingthat the flow along the Gulf entrance was less intense than from CaboSan Lucas to Cabo Corrientes. An exception occurred during April whena portion of the California Current changed direction and swung into theGulf of California.

Abrupt temperature and salinity gradients or fronts often formed atthe sea surface and upon occasion beneath the surface. In April, for ex-ample, a submerged salinity front was found between the low salinitycore of the California Current and the near-surface outflow from the Gulf.The front was most intense at 50 m depth where the horizontal gradientwas in 48 km (27 mi). It is expected that within the frontal boundarythe relative rate of advection was greater than the dissipative effects oflateral and vertical mixing. The interaction of the outflow of Gulf waterwith the California Current offers an opportunity to study rates of mixingand advection in the frontal boundaries. Such additional studies may,in turn, yield information about spatial changes in the concentration oflarval fishes due to natural dispersion.

Although limited numbers of biological measurements were made,several inferences can be drawn from the data. Median productivity forthe months of April, June and August was 5.6, 17.8 and 2.6 mgC/m3jday,respectively. Productivity at those stations located offshore from thevicinity of the Gulf were one-third or less than the productivity measurednear the entrance to the Gulf. Median productivity averaged for the threemonths was 5.5 mgC/m,'/day. Median concentrations of surface chlorophylla for April, June and August increased progressively, 0.65, 0.78 and0.97 mg /m'. The following relationships are from correlationcoefficients:

1) Less turbid water corresponded with greater mixed layer depth(r==O.72). The significance levels, however, were due to the fewavailable observations.

2) High productivity was associated with high standing cropsalthough the correspondence became progressively less from

April (significant at the 1. level) through August (not significant at thelevel) .

The water turbidity was directly related to the fertility (chlorophyllmeasurements) of the water (r 0.81 at about the 5 significance level).

4) The productivity was highest near Islas Las Tres Marias andsecond highest near Cabo San Lucas. The most productive locations wereoften 10 times as productive as the less fertile ones.

From April through August the chlorophyll a/phaeophytin a ratiochanged from 14:1 to 4:1 to 2.5:1 suggesting that the phytoplankton pop-ulation was gradually subjected to heavier and heavier grazing pressurethrough spring and summer. This conclusion is supported by the factthat low chlorophyll a/phaeophytin a ratios were associated with largezooplankton concentrations in summer (r 0.67; significant at thelevel) .

407

200

OCEANOGRAPHY -- GULF OF CALIFORNIA ENTRANCE

105°

22° 22°

l \.TRES

MARIAS

21° 21° 10

14

C. CORRIENTES 12

20°

13

190L-----L- ....1....- ---I.. .l-..- -----l.. -+-- --' 19° 1070 1060 105°

FIGURE IA

FIGURES IA to IH. Cruise tracks and station locations for the eight cruises.

FIGURAS IA a IH. Rumbo de los cruceros y localidades de las estaciones de los ocho cruceros.

408 STEVENSON

1070 105°

TRES MARIAS

\

15

200

10

12

14

13

c. CQRRIENTES 20°

190

L--__.i--. .J....1

1100

--l.1

1090

---L1

108°

FIGURE I B

J......I.

107°

__~ ~19°

\050

409 OCEANOGRAPHY - GULF OF CALIFORNIA ENTRANCE

22° 17 22°

21°

10

TRES MARIAS

•

l '

21°

20°

12

13

14 C. CORRIENTES

20°

1901...----'------_--i -'- J..

FIGURE IC

'

107° --+

106° '--' 19° 1050

410 STEVENSON

111 0 110° 10~ 108° 1070 1060 105°

230

16

23°

22°

10

15

12

\. TRES \.

MARIAS

22°

21° 13 21°

200

C. CORRIENTES

20°

19°'-------'--- __-I.I

1100

J...I

1090

---LI

108°

FIGURE I D

...LI 107°

~II_-----

106°

19° 105°

411 OCEANOGRAPHY --- GULF OF CALIFORNIA ENTRANCE

105°

22°

21° 10

14

13

•

l TRES

\.

MARIAS

22°

21°

20°

12

C. CORRIENTES

200

FIGURE IE

412 STEVENSON

1050

22°

21°

10

12

TRES MARIAS

•

l \

22°

21°

200 C. CORRIENTES

200

190L.---..L... .L. ..L. .L.

FIGLIRE IF

--I --+--:- ---' 19° \05°


22° 7 22°

8 TRES

MARIAS

~

21° 18

21°

19

C. CORRIENTES

20° 20°

12 13

FIGURE IG

414 STEVENSON

105°

23°

22° 7

8

9

14 TRES

MARIAS

22°

21° 10

II

2/°

200

C. CORRIENTES

200

I~L-..__.L. ...L. ....L ...L....

FIGURE IH

--I-~- --+-=- ~ 19° 105°

APRIL MAY JUNE

800

700

~ /\ o (1

~

600 M ~

o > ~ ~

~ 500 o E Q ,o ~

~ 400 ' >I ...... t-c1 ......- I...... ~

z ' ...... ~ ............. o 800 .................. --- !

i= 500 I~ t'-I H

o ~

400 25°N ~

Z H

300 " ! ! , , , , , I , , , " > JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC M

Z 1967 ~

~

FIGURE 2. Maximum insolation with a clear sky (gm cal cm-2 day-l). Data for the two curves were taken from Roden (1959L >ZThe area of the investigation lies between the 20° and 25°N radiation curves. The upper panel shows the daily (1observations made at Mazatlan (about 23°15'N); the smooth curve represents maximum incoming radiation for clear trj sky conditions.

FIGURA 2. Insolaci6n maxima con cielo despejado (gm cal cm-2 dia-1 ). Los datos para las dos curvas fueron obtenidos segun Roden (1959), EI area de investigaci6n esta ubicada entre las curvas de radiaci6n de 20° y 25°N. EI panel superior indica las observaciones diarias realizadas en Mazatlan (a unos 23°15'N); la curva suave representa la radiaci6n ~

~maxima entrante de las condiciones de un cielo despejado. CJl

416 STEVENSON

o. -\IJ IX: ::>

~ a: LIJ Q. :E LIJ ....

FIGUI{E 3.

FIGURA 3.

SALINITY (%0)

34 3530 ,...,...100 /"

.,/ /' ,/ /'

,...,.../'" ........

/'" /'" .-/ /'

,../ ,../ ......,...

,.../'

/' /'

./ /'

./f::JOO ",.... ./ ./

/' ./ "./ ./ /'/'

/',./ "./

25 ./ ",....

./

/'

i'"'' """ ."""" """ "",,"",," " ,,, .. ,,,,,, """

" """""""""... """"" """ "" """"",,"",, '::n~:n~ U~~~~j~~~ ~~~~~~~:

../ ./

,,/

20

;: :GULF WATER """""" "" ,/" """"".""."""" ."· "."""""."." ...." """"." ".. /'. "." ...." " "."" .. ." · ""." . /"... "."" .,/ · ."" ....... " .. " .. ./" "" .. ".,

,/

/"',/

,/ ,/

"/' .. " .. CALIFORNIA CURRENT'·

0°/15

~

/' /'

./ /'

./ /'

,/ ./

/'

/ ./

10 / /

/ /

/ /

T-S envelopes based on observations in Griffiths (1965), A central tendency was determined for each water mass by inspection of the station T-S curves and a smoothed envelope constructed about each group of curves. The numbered curves represent isopycnal surfaces. Numbers shown are for thermosterie anomaly (centiliters/ton). Cubiertas T-S basadas en observaciones descritas por Griffiths (1965), Se determin6 una tendencia central para cada masa de agua mediante la inspecci6n de las curvas T-S en la estaci6n y por la construcci6n de una cubierta suavizada encima de cada grupo de curvas. Las curvas numeradas representan superficies isopicnales. Los numeros que se presentan son para la anomalia termosterica (centilitros/tonelada).


111° 110° 1()90 108° 10]0 106° 105°

C. SAN

LUCAS MAZATLAN

23° 230

· ;;;;;:?:;/CJ.~f/!j~J l'5B~

22° 22°. ~: l/: .60~ . TRES~

MARIASQ .

• 1

21° 21°40SOL

~ 6 30 0 ~4 20

.2 10 ....20" -230 0 0 C. CORRIENTESa 10 20 30 40 50 60

200 ( MILES) 2CP

0 2'0 40 60 80 100 (KILOMET ERS)

DISTANCE BETWEEN CONTOUR INTERVALS

190190 111° llCO 1090 108° 106° 105°

FIGURE 4A

FIGURES 4A to 4E. Surface geostrophic circulation referenced to the 250 db surface, for five cruises. Speed at a location is determined by measuring the separation between nearby adjacent contour lines and comparing the distance to a nomogram curve at the correct latitude.

FIGURAS 4A a 4E. Circulaci6n superficial geostr6fica atribuida a la superficie de 250 db. La velocidad en una localidad se determina al medir la separaci6n entre las lineas de contorno adyacentes y comparando la distancia de una curva nomograma en la latitud correcta.

418 STEVENSON

111° 110° 10~ 108° 1070 106° 105°

23°

=: \\10\8 . 23°

22° • 66~-6:, .6/.62

.64 .62 ,--. ~ TRES

n°

· J .6f 0. .. MARIAS 21° 0

50 21° UJ UJ .8 ~ 40 0.. U (J) ~.6 ~ 30 ..... 0 "

~ ~.4 ~ 20

200

a:: ::) U

.2

0

10

0 0

......20t> -23t>

10 20 30 40 50 60

CORRIENTES\...0.

200 ( MILES)

o 20 40 60 80 100 (KI LOMETERS)


19'> 111° 1100 109'> 108° 106°

190 105°

FIGURE 48

OCEANOGRAPHY -- GULF OF CALIFORNIA ENTRANCE i19

111° 110° Ioct' 1080 1070 106° 105°

23°

• I'\1162 I ~O 23°

.56 22° 220

~.56j .~ TRES ~O MARIAS

~

21° 0 w w .8

50

,..., 40 .~ 21°

0.. u en ~.6 ~ 30 r-o ........ ~ ~ 4 ~ 20 a:: a:: ~ u

.2

a

~

10

a ,..200

.... 230 C. CORRIENTES

200 a 10 20 30 40 50 60 200 (MILES)

a 20 40 60 80 100 (KILOMETERS)


190 111 0 1100 109° 108° 107° 106°

190 105°

FIGURE 4C

420 STEVENSON

MAZATLAN

.62

~::\ .60

TRES MARIAS~

0.~~. / / ~66

/ / / /

I / C. CORRIENTES I /

....... / I

/ /

01.~ .8 'C 4050~ 20°

-~ / /

/ : ~ 6 ~ 30 zO '-..

/ ~ ~ 4 :::E 20 ""'--- .,,~

/

B g 10 )8 0 o -20 0

.66 o 10 20 30 40 50 60

\ (MILES) o 20 40 60 80 100

(KILOMETERS)

DISTANCE BETWEEI\J CONTOUR INTERVALS

.72 .7) ). .t

1100

FIGURE 40


111° 110° 109'> 108° 1070 106° 105°

23° 230

·.6/~ .~

22°

/~'60~ ,/ .62 .6~' . ~ ::--- 22°

• /-:::: .68 .70 ~ \

• TRES

/~ ••7~~Q.... MARIAS' 21° 50

I ...... -------- .. -=:;-----. 21°

200

.8

~ .6 0

~~

.2

0 L'20 0 """: 40 u w CJ) 30 "~ 20

10

0 '230

0 10 20 30 40 50 60

~~

'" C. CORRIENTES 200 (MILES) 0 20 40 60 80 100 (KILOMETERS)


19° 111 0 1100 1090 108° 107

0 106° 19°

1050

FIGURE 4E

422 STEVENSON

111 0 1100 1()90 108° 1070 1060 1050

C. SAN

LUCAS '-6-!...23° .26 230

.. ~/·) r---.30 .:8(~ .9 .27 .·.31~ .28220 ~ 220

I. .~ TRES MARIAS. ..

.5 2521° 21 0

~ 3 0 ~ .2

.....200

-230 0 C. CORRIENTES00 10 20 30 40 50 60200 200(MILES)

0 20 40 60 eo 100 (KILOMETERS)


190 111 0 1100 1090 1080 10]0 1060 1050

190

FIGURE 5A

FIGURES 5A to 5E. Geostrophic circulation at 100 m referenced to the 250 db surface, for five cruises. Speed at a location is determined by measuring the separation between nearby adjacent contour lines and comparing the distance to a nomogram curve at the correct latitude.

FIGURAS 5A a 5E. Circulaci6n geostr6fica a 100 m atribuida a la superficie de 250 db. La velocidad en una localidad se determina al medir la separaci6n entre las lineas de contorno adyacentes y comparando la distancia de una curva nomograma en la latitud correcta.


105°

23°

.~~ TRES \.~~'.29

MARIAS

.28~\I.\\ 21°.5 25 \ .2\:

.....200

C. C ORRIENTES

200 1----J 23°o 20000 10 20 30405060 (MILES)

o 20 4'0 60 80 100 (KI LOM ETERS)


FIGURE 58

424 STEVENSON

111 0 1100 1()90 IOSo 1070 1060 1050

23° 230

0.28 \~ .27

220 220

.~ TRES MARIAS

.26 O . .. 06.5 25 21 0 0 2/ 0 w .4 _20w

ll. u (I) ~.3 ~ 151-0

~ ~.2 ~ 10 Ct: :::> ..... 20·

-230 u

C. CORRIENTES0 0 200 0 10 20 30 40 50 60 200

(MILES)

o20 40 60 80 100 (KILOMETERS)


190 190 111 0 II CO 1090 1080 1060 1050

FIGURE 5C

425

1050

OCEANOGRAPHY - GULF OF CALIFORNIA ENTRANCE

_--..,-----l""'"""-....---_-~~-----__-----_-__---_-----_ MAZATLAN

I ~, .. , ,//.26

' .... -.,/

~822°

TRES MARIAS

J.27 / ... --' ...... ,o

}7 " :+) J,-.26 C. CORRIENTES/ .5 25

10

L~ 20°

,/ ~ .4 U 20

~" / ~~.3 ~ 15 , __",/ ~~.2 ~ 10

a:: ::::> u

, o o o 10 20 30 40 50 60 , , (MILES)

~ \

\28 o 20 40 60 80 100 \.29 , (KILOM ETERS )

I DISTANCE BETWEEN I CONTOUR INTERVALS

~~_/

FIGURE 50

------

426 STEVENSON

111 0 ICWO 1080 1050

'/26 .25'/ )

/ .25

. / .27 .28 ............. TRES

I

I / ~ MARrAS I , (j..

II {._:_-~

I

\. -------.--.2~.S

c ~ .4 0 20 :~.3 ~ ~ ~.2 ~ 10 ~ .1 C. CORRIENTES u .... 20

0

200 o o '230 200 o 10 20 30 40 50 60

( MILES)

o 20 40 60 80 ,00 (KILOMETERS)


1901..-__ ......l- L..- ....L..- ----J'-- -+- ----Jlc)o~ 1060 1050

FIGURE 5E


MAZATLAN

50 "-. /

10 ) ~ TRES

13

428 STEVENSON SALINITY (%0)

30 .....__..,....__3..,4 ",..,.,~-----__,3r_5___.,---- __

1 00 ",..,.,"""" ",..,.,/

/",..,.,,.,.,.

// Sto. I .

5to. 2 - --•••

o~ 5to. 3 --.roO Sto. 4 •..••...•....

Sta. 6 -.-.-Sto. 7 •••••...•••••

",,/25 ",,/.,/' 5to. 8 --e- --e--

/ S'o. 9 ---e---e

,///

~OO // /"

/"/

/",/"

/"/"

u ~ 20 w 0:: :::>

~ ,/" ./a: w /"a... ~ w .....

15

/'/'

./'/

10

//

FIGURE 7A FIGURES 7A to 7B. T-S curves from the January 1967 cruise (MZ-4). The influ

ence of the three general water masses (Roden and Groves 1959; Griffiths 1965) may be seen from a comparison with the T-S envelopes shown in Figure 7A (stations 1-9). and in Figure 7B (stations 10-16) on the adjoining page.

FIGURAS 7A Y 7B. Las curvas T-S del crucero de enero 1967 crucero (MZ-4L La influencia de las tres masas generales de agua (Roden y Groves 1959; Griffiths 1965) pueden verse segun una comparaci6n con las cubiertas T-S presentadas en la Figura 7A (estaciones 1-9), Y en la Figura 7B Cestaciones 1016) en la pagina siguiente.

http:�..��...�


30

25

SALtNITY

34

100 ///

//

/"../

/" Sta. 10 ..·0 ..··0···· Sto. II --0 - --0---

Sto 12 --(l-a--/

~OO 5to 1'3 ··.6····6····

StC. 14 - -6- -6- --

Sto 15 -6-6

/" 5to 16 •••r)····O···· ./

/"

(%0)

35

u ~ 20 w 0:: ::::> ~ 0:: W a... ::E w. /

,/

/ /

/;. ,/

,/

. \

15

/' /'

.//"

././

././

/' /'

10

/ /

/ /

/ /

/ /

FIGURE 78

wo 450 ~

o 35.4

E375 E35.2 o ~

~ 35.0 300 0z

I t=r I I>t: 34.8 ~

z • ~ 225 a.. ~ 34.6 u wI~Il-l~f-

!::

(J) 150 a:: a..34.4

75 34.2 [ 1 0 34.0 C Ca c-o~--a c/ I0 0

T U1 ~T M28 120IM'0 r~t

E:r:

• 80 ~

1 :24 a:: n: ~: T w w 60 ~

Q.. T~ ; T

4l

""r I I 0 I

W 22 ~ 40 w

I X 0

...I.. :I 01- I i~--t---t~l~1

~

20 .L :

II 20

J.. I I

I '8 """""" 0

OCT NOV DEC JAN FEB MAR APRIL MAY JUNE JULY AUG ANNUAL

FIGURE 8. Weighted mean monthly surface temperature and salinity, mixed layer depth (MLD) and surface precipitation at Mazatlan for the period of eight cruises. The range about each weighted mean value is indicated by a vertical line; annual conditions are shown on the right side of the charts.

FIGURA 8. Los valores de la media mensual ponderada de la temperatura superficial y salinidad, profundidad de la capa mixta (MLD) y la precipitaci6n superficial en Mazatlan se indican para el periodo de los ocho cruceros. El alcance respecto a cada valor medio ponderado se indica par una linea vertical; las condiciones anuales se presentan al lado derecho de los diagramas.

431

200

OCEANOGRAPHY GULF OF CALIFORNIA ENTRANCE

111° 105°

~ MAZATLAN

23°

\TRES MARIAS

C. CORRIENTES

2CP

FIGURE 9A

FIGURES 9A to 9R. Surface temperature (OC) in contour intervals of 0.5 c'.

FIGURES 91 to 9P. Surface salinity (%0) in contour intervals of 0.1%0'

FIGURES 9Q to 9X. Surface density (sigma-t) in intervals of 0.1 sigma-t units.

FIGURAS 9A a 9R. Temperatura superficial (DC) en intervalos de contorno de 0.5 0 •

FIGURAS 91 a 9P. Salinidad superficial (%0) en intervalos de contorno de 0.1%0'

FIGURAS 9Q a 9X. Densidad superficial (sigma-t) en intervalos de unidades de 0.1 sigma-to

432 STEVENSON

.. l TRES . \. ~ MARIAS(j,

27.5/

/ ./"

./27.5

.,28.0


111° 110° 109° 108° 1070 106° 105°

23° 23°

/ 24.0/

n° 22° 24.5

l \.

;: TRES25.0 ~

MARIAS(

434 STEVEN·SON

FIGURE


111° 110° 109° 108° 107° 106° IOSOl

23°

· '--2~) J. 22.5------.... 23°

23.0~

________ 2/.5

22°

~22.0 ~

23.5

l

22°

r~ TRES \.

2\.

436 STEVENSON

MAZATLAN

'-J~ \ \2;.0 \ 21.5 ~~:~ "'I~

-:2~~2.0

\\\' 22.5 TRESr:-'t'l

\ \ \) r:-- 230 240 -'"-J ~ MARIAS 4.~\ •

O

2 .)

.._.~

437 OCEANOGRAPHY GULF OF CALIFORNIA ENTRANCE

MAZATLAN

/

TRES MARIAS

C CORRIENTE S

\ \ \ \ \ \ \ \ I

I I,

I

I

"

FIGURE 9G

438 STEVENSON

111° 110° 109° 108° 107° 106° 105°

MAZATLAN

23° 23° 28.0~ 280~

285• 29{·/ / --- 29.0 .~285 ~

22° 22°.~!f .~ TRES

) 28.5 MARIAS0 •

21° 2/°

C. CORRIENTE S

200 200

190L....----~--_---IL_ ____II ____I....I _____LI __+I--- 19° 1100 1090 1080 1070 1060 105°

FIGURE 9H


111° 1100 109° 1080 1070 1060 1050

MAZATLAN

23° 23°

~~~O348 22° 34.7 22°

/J5'/

~ TRES O. MARIAS ..

l \.

21° 21°

~.~ ~ /45/ , /34.4 C. CORRIENTE S

200 • .,...-34.4_ 2(P

FIGURE 91

440 STEVENSON

1110 1/00 1090 108° 1070 1060 1050

23° 230

34;//. /34.7

22° 34.8 ____ 220

34.7

~ TRES MARIAS \.

21°

I / 34.8 (.o.~

21 0

34.7

200

) /'34.6

./ C. CORRIENTES

200

111 0

FIGURE 9J


111° 110° 10~ 108° 1070 106° 105°

. 23° . 23°

35.0 3~.8~~~ 3~r5~1 34.93'6) 350 \34.8

22° 22°. 34.9 TRES \.

{J., MARIAS.3\ ~ •

-..21° 2/°

I • I

~.\_._--~~35./_ C. CORRIENTES

20° 20°

FIGURE 9K

442 STEVENSON

35~ 34.8

349~6\ " ~t'1. ~ \."V3~.8 TRES \.61 /

MARIAS ( 1,34.6

I ~34.4

IOSO

FIGURE 9L


111° 110° 1090 108° 1070 1060 105 0

MAZATLAN. 23° 23°

~4.5 J. ( ~~9! /34.6 348

22° 22°I:34.7 34(fr~ \.TFiES

~ MARIAS\:

444 STEVENSON

FIGURE 9N


MAZATLAN

3.4~(;: 22° 34.5

TRES MARIAS

34.9

I

, ", , C. CORRIENTES ,

, .\ "

I

,I t t

,

i

,~~,J'\ "'... .. ..-.. -. -... '

.....

FIGURE 9 a

446 STEVENSON

,

"'''"-- .'

447

200

OCEANOGRAPHY - GULF OF CALIFORNIA ENTRANCE

1110 1100 1090 1080 1070 106° 105°

MAZATLAN

23° 23°

. . "}f)220 ~\2;~ ~226 22.1

22° 22°

~ \ 225 l 22.2 \.

~ TRES MAR!AS

., )"-,•· \ 22f 22.3 %21° 21° 1~7· /;/218 /

C. CORRIENTES21.8 20°

• / 2\.9 ~

190L--------L.. . ..I......- ---.l..- --L- ....L......- -+- ----' 19° 107° 106° \05°

FIGURE 9Q

.~ TRES' \

23.2--------- 21° _________23.1:J _

448

450

STEVENSON

\ MAZATLAN

) . 22.8~

?£lI't /

22.7 2/~"'JO..TRES

MARIAS

2~/225 \

;/ 223

. / 2'24 .~22

/22.1

STEVENSON

111 0

\ MAZATLAN c. SAN \ • //_

LUCAS 1. I L243 / / 242 ~ J.~~!/; V / /.~ 24.24.1 2 _::/4.1_

23.9 24.1 -y;. '. 24.0 >4(;23.7 TRES)-- \

~23.9 (!/f;32::ARIAS .. r;21 0 21°


1050

MAZATLAN

452 STEVENSON

FIGURE 9V


MAZATLAN

\ \ \

" '.1 ' ':;j

/ TRES~ 'V: MAniAS 22.8 j((4It

232 ~ .~29 (7 22.6 225

................ \\ C. CORRIENTES 20°

~~3322~\~~~ . \ ~~~

10e O

FIGURE 9W

454 STEVENSON

9X

OCEANOGRAPHY - GULF OF CALIFORNIA ENTRANCE ·155

MZ -I OCT. 1966

STATION NUMBER 4

a

50

100

150

:I: lll. W 0

200

250

300O"""'-----L..-----------'------------.L------ -L-----l 1101> 109"

LONG ITUDE

__

NORTHERN SECTION: TEMPERATURE

STATION NUMBER 7 10 12 13

0..---.---_._-----.:::::::---..---...-----------.::-_------.-_._-.------ -...,....------..... '-~~____ ----27.0"-_ 28.0

26.0 25;---=---=~~ ~ ---=--- ~----:=:===-==--=--==---=======-----

50

..L

300 '-----1-'-10-o---------'...L-09-o---------1-'-0-ao------------'-'0-7-l.O°C.

FIGURAS 101 a 10M. Perfiles verticales de salinidad (%0) correspondientes a cinco cruceros en las mismas areas descritas en las Figuras lOA a lOR.

----- ------

456 STEVENSON

MZ-2 NOV. 1966

STATION NUMBER

a 6 3 2

26.0 270 /~

-------==-==~~:~~--====---------50

----------18.0--- -----////--------16.0---_____--------

_____-:///'_~ __~~///~-15.0----=----------=---100

u; ----·----14.0-----------~--5 ~ ---130~l.IJ ~

150 :I: ~

~ 12.0_________ 0

200

...L II.O~ -L250 , -,-...-....==J--, ..L

300

LONGITUDE


0

50

100 (i)

~ Il.IJ

~ 150 I ~

~ 0

200

250

300

LONGITUDE

SOUTHERN SECTION: TEMPERATURE

FIGURE 108


MZ -3 DEC. 1966

STATION NUMBER 6 4

0

50

'00

458 STEVENSON

MZ-4 JAN. 1967

STATION NUMBER

50

en a: 1&.1 .... 1&.1

! :I: .... 0. W a

100

150

200

250

300 110

NORTHERN

LONGITUDE

SECTION: TEMPERATURE

STATiON NUMBER

200

250

300 l-_----I. ..L...

10~

SOUTHERN

....L__•

108LONGITUDE

SECTION' TEMPe:RATURE

FIGURE 100

. -..I. ~.__"_._.J

_________


MZ-5 FEB. 1967

STATION NUMBER 6 5 4 3 2

Or--w-----r-r-----..__--o::----.....,.--_.__-------,--,r---,..--------.....---------. _

50

-----~--15.0--------------

14.0------en 0:: W ~ W

~ 150 :I: ~ a.. w o

200

...L

250

300 '--------'-------------'------ ---L_--I..I....

LONGITUDE


STATION NUMBER

7 8 9 10 II 12 o -..----------------.-----.------------....2-3.-0---.//..-/-----r---....,

--~~

22.050

-----13.0

200

..1

250

3001--""--------------1..------------L...----------"'----' I071t

LONGITUDE


FIGURE 10 E

460 STEVENSON

MZ-6 APR. 1967

STATION NUMBER

~O

(i) a: I.LJ t-w ~

x t-Q. I.LJ 0

100

I~O

200

....L

2~0

300 L.I.....

1100 10~

.....1.

LONGITUDE

.....L --l.-__---J


0 9

STATION

10

NUMBER

12 13

50

u; a: ILl t-ILl

~

:t: IQ. ILl 0

100

150

200

/2.0

...L .L

250

300 L-----l

1100 --..I.

SOUTHERN

..J.

LONGITUDE


---1 -----J

FIGURE IOF

-----------

-------- ---------


MZ -7 JUN. 1967

STATION NUMBER

6 5 4 2 OI'TW'------~.-----r---- _.___.______.______r-__ --__.,........__-__.__ -----___.

50

en 0:: w .... w ! :J: .... Q. w 0

100

150

200

250

300

/////

/12.0

...L ___II.O~

:.L ..L ...L ~ ..L ..L

110· 109 107"LONGITUDE


STATION NUMBER

o n7........-~__......,8.........~_--..--~~1 ...7---. o::--....,I...8 .--_1__9-----__2.,..0-------_2......1 24.0~~_ -----26.0--- ~ -_.~

22.0 _----------

50 _____18.0~--- .

- -- 16.0 //~~ ___ ___15.0~ ~

100 14.0 u; 0:: W .... ~/I 13.0--- '/11//. W

~ 150 :t: .... Q..

W 0

200

250...L

300 L....---L-

110°

.5 ~12.6___

12.0

11.0

~ ...L ~

.L- --L

108 0 LONGITUDE


FIGURE 10 G

-----=======~==

----- ______ ~

~

----l .....l

462 STEVENSON

0

MZ-8 AUG. 1967

STATION NUMBER

4 3 2

50

en ll: w I-w ~

::r: I0.. UJ c

100

150

200

250

300 ~ ____L ----.l.. _____1.___.J

LONGITUDE


STATiON NUMBER

0 10 12

50

en a: w IW

~

:r I0.. UJ C

100

150

200

---~--12.0

110 ________

250 ....L ~

110°

300 "------I. ----L

SOUTHERN

-----.l

LONGliUDE


~1-----

1070

FIGURE 10 H


MZ-4 JAN.1967

STATION NUMBER

50 1--------

~.~~)\ I ®~, .-34.8'----"34.1 } \ ~ 100 r~4~4" 34.5 ~ 1~·6

~150'~ :J: t 34.7 o.. LIJ Q ~/

200

2501-----~

~ 34.7~ 34.6

300 u..... --.l.. ----JL.- ....l....-__-'

LONGITUDE

NORTHERN SECTION: SALINITY

STATION NUMBER

7 10 12 13 O...---~--=--:--:::---=_--~--T"""""""'~~~50 ~:~~---------~--345---===~ ~

_ _~===--34.3 /34.2 345

100 ••C::::3~3'9 .~ (/) --------34~ 34.6

~ ------ . ~ ------ -- ------. ~34.7 ~ 150 ---- • •

:J:

5 o 200

134.7250 I. ""

300L..------"-----------.L...----------....1...--------------"---------~

108 0

LONGITUDE

SOUTHERN SECTION: SALINITY

FIGURE 10 I

• • • •

• •

464

0

50

100 Ciia:: LiJ ..... LiJ

~ 150 z ..... a.. LiJ c

200

250

300 110·

50

100

~ w ..... w ~ 150

:I: ~ a.. w 0

200

250

300 110·

STEVENSON

MZ-5 FEB.1967

STATION NUMBER

4 2

~. . . . 34.2~ ~ " _ 34.8 •

• 34.4 34.5 • -~ •

34.8~ry' _~34.7~ ~ ~ 34.6 _34.4 34.5 • • 34.6~ •

-------- ~---- • ------!:----- •___________ 34.7___ • •

~-34.~•

LONGITUDE

NORTHERN SECTION: SAL I N ITY

STATION NUMBER

8 9 10 12

.34.7 34.7------

/ 34.7

• • •

109· LONGITUDE


FIGURE 10 J


MZ-6 APR. 1967

STATION NUMBER

o r------..-------r--------4__--......-----,3.-r----r-----r--T----r-..--..r--r---"Tr----r-__. ~34.3

..... - -~---~-..--~_--34.1__:::::=__::::~-

50

• 33.9~. 34.0 -------- ~ 100 .-= 341 ~

____________~ 3 4.2 C/)

a:: w ~.3 W f

~~4.4~ 150 iE

t\ 34Z ~ a.. cw ) 34.5

200

'--.. . 34.7-250 34.6 ..\ 300 '--- L.._.. ....L.-__--J~

lOS-

LONGITUDE


STATION NUMBER

10 12 13

• - ~ 34.8__-_ ~ • 34.7~ ~ __

0

e--....:::-..............~~-m ·// · 50 ~ J -/ 0::: ---.34.2 34.6 34.2 34.4 -.,; 34.5 ·34.3

,';_._.. . 100 \, ~34.5rn

a::: w 3\ 34.4 ( ( 34.6--I W

~ 150 .~~~ C--34.7 ~ 348:r: l e.. \w 0 .~200

34.7

250

300 1090

LONGITUDE


FIGURE 10K

• •

__ __

466 STEVENSON

MZ-7 JUN. 1967

STATION NUMBER

3 2 I 0

}4.7 ?4.6

50

100 en a: w r w ~ 150 :~

34.7 :J:

/I0.. 1LI a 200

250 :\ 34.7

300 110· 109· lOS· 107·

LONGITUDE

NORTHERN SECTION: SA L INITY

STATION NUMBER

7 S 17 18 19 20 21 0

-3 - 344 346 _ ~5.0; ---:::::348 ------ -! I • ;4.5 / • " ~4.5~ 34.~ 34.~ 34~ _/ __~:~---343~~ ----leL-----~

50

100


MZ-8 AUG. 1967

STATION NUMBER

6 4 3 2 0

~______ -34.8 ~4.9_-7- .}4.7 "::C:J 934.5 34.~ 34.7 ~-r-

~i~-~~~~@~~1t~f:~~~~~~~~~~~:::::~~~~2~50 100 en

It: LLI

! LLI

150

:I:..

..-

iU---- :~~~~; 0..

0 W . .\

200

~'8 34.7

250 .~ 300 L...-L- ----' ...I....- ---.l..- ---L__--l

1100

LONGITUDE


o

50

100

150 :I: ~ 0.. W o

LONGITUDE


FIGURE 10 M

468 STEVENSON

30C. CORRIEN TES

200 / 35 200

~

190 L-----I~IIO--·-----11 iao-----lo...L.19o-----I08...J..I-o-----,..L.d7-0 ---'-~IOf-:6°:-----------IO~: FIGURE II A

FIGURES 11A to 11R. Mixed layer depth (MLD; depth of the surface isothermal layer) for eight cruises. Contour interval: 5m.

FIGURAS 11A a 11H. Profundidad de la capa mixta (MLD; profundidad de la capa isotermal superficial). Intervalo de contorno: 5 m.


111 0 1100 1090 /080 1070 1060 1050

230 230

.~~~~/:~-"'\\01

,'':

25 0/ 30 30/22° 22°

/40 . ~ TRES

45 MARIAS re \ Q...

2.5.

21° ~ 21°

C. CORRIENTES

2QO 200

IC}OL..-------L..- ----! ..L-- ...L- ----L. ----i ----J 190

1050

FIGURE liB

470 STEVENSON

FIGURE lie

471 OCEANOGRAPHY - GUL14' OF CALIFORNIA ENTRANCE

111° 110° lolf> 108° 107° 106° 105°

. 23° 23°

45r\\~(~:/'\ \ ) t\O0 I 5,5 .... ". '" ,%70 5: 60 55 45 ". --:::.- -~ 1 ~ \ 40 )22° ---. -_.- 1'1 , 1~0 22°

1.\

\ ( ~(. TRE~ ./ ~'~fda MARIAS 21° \e 21°

c. CORRIENTES 2CP 2CP

1070

FIGURE 110

472 STEVENSON

FIGURE liE


21 0

200 200

I90L-----1..-----L..-..----...L-------l-----~~---___:_:~---_:::IO~: 111 0 llCO 1090

FIGURE II F

474 STEVENSON

__--,......----......,r-------......,r----_---......,-----_105° _--~-----r--,....---MAZATLAN

TRES

MARIAS

C CORRIENTES 20° 200

19° 19° ~ ....,,/\30__.

25

ISo .\ . . 18°

111 0 1100 lOgO loe o 1070 106 0 10~0

FIGURE II G


111° 110° 109° 108° 107° 106° 105°

MAZATLAN

23° 23°

} 10

15

"\ ) I~ (

22° 20 22°:/('1 \

TRES MARIAS

~)O ~ Q ..

21° 21°

~ ........... C. CORRIENTES

200 200

190L-----I.II-10-------l110-0-----1.....l09-0-----10...L8o---------L.::----~---=------~IO~:

FIGURE" H

476 STEVENSON

1050

30~2/) 1.0 .5) .5 ~

J • (10~ TilES~~ \/ C:-' ~ "'ARIAS • 1.5~ ...

~ .. .~--------:.,\ 21 0 '-- /;5 i

(

-/ ) C, CGRRIENTES

200 200

190 '---_--L.__ --IiL..OO-----,o...J..9-0----I--l.O~-O------I.-LOI7-0---·--IOt-60~------'\O~~0

FIGURE 12 A

FIGURES 12A to 12C. Surface chlorophyll a (mg/m3 ), for three cruises. Contour intervals: 0.5 mg/m3 •

FIGURAS 12A a 12C. Clorofila superficial a (mg/m3 ), de tres cruceros. Intervalos de contorno: 0.5 mg/m3 •


111° 110° 109° 10So 107 0 106 0 105°

·l.o'- _0. .... MAZATLAN

23° 23°ylO/' 2.0

"'1.5

22° 22° ~ ~ TRES MARIAS

( ,,~)'

.5

~

21° 21°

1.0

,~ ~ , , " , ~ C. CORRIENTES

,20° , ~ 20°

",,';,'

.. " ~~

/--....... "

.5 19° 19°

.......--; ' ... _- -."."

FIGURE 12 B

478 STEVENSON

\0AZATLAN

1050

~o

triARIAS TRES \

21 0

200

C. CORR!ENTfS

200

1901..------1--llao

FIGURE 12 C

_---1...1 1070

--+1060

....J 190

1050


.8 MZ-6

.7 ... APRIL

-til E "C' E

.6

.5

-

...

r =0.97 P =0.5%

~

z .4 ~ I>:r Q. 0 iLl

• •

•

• •

480 STEVENSON

111° 110° 1Q90 108° 1070 106° 105° I

~AZATlAN 23° e 2JOe

• e

e 22° 22°

~ l'RES MARIAS • 0 ..

21° 21°e

C. CORRIENTES 200 200

,190 190 1 111° 1100 1090 108° 1070 106° 105°

111° 110° 1Q90 108° 1070 105°

23° 230

e •

• e 22° 22° e 'TRES MARIAS

e

• ~

0

21° 21°e 400 • C. CORRIENTES 200 ml/IOOOm3 200 190

105°

FIGURE 14. Zooplankton abundance (ml/l03m~~) for two cruises. Organisms represented are S30 mm in length. Net tows were made from about 75 m to the surface.

FIGURA 14. Abundaneia de zooplancton (ml/103m:3 ) correspondiente ados cruceros. Los organismos representados tienen S 30 mm de longitud.Los arrastres de la red se realizaron a unos 75 In de la superficie.


TABLE 1. Productivity estimated from the uptake of radiocarbon (C14 ). Refer to Figure 1 for the location of stations. Rates of carbon fixation (in mgC/m3/day) are for surface water samples incubated under both incident light and light reduced to 50% of the incoming intensity. The reader is referred to the Data Report for more details of the method and procedure used (Leet and Stevenson, 1969). Station numbers followed by an asterisk (*) were part of the cruise but were located outside of area normally surveyed.

TABLA 1. Productividad estimada de la absorci6n de radiocarbono (C14 ). Refierase a la Figura 1 para la localidad de las estaciones. Las tasas de la fijaci6n de carbono (en mgC/m3/dia) son para las

inter-american tropical tuna commission comision ......a study of the temporal and spatial...

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