Thermohaline variability and geostrophic circulation in the southern portion of the Gulf of...

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doi:10.1016/j.ds

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rdurazo@uabc.

(A. Mascarenha

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Deep-Sea Research I 53 (2006) 188–200

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Thermohaline variability and geostrophic circulation in thesouthern portion of the Gulf of California

Ruben Castroa,�, Reginaldo Durazoa, Affonso Mascarenhasb,Curtis A. Collinsc, Armando Trasvinad

aFacultad de Ciencias Marinas, Universidad Autonoma de Baja California (UABC), Apartado postal 453, Ensenada, Baja California,

C. P. 22800, MexicobInstituto de Investigaciones Oceanologicas, Universidad Autonoma de Baja California (UABC), Apartado postal 453, Ensenada,

Baja California, C. P. 22800, MexicocDepartment of Oceanography, Naval Postgraduate School, Monterey, California, 833 Dyer Road, Rm 328, Monterey, CA 93943, USA

dCentro de Investigacion Cientıfica y Educacion Superior de Ensenada, Estacion La Paz, La Paz, B.C.S., Mexico

Received 25 October 2004; received in revised form 1 July 2005; accepted 20 September 2005

Available online 15 November 2005

Abstract

Data from seven oceanographic cruises in the southern Gulf of California from 1997 to 2002 are used to describe the

thermohaline variability and the geostrophic circulation. Baroclinic patterns exhibited spatial and temporal variability. A

deepening of isotherms at the center of the section was evident in February 1999, suggesting anticyclonic flow. In May 1998

and November 1997, cyclonic flow was suggested by shoaling of isotherms at the center of the section. Other cruises

showed alternating cores of flow into and out of the Gulf (August 1998, September 1997 and October 2002). Neither a

seasonal nor a spatial pattern in geostrophic flows was apparent, suggesting that the exchange of waters between the

cyclonic flow of Pescadero basin and the interior of the Gulf is complex. Relatively high salinities were recorded during

most of the cruises indicating that Gulf of California Water (GCW) was present most of the year. Higher salinities were

observed during winter and spring, although during summer, relatively high and low salinities were both observed as

surface and subsurface cores. Temperature and salinity characteristics of California Current waters were observed only in

August 1995 when they reached as far north as Cerralvo Island at �50 dbar. During El Nino conditions in November 1997,

a mixed layer (�70 dbar) and deepening of the thermocline (�50 dbar) characterized anomalous conditions; during this

cruise an asymmetric salinity pattern was observed with low salinities characteristic of Tropical Surface waters at the center

and east of the section, while maximum salinities (34.9oSo35.0) and Gulf waters were located in an 80 km wide core next

to the Baja California Sur shelf as far north as San Jose Island.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Gulf of California; Hydrographic measurements; Thermohaline structure; Cyclonic and anticyclonic flow; El Nino

e front matter r 2005 Elsevier Ltd. All rights reserved

r.2005.09.010

ng author. Tel.: +6461744570;

03.

sses: [email protected] (R. Castro),

mx (R. Durazo), [email protected]

s), [email protected] (C.A. Collins),

x (A. Trasvina).

1. Introduction

The Gulf of California (Fig. 1) is a marginal seaof the Pacific Ocean surrounded by the semi-desertof northwestern Mexico. Its evaporation rate is

.

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Fig. 1. Chart showing location of the hydrographic sections for cruises listed in Table 1 and Hurricane Flossie track during August 1995.

Section A (+), section B (m), section C (K) and section D (’). Section E (E) was used in Castro et al. (2000). Isobaths depths are in

meters.

R. Castro et al. / Deep-Sea Research I 53 (2006) 188–200 189

high, �1myr�1, but so is its annual mean heat gainfrom the atmosphere, �120Wm�2 (Castro et al.,1994). This implies that the thermohaline exchangewith the adjacent ocean has a different behaviorcompared to typical evaporative basins, e.g. theMediterranean and Red Seas. These basins are animportant source of deep water in the adjacentocean, imprinting their signature far from thebasins. On the other hand, the water masses whichoriginate in the northern Gulf are distributed andspread in the upper layer (o200m) along the entirelength of the Gulf (Bray, 1988; Torres-Orozco,1993; Castro et al., 2000) because of their relativebuoyancy. However, these waters do not contributeas a source of high salinity intrusions in the deeper

waters of the Eastern Pacific Ocean (Neumann andPierson, 1966).

The connection to the Pacific Ocean at the south-ernmost region of the Gulf of California lies in anoceanographic transition zone where several watermasses commingle (Table 2). At the surface, watersfrom the Pacific Ocean, including California CurrentWater (CCW) and Tropical Surface Water (TSW) aswell as outflow waters with temperature and salinityranges typical of the inner Gulf (Gulf of CaliforniaWater, GCW), are found. The relative volumes ofthese waters change seasonally (Torres-Orozco, 1993).Below these water masses, with less temporal varia-bility, three waters masses of Pacific Ocean origin arefound, namely, Subtropical Subsurface Water (SSW),

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Pacific Intermediate Water (PIW) and Pacific DeepWater (PDW) (Griffiths, 1968; Stevenson, 1970;Castro et al., 2000). Castro et al. (2000) found asubsurface water identified as Central Gulf Water(CGW) next to the Baja California Sur (Fig. 1; LineE), with salinity equal to 34.83 and potentialtemperature (y) equal to 15 1C. The CGW is similarin y/S ranges to SSW (Bray, 1988), and these twowater masses can be confused at the entrance to theGulf.

The thermohaline properties of water masses at theentrance have been defined differently by differentauthors. While Torres-Orozco (1993) defined GCWwith salinity SX35 for the entire Gulf, Castro et al.(2000) suggested that at the entrance to the Gulf,S ¼ 34:85, is a better boundary to differentiatebetween waters formed in the Gulf from thoseentering from the Pacific Ocean. Other authors havedefined Gulf California waters with S434.9 (Warshand Warsh, 1971; Alvarez-Sanchez et al., 1978) andS ¼ 34:9 (Alvarez-Borrego and Schwartzlose, 1979;Alvarez-Borrego, 1983).

Circulation studies at the mouth of the Gulfindicate a complex flow pattern: Roden (1972)computed the baroclinic flow at the entrance tothe Gulf relative to 1500 dbar for December 1969.He observed that most of the outflow took place inthe western part of the Gulf, as a narrow jet 30 kmwide and �1000m deep, with surface velocities of0.55m s�1 while currents of 0.40m s�1 reached600m. The slower inflow took place in the centralpart of the Gulf over a broad area. Collins et al.(1997) used in situ transport and current observa-tions during April, May and December 1992 (Fig. 1,section E) and suggested that inflow occurred nextto Sinaloa and the outflow next to Baja CaliforniaSur (BCS). Mascarenhas et al. (2004), along thesame line, computed the mean geostrophic flow andfound that winter and spring baroclinic outflow wasfound near the BCS coast as a deep (�700m) core�45 km wide, while inflow occurred either throughthe center of the section or along Sinaloa. However,they also found that summer and fall had a rathercomplex exchange pattern of alternating inflow andoutflow, although inflow along Sinaloa was ob-served during all cruises. It is not clear, however,whether this exchange pattern also occurs furthernorth.

Alvarez-Sanchez et al. (1978) conducted twohydrographic transects between La Paz, BCS, andTopolobampo, Sinaloa (Fig. 1), in early spring of1970. Using Wyrtki’s (1967) criteria, modified by

Warsh and Warsh (1971), they found GCW on theeastern side, CCW next to BCS and TSW at thecenter of the section. Geostrophic flow referred to500 dbar resulted in an outflow over a broad regionon the eastern side and an inflow in the middle partof the two sections, suggesting an anticyclonic gyre(Alvarez-Sanchez, op. cit.). On the other hand,Emilsson and Alatorre (1997) used drifters betweenEspıritu Santo and San Jose Islands and Sinaloa(Fig. 1) to show evidence of a cyclonic gyre. Theyalso computed the geostrophic flow (Fig. 1, sectionA) relative to 500 dbar for one section and observedcharacteristics of cyclonic flow, with fresher waters(34.6–34.8) flowing adjacent to Sinaloa and highsalinity water (435.0) flowing southward near BCS.Amador-Buenrostro et al. (2003), using directobservations (ADCP) of currents and geostrophyduring November 22–24, 1997, and a satellite imagefrom November 17, 1995, described a 120 kmdiameter near-surface cyclonic eddy in FarallonBasin. Amador-Buenrostro et al. (2003) also ob-served an intense and narrow coastal jet flowingnorthward next to Espıritu Santo Island.

The differences in air–sea fluxes between thePacific Ocean and the Gulf California, wind stress,coastal-trapped waves, and remote Pacific forcingdrive the exchange of water and properties at theentrance to the Gulf of California. What we knowabout the exchanges is based upon hydrographicsections across the entrance to the Gulf.

The objective of this study was to compare theseasonal variability of the thermohaline structure inthe southern portion of the Gulf of California withobservations at the entrance to the Gulf. Directlymeasured and geostrophic currents are also com-pared.

2. Data and methods

From 1994 to 2002, water properties weresampled with a CTD for seven oceanographiccruises between BCS and Sinaloa (Table 1, Fig. 1).To resolve mesoscale variability, the CTD stationswere closely spaced (�10 km). Although not allobservations were collected along the same geo-graphic section, they were combined to complete anannual cycle. Two sections in November 1997 weresampled during the 1997–98 El Nino. El Ninoconditions in the Gulf are known to result infreshening and heating of upper waters (Robles andMarinone, 1987; Lavın et al., 1997; Castro et al.,2000), modifying the annual cycle.

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Table 1

Dates of cruises performed in the Gulf of California

Dates Research vessel Section Casts

February 25–26, 1999 Francisco de Ulloa C 16

May 27–28, 1998 Francisco de Ulloa C 16

August 06–07, 1998 Francisco de Ulloa C 16

August 12–13, 1995 El Puma D 16

August 13–15, 1995 El Puma E 21

August 20–21, 1995 El Puma C 12

September 13–15, 1997 A. de Humboldt B 14

October 11–12, 2002 Rıo Suchiate C 16

November 17–18, 1997 Francisco de Ulloa A 13

November 15–16, 1997 Francisco de Ulloa C 11

Fig. 2. Potential temperature and salinity diagram for CTD

soundings from cruises listed in Table 1 (sections A–D).

TSW ¼ Tropical Surface Water, GCW ¼ Gulf of California

Water, CCW ¼ California Current Water, SSW ¼ Subtropical

Subsurface Water, PIW ¼ Pacific Intermediate Water and

PDW ¼ Pacific Deep Water. The thermohaline boundaries

between water masses are similar to Torres-Orozco (1993), except

the salinity limit for GCW (S434.9) as shown by the dashed line.


Factory-calibrated SeaBird-25 and SeaBird-911

CTDs were used to collect the data. Salinity wascomputed according to the 1978 Practical SalinityScale. Salinity accuracy was maintained by compar-ing CTD data with those obtained from watersamples at �1000m depth. CTD casts were carriedout from the surface to the bottom, except for theOctober 2002 and September 1997 cruises, when themaximum observation depths were 750 and 1200m,respectively, due to the length of the hydrographiccable available. Potential temperature, y (1C), anddensity anomaly, gy (kg/m3), were computed fromthe 1980 equation of state (UNESCO, 1991).Geostrophic velocity, V (m s�1), was based on usingthe deepest common depth as the reference level.

Along the C and E sections in August 1995,velocities were observed with a self-contained150 kHz broadband acoustic Doppler current pro-filer (ADCP). The ADCP was affixed to the CTDrosette and lowered from the surface to 50m abovethe bottom and is subsequently referred to asLADCP. Absolute velocity profiles were derivedaccording to the methods described by Fischer andVisbeck (1993). The start of the August 1995 cruisewas delayed a few days by the presence of HurricaneFlossie in the study area (Fig. 1).

3. Results

The water mass characteristics for the sevencruises are shown in Fig. 2. Note that thermohalineboundaries between water masses are similar toTorres-Orozco (1993), except for the lower limit ofGCW, which was modified to S434.9. This changein the thermohaline index will be discussed below.Similar to Castro et al. (2000) (section E, Fig. 1),two regimes of variability were clearly observed.

Waters with potential density anomalies exceedinggy ¼ 26.5 kg/m3 exhibited uniform thermohalineconditions, while waters with density anomalies lessthan this value showed considerable variability inthermohaline properties. GCW was present duringall cruises, with the lowest salinity value of �34.9observed in September 1997 and November 1997,two El Nino cruises. Waters with characteristics ofCCW were observed only during August 1995, moreclearly in section D than section C (Fig. 1), whereonly a few samples were near boundaries with TSWand SSW. During this cruise, Castro et al. (2000)found a large amount of CCW at the entrance to the

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Table 2

Thermohaline ranges of the water masses and their abbreviations at the southern portion of the Gulf of California. Ranges are based on

Torres-Orozco (1993), with the exception that salinity range for GCW was shifted to S434.9

Water masses Salinity Temperature (1C)

California Current Water CCW Sp34.5 12oTo18

Tropical Surface Water TSW So35.0 TX18

Gulf California Water GCW S434.9 TX12

Subtropical Subsurface Water SSW 34.5pSp35.0 9.0pTp18.0

Pacific Intermediate Water PIW 34.5pSp34.8 4.0pTp9.0

Pacific Deep Water PDW S434.5 To4

R. Castro et al. / Deep-Sea Research I 53 (2006) 188–200192

Gulf (section E, Fig. 1). TSW was present during allcruises except February 1999 (Table 2).

To further visualize the seasonal variability,temperature and salinity sections from the surfaceto 500m are shown in Figs. 3 and 4. A relativelycold, surface layer was well developed in February1999 (Fig. 3a), when the 19 1C isotherm was foundnear the surface. The same isotherm was foundbelow 100 dbar in September 1997 (Fig. 3e).

The temperature fields (density is not shownbecause it exhibits similar behavior) display bar-oclinic signals with large space and time variability,suggesting changes in the patterns of flow. For somecruises, in spite of small-scale spatial variabilitypossibly due to internal waves, isotherms shoaled atthe center of the section as in May 1998 andNovember 1997 (Figs. 3b, g, h), suggesting cyclonicflow. The shoaling was most pronounced inNovember 1997 sections (Figs. 3g, h), when thedoming of isotherms showed slopes of 1� 10�3 overthe central portion of the Gulf between 150 and700 dbar. This feature was also reported by Castroet al. (2000) (section E, Fig. 1) in May 1992, May1998, November 1997 and December 1992, withlargest slopes in May 1992 (�2� 10�3). Deepeningof isotherms at the center of the section, indicativeof anticyclonic clockwise circulation was observedin February 1999 (Fig. 3a) and below 100 dbar inAugust 1995 and September 1997 (Figs. 3d, e). Thispattern was not observed along section E in Castroet al. (2000). Upwelling was suggested in August1995 (Fig. 3d) by the rising of isotherms towardBCS between 0 and 60 dbar. This pattern was seenin an AVHRR image on 15 August (Fig. 5). Castroet al. (2000) observed coastal upwelling featuresnear the BCS coast on section E. The isothermsbetween 200 and 400 dbar were level in October2002 (Fig. 3f), while isotherms sloped upward to theeast in the upper layer and in the opposite directionbelow 400 dbar. This structure, when compared

with salinity fields, suggested that surface outflowwas composed of GCW while mid-water inflow wascomposed of SSW. Cruises carried out during ElNino in 1997 (Figs. 3g–h) showed a deep mixedlayer (70 dbar) with a deepening of isotherms about�50 dbar compared to other cruises. Castro et al.(2000) reported similar conditions at the entrance tothe Gulf, but GCW was not present (this wasnotable in salinity fields described below).

Salinity did not show a clear seasonal signal. Thisis depicted by the sections shown in Fig. 4.Relatively high salinities were recorded during mostcruises indicating that GCW was present yearround. High salinities were confined to the surfacelayer in February 1999 (S ¼ 34:9235:3, 0–170 dbar)and May 1998 (34.9–35.1, 0–100 dbar) (Figs. 4a, b).During the summer, high (35.1) and low (o34.8)salinities were observed as surface and subsurfacecores. The upper layer (0–40 dbar) during August1998 (Fig. 4c) showed salinities of 34.9–35.1, whilebetween 40 and 60 dbar the salinity was lower than34.9 with a minimum of 34.7 near Sinaloa. Between70 and 170 dbar, the salinity increased again to 34.9along the section, including a 30 km wide core ofS ¼ 35 at 120 dbar near Sinaloa. The characteristicy/S values of CCW (Fig. 2) were observed only inAugust 1995 when relatively cold and freshwaters reached as far north as Cerralvo Island(Fig. 1). This is shown as a low salinity core(34.2–34.6) between 30 and 80 dbar next toBCS in Fig. 4d intruding below the upper layer(S�34.9, 0–200 dbar). The subsurface layer salinity(S ¼ 34:9) during September 1997 (Fig. 4e) showedcharacteristics similar to August 1998, althoughnear the surface (0–30 dbar), cores of low salinity(34.6–34.8) and a small core of S ¼ 34:9 appeared inthe center and 30 km off Sinaloa, respectively. Theupper layer salinity (0–180 dbar, S ¼ 34:9235:4) inOctober 2002 (Fig. 4f) was similar that in February1999, with the exception of two subsurface salinity

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Fig. 3. Temperature sections (1C). The contour interval is 1 1C. See Table 1.


minimums (34.8), a 60 km wide relative minimumbetween 40 and 80 dbar near BCS and a 20 km wideminimum between 100 and 180 dbar near the centerof the section. These salinity minimums were notCCW. During El Nino in November 1997, anasymmetric pattern was observed, with the salinitymaximum (34.9) observed in a core 80 km wide and160 dbar deep next to the BCS shelf and San JoseIsland (Figs. 4g, h). As suggested by Castro et al.(2000), this core could be associated with an outflowof both GCW and CGW. The pattern of cyclonic/anticyclonic flow shown by salinity sections was

consistent with that derived from potential tem-perature for all cruises.

Geostrophic flow clearly showed seasonal varia-bility characterized by anticyclonic flow in February1999 and August 1995 (Figs. 6a, d) and cyclonic flowduring May 1998 and November 1997 (Figs. 6b, g, h).This was different from the variability at the entranceto the Gulf; about 200km to the south, wherecyclonic circulation was observed year round (Collinset al., 1997; Mascarenhas et al., 2004). DuringFebruary 1999, the inflow along the western sidewas approximately 100km wide, with maximum

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Fig. 4. Salinity sections. The contour interval is 0.1. The shaded region indicate S434.9. The hatched isohaline is S ¼ 34.5. See Table 1.


near-surface velocities of 0.2–0.3m s�1. The 60 kmwide outflow was deeper, with the 0.1m s�1 isotachreaching 800 dbar. In August 1995, the outflow waswider (�100 km) and shallower (�500 dbar) thanthe 50 km wide inflow, with 0.1m s�1 reaching1200–1500 dbar.

When cyclonic flow patterns occurred (as in May1998 and November 1997), they were not as wellorganized as the anticyclonic flow observed inFebruary 1999 and August 1995. In May 1998(Fig. 6b) the 60 km wide inflow was better definedthan outflow to the west. Inflow showed maximum

velocities of 0.3m s�1 at �90 dbar and the 0.1m s�1

isotach reached 800 dbar. During this cruise, therewas also evidence of a shallow jet of inflowingwaters about 30 km east of BCS with maximumvelocities of order 0.3m s�1 at the surface. InNovember 1997 (Fig. 6g), cyclonic circulation wasclearly observed in the upper 600 dbar, with a 40 kmwide inflow core next to Sinaloa and an outflowcore which was 70 km wide next to BCS. While theinflow had maximum velocities �0.3m s�1 at thesurface and 150 dbar, the outflow exhibited near-surface currents larger than 0.4m s�1 between 0 and

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Fig. 5. Sea surface temperature observed by AVHRR on 15 August 1995.


100 dbar. Below 700 dbar (Fig. 6g) the baroclinicstructure showed an inflow with 0.1m s�1 velocitiesreaching 1500 dbar. Section A (Fig. 1), also occu-pied in November 1997, showed similar cycloniccharacteristics, although better defined along theentire section (Fig. 6h). The inflow was wider(70 km) and shallower than the outflow, with themaximum velocity (0.7m s�1) located at 90 dbarnear the center of the section. The outflow wasnarrow in the upper 500 dbar but widened belowthat depth with values of 0.1m s�1 reaching1300 dbar. The maximum velocity (0.9m s�1) wasfound at 90 dbar about 30–40 km east of San JoseIsland.

Other cruises showed greater variability in theflow patterns, e.g., the alternating flows observed inAugust 1998, September 1997 and October 2002(Figs. 6c, e, f). In August 1998, (Fig. 6c) most of theinflow was located at the center of the section in adeep, narrow jet with maximum velocities of0.3m s�1 between 0 and 200 dbar and velocities ofup to 0.1m s�1 at 1200 dbar. The outflow wasweaker and disorganized with the maximum velo-cities (0.1–0.2m s�1) observed near surface at the

center of the section. In September 1997 (Fig. 5e)the outflow was strongest and occupied a broaderarea.

During the August 1995 cruise a LADCP wasincorporated into the CTD casts (Fig. 1, sections Cand E). This provided the opportunity to comparethe geostrophic and observed velocity fields (Fig. 7).These figures showed flow into the Gulf along theBCS side with a deep core (40.1m s�1) extendingfrom 600 to 1800 dbar (Figs. 7a, b). This deep inflowconsisted of Pacific Intermediate Water and wasconstrained by its density anomaly and the topo-graphy. The outflow of these Intermediate Watersalong Sinaloa also reached deep levels (�1300 dbar)with velocities of the same order of magnitude as theinflow. The agreement between the velocity ob-served by the LADCP and the geostrophic velocitywas remarkable. Observed differences were due tothe higher resolution of the vertical structure shownby the LADCP.

The good agreement between LADCP andgeostrophic velocities for section C was notreplicated for section E, approximately 200 kmsouthward (Figs. 7d, e). This southernmost section

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Fig. 6. Geostrophic velocity (m s�1) using the deepest common pressure as a reference level. The contour interval is 0.1m s�1. Solid lines

indicate inflow and dashed lines indicates outflow (shaded area): (a) February 1999; (b) May 1998; (c) August 1998; (d) August 1995;

(e) September 1997; (f) October 2002; (g) November 1997, section C; (h) November 1997, section A.


showed strong and narrow geostrophic flows(�0.5m s�1) with alternating cores of inflow andoutflow. The observed velocities (LADCP) showedsome of the cores but not with the same intensityand definition. At the Gulf entrance, several watermasses commingle (Castro et al., 2000) and createfronts, small and mesoscale eddies and frontalinstabilities (Collins et al., 1997). These featuresare usually accompanied by a high internal waveactivity (Torres et al., 2004). Reasons for thediscrepancy are the aliasing caused by samplingthe water column at time scales larger than internalwaves and due to the fact that the LADCP measuresabsolute velocities while geostrophy ‘‘averages’’density slopes over the distance between oceano-graphic stations.

Across-Gulf velocity fields (Figs. 7c, f) depict aflow towards the west at a core centered at 250mdepth at the center of section C, and two cores ofeastward flow at the same depth in section E.

Although there is no way to directly compareLADCP measured velocities to geostrophic esti-mates with the data available, the observed flowpattern was consistent with previous observationsusing a Pegasus drop-sonde (Collins et al., 1997,their Fig. 3). This suggested the presence of ancyclonic mesoscale eddy that drove CCW into theGulf near the entrance (eastwards) and turnedwestward further inside the Gulf (section C). Similarbehavior was shown in Fig. 5 above.

4. Discussion

The across-Gulf thermohaline structure in aregion about 200 km north of the entrance to theGulf of California was described by a series ofmesoscale-resolving (10 km by 1 dbar) sections.Compared to previous studies (Alvarez-Sanchezet al., 1978; Emilsson and Alatorre, 1997), the datareveal mesoscale features of temperature, salinity

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Fig. 7. August 1995 velocities (m s�1), the contour interval is 0.1m s�1. Geostrophic velocity (a) and (d), using the deepest common

pressure as a reference level. Observed velocity components: (b) and (e), v component; (c) and (f), u component. Upper panel, section C

and lower panel, section E. Positive values indicate inflow and negative values indicates outflow (shaded area).


and geostrophic flow. In contrast to the entrance,waters with relatively high salinity (S434.9) werefound to be distributed in the upper layer along allsections. During winter and spring, these watersappeared as a well-defined surface layer with salinitydecreasing with pressure. However, during summerand fall, relatively fresh waters signaling thepresence of TSW (or CCW) appeared as subsurfacecores (or layers), bounded above and below by moresaline Gulf waters. The presence of CCW has rarelybeen reported for this region. For example, Torres-Orozco (1993), who analyzed existing hydrographicdata, did not find CCW, probably because lack ofdata at these latitudes. Other studies (Castro et al.,2000) have shown that in summer, CCW waspresent at the entrance (section E, Fig. 1) but thatit was constrained to the center of the section andtowards the Sinaloa shelf. CCW was also observedalong section E during other seasons (Griffiths

(1968), during May 1960). Castro et al. (2000)reported CCW during October 1994 but noted thatits presence diminished greatly toward the innerGulf (also see Blanco-Betancourt, 1996; Castro-Valdez, 2001). Their findings and the data shownhere suggest that the northward excursion of CCWinto the Gulf was maximum during the summerseason. The fact that CCW was found to the east atthe entrance and to the west on the northern sectionanalyzed here may be indicative of exchange ofwaters through cyclonic and anticyclonic pairs (seeFigs. 8d–f). Alternating gyres along the axis of theGulf of California were observed by Pegau et al.(2002). One possible mechanism by which the watercould reach higher latitudes in the Gulf is byentrainment of water in these alternating gyres.

The shoaling of isotherms next to BCS wasobserved in Fig. 8a (section E), 8b (section D),and 8c (section C). This suggested a region of at

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Fig. 8. August 1995, sections. Upper panel, temperature, the contour interval is 1 1C. Lower panel, salinity, the contour interval is 0.1. The

shaded region indicate S434.9. The hatched isohaline is S ¼ 34:5. (a) and (d), section E; (b) and (e), section D; (c) and (f), section C.


least 200 km of coastal upwelling. A satellite imagefor this period (Fig. 5) showed cold water from LosFrailes to the southern tip of Cerralvo Island, whichindicated the intensity and spatial extent of theevent. Hurricane Flossie reached the vicinity of thestudy area on August 11 (Fig. 1) (Navarro-Olacheet al., 1997). Observations covering sections E andD started on August 12 and ended on August 15.Section C was completed between August 20 and 21.During the observation period, 2.5–5.2m s�1 north-westward wind predominated (Blanco-Betancourt etal., 1995). Stronger northward winds (18–21m s�1)were observed prior to the cruise, delaying departure

from La Paz. The divergence due to the Ekmantransport caused by these winds was responsible forupwelling along BCS and possibly aided the trans-port of CCW to the inner Gulf. Along Sinaloa, theresponse of the ocean to Flossie was characterized bysea-surface temperature cooling associated withmixing of warm upper waters with cooler deeperwaters and a deepening of the seasonal thermoclinedue to Ekman transport toward the coast. Thepresence of the subsurface salinity minimum at50–60m also means that freshening of the mixedlayer should occur as the upper ocean waters mixnear Sinaloa.

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While the baroclinic structure at the entrance tothe Gulf is characterized as cyclonic, that is, inflowthrough Sinaloa and outflow along BCS (Collinset al., 1997; Castro et al., 2000; Mascarenhas et al.,2004), the baroclinic patterns for the regionimmediately to the north suggested that flow canchange from cyclonic to anticyclonic (Fig. 6).However, a seasonal signal associated with thealternating flow could not be clearly distinguished.Satellite imagery of the region has shown evidencefor both cyclonic (entrance) and anticyclonic (eastof Cerralvo Island) circulation (Collins et al., 1997;Pegau et al., 2002).

There are several forcing mechanisms that coulddrive this type of alternating circulation, amongothers, wind stress curl, topography and coastallytrapped waves. Evidence of wind stress as agenerating mechanism has been provided by Martı-nez (2002), who modeled the Gulf circulation usingsatellite-derived winds (see also Marinone, 2003).An important feature of satellite wind observationswas that the along Gulf component was dominantlysoutheastward, while winds in opposite directionoccurred mainly in summer (Marinone et al., 2004).The southeastward wind drives Gulf waters(GCW–CGW) along the Gulf most of the year.Martınez (2002) found a complex circulationpattern generated by these winds in the southern-most portion of the Gulf with the circulationdominated by cyclonic and anticyclonic mesoscaleeddy pairs along the basin. These alternating eddiesappear to be fixed to basins within the Gulf andresult in a complex exchange and recirculation ofwater masses along the Gulf. During February1999, the surface layer (0–120 dbar) along section Ccontained GCW, indicating a possible recirculationof these waters; however, in August 1995 theexchange between the Pacific Ocean and the Gulfwas more evident, possibly because Flossie.

The thermohaline limits for GCW (SX35.0 andT412 1C) suggested by Torres-Orozco (1993)would indicate that the salty core (SX34.9)observed adjacent to BCS at about 100 dbar(Figs. 4g, h) in November 1997 was a mixture ofTSW and StSsW. In order to avoid such ambiguity,a better definition of GCW salinity is needed sinceGulf waters freshen by mixing on their way to thePacific. As has been shown by Castro et al. (2000),waters with SX34.85 found at the entrance arecharacteristic of GCW. Farther to the north, in theregion studied in here, one would expect less mixingwith fresh Pacific waters, so SX34.9 seems to be a

better threshold for GCW. It is thus expected thatSX35, as suggested by Torres-Orozco (1993), is agood GCW limit for waters found within the Gulfto the north of San Jose Island and for those watersfound adjacent to a coastal evaporative embayment.

El Nino 1997–98 affected the coastal waters in thenortheastern Pacific Ocean (Durazo and Baumgart-ner, 2002). At the entrance to the Gulf (section E),Castro et al. (2000) observed a deep mixed layer(�70 dbar) and a deepening of the thermocline byabout 50 dbar in November 1997. The maximumsalinity measured at the entrance was 34.8 indicat-ing the absence of GCW. This suggested intrusionof TSW which reduced high surface salinities. In thesections further north analyzed here, a deep mixedlayer and deepening of isotherms was also observed.The salinity field showed an asymmetric patternduring November 1997, with TSW on the easternflank and S434.9 adjacent to BCS at about100 dbar. These relatively high salinities indicatepresence of GCW and CGW. Their absence fromthe entrance on the same cruise could suggest thatone of the El Nino effects is to impede the southernadvection of Gulf waters. However, late summerand early fall is when TSW overruns Gulf waters atthe entrance (Wyrtki, 1967; Fiedler, 1992). Thus,it is not clear whether this effect was caused byEl Nino or seasonal processes.

Acknowledgements

This project was supported by CONACyT (project4271P-T, 38797-T), Universidad Autonoma deBaja California (project 1354-333, 0550, 0324) andSecretarıa de Marina de Mexico. E. Gil, S. Larios,R. Blanco and A. Sanchez Devora helped collect thedata. We are also thankful for the strong supportof the masters and crews of B/O A. Humboldt,B/O Francisco de Ulloa, B/O Rio Suchiate and B/OEl Puma.

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