OCEANOLOGICA ACTA 1985 - VOL. 8 - N• 1 ~----

Spring and summer in the Gulf of California:

Gulf of California Satellite

Thermal pattern Upwelling

Fronts

observations of surface thermal patterns

Golfe de Californie Satellite

Structure thermique Upwelling

Fronts

ABSTRACT

RÉSUMÉ

A. Badan-Dangon a, C.J. Koblinsky b*, T. Baumgartner a

a CICESE, Apartado Postal 2732, Ensenada, B.C., Mexico. b Scripps Institution of Oceanography, La JoUa, CA 92093, USA. * Present Address: Code 921, NASA/GSFC, Greenbelt, MD 20771, USA.

Received 7/11/83, in revised form 29/6/84, accepted 12/7/84.

A sequence of infrared images of the Gulf of California, taken from the NOAA-6 and TIROS-N satellites between 24 March and 22 August 1980, reveals well defined mesoscale processes whose distribution in time and space reflects the elima tic transition from the spring to summer wind regimes. The most conspicuous structure is a persistent pool of cool water along the western side of Angel de la Guarda Island extending through the Ballenas Channel. This pool does not depend on the wind and is probably a result of strong tidal mixing in the Channel. lt is the coolest surface water in the Gulf, delimited by a system of fronts whose configuration depends on the direction of the local wind. In the spring, northwes­terly winds drive highly visible plumes of cool upwelled water across the Gulf from the east coast. A transition takes place in late May and early June, after which coastal upwelling appears along the west coast. The summer upwelling is less intense than that in the spring.

Oceanol. Acta, 1985, 8, l, 13-22.

Printemps et été dans le Golfe de Californie: observation des structures thermiques de surface. Une séquence d'images infrarouges des satellites NOAA-6 et TIROS-N du Golfe de Californie a été obtenue, entre le 24 mars et le 22 août 1981; ces photographies montrent une abondance de structures thermiques d'échelle moyenne, dont la distribution temporelle et spatiale manifeste la transition du régime des vents de printemps à celui d'été. La structure la plus remarquable est une tache persistante d'eau froide entre l'île Angel de la Guarda et la péninsule de la Basse-Californie, qui s'étend le long du Canal des Baleines~ La présence de cette eau froide ne paraît pas dépendre du vent, mais résulte probableme~t du mélange vertical induit par la forte marée dans le canal. Cette région est la plus froide du Golfe, délimitée par un système de fronts dont la configuration dépend de la direction du vent. Au printemps, les vents de Nord-Ouest causent des remontées d'eau froide le long de la côte est du Golfe; la manifestation en surface de cet upwelling, clairement visible dans les images, s'étend jusqu'à la côte opposée. Une transition atmosphéri­que s'opère fin mai et début juin, et l'upwelling réappâraît le long de la côte ouest du Golfe. L'upwelling d'été est moins intense que celui de printemps.

Oceannl. Acta, 1985, 8, 1, 13-22.

0399-1784/85/01 13 10/$ 3.00/© Gauthier-Villars 13

A. BADAN-DANGON. C.J. KOBLINSKY. T. BAUMGARTNER

INTRODUCTION

The Gulf of California is located between the Baja California peninsula and mainland Mexico. In addi­tion to supporting a substantial fishery, the Gulf bas long been recognized as an important area for oceanographie research because of its significance as a marginal sea. Although few direct observations. have been made in the Gulf, it is often cloud free, which makes it ideal for observations using satellite remote sensing techniques. This communication des­cribes the predominant sea surface temperature fea­tures that occur in a sequence of satellite infrared images of the Gulf between Marchand August of 1980. Satellite infrared imagery bas emerged as an impor­tant tool for the identification and study of oceano­graphie processes. It provides qualitative informa­tion of remote regions as weil as synopticity for better known areas of the ocean. Koblinsky et al., (1984), among others, have demonstrated that the dominant surface variability seen in satellite infrared images is often the result of advection that cao be associated either with active subsurface processes through geostrophy, or with the wind through Ekman dynamics, depending on the strength ofthese forcing functions. ln the Gulf, Vonder Haar and Stone (1973) used a sequence of orbital photographs from the Gemini and Apollo space missions to examine the shallow water circulation and associated turbidity patterns. The Gulf of California is roughly 1000 km long, oriented Northwest to Southeast, and varies from lOO to 200 km in width (Fig. 1). lt bas an extremely variable bathymetry, which plays a critical rote in the circulation processes seen from space. The two principal physiographic and oceanographie provin­ces, the upper and lower Gulf, are separated by a group of islands at about 29°N. The upper Gulf is essentially a semi-circular continental shelf enclosing the deeper regions around the pedestal of Angel de la Guarda Island. It is approximately 300 km long and 125 km wide, with 75% of its area covered with less than 200 m of water. The lower Gulf consists of a series of silled basins which progressively deepen from about 2000 rn at the center of the Guaymas Basin in the north to depths of 3000 rn at the mou th of the Gulf. The margins along mainland Mexico (hereafter referred to as the eastern coast) of the lower Gulf have a wider shelf and a more variable coastline than those along Baja California (the western coast). Communication between the upper and lower Gulf is primarily through the Ballenas Channel. This Channel is bounded by Baja California on the west and a long continuous submarine ridge from which rise Angel de la Guarda, San Lorenzo and smaller islands to the east. The Channel is nearly 125 km long and narrows to less than 20 km; depths in the Channel exceed 1600 m. The Salsipuedes sill at the south end of the Channel lies between three islands where depths shoal to between 420 and 600m.

14

; \ BAHIA DE toS

Figure 1 Confïguration and generali=ed bathymetry of the Gu(f'llj'Cal[fornia (in rn). Also shown is the areal col'erage of the satellite images presented in this paper. The numbers in the brackets refer to the figure number.

The intense forcing by tides, winds, solar heating, and interactions with the open Pacifie Ocean creates a vigorous circulation in the Gulf. This circulation, in turn, interacts with the variable bathymetry to create a rich space-time kinetic energy spectrum, much of which occurs over periods less than a month. These motions advect the sea surface tempe­rature field measured by a satellite radiometer. Tidal forcing, mostly by the dominant co-oscillating M2 component (Hendershott, Speranza, 1971), is very important. Tidal ranges as large as 7 rn at the head of the Gulf and 4 rn in the Ballenas Channel. are common. This causes strong tidal flow through the Ballenas Channel, with a substantial generation of

trains of internai waves over the sills (Fu, Holt, 1984), internai tides near the continental margins, and rectified residual flow around the islands (Qui­ros, 1983). In addition, the tidal range more than doubles from neap to spring tides. The character of the direct atmospheric forcing is poorly known and no studies of its spatial distribution exist yet in the literature. The variability of the lower atmosphere, determined from radiosondes, is dominated by ener­getic events with periods of a few days to a couple of weeks, with amplitudes Iarger than 10 rn/s. Over longer time scales, the alongshore wind switches direction in a major transition that takes place in late May or early June (Fig. 2), which is associated with the migration of the high pressure anticyclone over the North Pacifie (Hastings, Turner, 1965). The unique clear dry air conditions over the Gulf make it the only evaporative basin of the Pacifie (Roden, 1 958). The evaporative effect is dominant in water mass generation (Sverdrup, 1941), so fronts and thermohaline circulations are important dynamical processes. Externat forcing from the Pacifie Ocean can genera te sea levet events in the Gulf with ranges of about 20 cm at 7 to 20 day periods (Christensen et al., 1983). These events may be generated by tropical hurricanes off the Pacifie coast south of the Gulf during the summer (Enfield, Allen, 1983). The mountain ranges along the Baja Californ'ia peninsula prevent low levet clouds from the Pacifie from influencing the Gulf. Consequently, the Gulf is cloud-free most of the time, but exceptions occur primarily in the summer when moist tropical air moves into the Gulffrom the south. Because ofthese clear air conditions, we have been able to collect and analyze over 50 infrared images of the Gulf, spanning six months. We document here a number of meso­scale processes that result from ti dai and atmospheric forcing. This application of remote sensing is an important step in understanding the dominant physi­cal scales ïn the Gulf.

THE SATELLITE OBSERVATIONS

For this study, we have monitored the infrared radiation propagating spaceward from the Gulf of California with the Advanced Very High Resolution Radiometer (A VHRR) on the polar orbiting NOAA-6 and TIROS-N satellites. This instrument senses the upwelled irradiance at wavelengths of0.7, 0.9, 3.7, and 11.0 rn at a distance of 800 km above the surface (Schwalb, 1 978). The spatial resolution of the A VHRR is 1.1 km at Nadir. The A VHRR observations were directly received and processed at the Scripps Satellite Oceanography Facility in La Jo lia. The images shown here have been contrast enhanced separately to improve pattern recognition. Hence, , intercomparison of the absolute brightness tempera­ture among any of the images shown in this paper is not possible. The images were mapped so that

15

SURFACE THERMAL PATTERNS IN THE GULF OF CALIFORNIA

EMPALME

MAR APR MAY JUN JUL AUG

Figure 2 Time series of winds measured at a height of 1000 mb from radiosonde launches at Empalme during 1980. The vectors have been rotated so that the general orientation of the Gulf is vertical on the page.

warmer temperatures are associated with darker grey shades; bence, clouds are white. The positional accuracy of the processed images is approximately ± 2 picture elements (pixels) or ± 2.2 km. The brightness temperature accuracy is on the order of ± 0.25 K. Within the period of 150 da ys for which we examined infrared imagery, we found 61 images that were clear enough for use in this study, which indicates that there is better than 30% chance of obtaining a usable image in the Gulf on any day. By comparison, there is less than 3% chance of collecting a clear image to examine mesoscale varia­bility in the mid-latitude North Pacifie (Van Woert, pers. comm.).

DESCRIPTION

The predominant patterns in the imagery are a cold water pool in the Ballenas Channel region, coastal upwelling along both coasts of the lower Gulf and south of Ti buron Island, exchange of water between the upper and lower Gulf, and warm water patches in the upper Gulf. The available sequence of 61 cloud free images was examined subjectively for the presence of these features and the results are summarized in the Table. Twenty-four images, wh ose areal co vera ge is indicated in Figure 1, have been retained for illustration purposes. Figure 4 (March-April) illustra tes the temporal deve­lopment of a major upwelling event off the east coast of the lower Gulf during a period of northerly wind, as weil as sorne patterns characteristic of the southward excursions of the Ballenas front. Figure 5 (May) documents the evolution over a few days of the pool of cooler water in the Ballenas-channel during the spring to summer wind transition. Figure 6 (June-July) shows the extension of cool water from the narrow region of the mid-Gulf islands into the upper Gulf, after the transition in wind has occurred. Figure 7 (August) illustrates the surface thermal patterns observed when winds are fully developed

' '·

A. BADAN-DANGON. C.J. KOBLINSKY. T. BAUMGARTNER

Table Graphical listing of dominant patterns observed in the images. Symhols are as fol/ou•s: -- indicates features not visible, general/y because of c/ouds; x indicates region visible butfeature not present. 0 indicates weak; • indicates strong.Jn column 2, 0 refers to east coast; LI refers to west coast.

2 3 4 5 2 3 4 5 2 3 4 5 MAR 25 - o - - ' MAY 8 • x 0 x - JUL 2 •

3 0

4 • 7 0

8 • 12 • 16 • 28 • 29 -30-

A x x 1

from the south and coastal upwelling is observed off the west coast. Figure 8 (April) shows enlarged portions of sorne images on Figure 4, illustrating warm patches in the upper Gulf. Figure 9 (April, May and August) summarizes the patterns observed for periods before, during, arid after the spring to summer wind transition.

APR

26-0- 9• x 0 x .. 28 • 0 0 x ' 12 • x 0 x 1

29 • 0 0 x 1 13 • x - 0 ' 1 0 • • x '

2 0 • • x 3 - • • • 1 4 - •• - J 5 • . . . \ 7 •

14 • x 0 - \ 15 • x 0 x \ 16exoo• 17 • x 0 0 ' 18exoo' 21 • - x 0 ~

23oxxo~

A x x 1 A x x .... x x x • A X )(

.. x - - THE BALLENAS CHANNEL REGION

x -.. x

8 • . . . ' . . . ' • • • 1

AUG 3 - A

Ali of the images we have examined show a persistent pool of cooler water in the Ballenas Channel region, with regard for neither direction nor intensity of the

10 • 12 • 13 0 14 0 15 0 17 • 18 • 22 •

.. - \ •• x Ill' 0 • x .... 0 0 0 \

0 0 • \

0 0 0 ' x - - ...

1 • BALLENAS POOL 2 • COAST AL UPWELLI NG 3 • Tl BURON UPWELUNG

Figure 3

JUN 5 o o o e \ 3 e A X X

14 - A - - /

15 e A 0 X 17 • x 0 0 -220AXX 24 e A X X -28 e A X X ~

29eAXX-

4 •OVERTURN 5• WINO

Summary of the principal positions observed of the Bal/enas Channel frontal system.

6 - - - x ' wind. Most of this region is deep (Fig. 1) and 7 0 8 • 9 • Il • 12 0

13 • 14 • 18 21 0

22 -

 x x "' A x o , the water column can be weil mixed throughout • x - -.. x x - x .. x .. x

.. x

x .... x ,. x -

x -

16

(Gaxiola-Castro et al., 1978). The brightness charac­teristics of the region demonstrate that the coldest surface temperatures of the Gulf are found here. This is supported by historical data compiled by Robinson (1973). The mechanism that brings the

29 March 4 April 8 April

AS

29MAR

Figure 4 Sequence from Marchand April illustrating the temporal evolu­tion of upll'elling plumes off the east coast and the southward extension of the Bal/enas Chan­ne/fronts. ln this and subsequent figures , the arrows indicate the ll'ind averaged from the two radiosoundings taken on that day. The diameler of the circle in the arrow pro vides the sca/e of 1 m. s- 1

•

cold water to the surface is unknown, but we suspect that tidal forcing is resp.onsible. There is, however, no detectable variation of the pool of cold water from spring to ueap tides. Figure 3 is a summary of the positions occupied by the front along the edge of the cold water, obtained from the IR data at various intervals. On 29 March a major upwelling event appears to be in its initial stages. The main' body of cool water occupies most of the region of the Ballenas Channel, surrounding Angel de la Guarda Island, flowing over the Salsipuedes sill, and following the western coastline for about 100 km to the south. There, it abruptly moves offshore to form a small plume with an anticyclonic configuration in the middle of the Guaymas Basin. A second patch of cool water, upwelled near San Esteban and Tibur6n Islands, stretches along the east coast to join an upwelling plume off Guaymas. This surface movement of cool water is probaoly driven by the northerly winds that are beginning to intensify (Fig. 2). By 4 April , the anticyclonic motion of the surface layer extends across the Gulf, roughly centered over

17

SUR FACE THERMAL PATIERNS IN THE GU LF 01 CALI FORN IA

the Guaymas Basin. This motion is traced by two plumes, the first one representing an extension of the Ballenas front projecting offshore from the western coast; the second is associated with the now much more developed upwelling plume off Guaymas and is connected to a patch of cool water offTibur6n Island. At this stage the main southward front from the Ballenas Channel still follows the west coast somewhat before moving offshore. On 8 April (Fig. 3), the two fronts have coalesced next to the eastern coast. The combined front has a pronounced concave shape with the cool water extending eastward from the west coast, following closely the shape of the upwelling plume off Guaymas. There is little lateral mixing in the process for it is possible to distinguish the water that extends from the Ballenas Channel and crosses the Gulf from the water south ofTibur6n Island. The evolution of the Ballenas front and the upwelling plumes off the east coast suggest that the surface circulation of the Gulf responds quickly to the wind, as is generally the case in other coastal regions. For example, a response time of 9 hours or

A BADAN -DANGON. C.J . KOBLINSKY. T. BAUMGARTNER

Figure 5 Sequenceji·om the mon th of Ma y sh01ring an occurrence of the soutln rard extension of the Ba/le­IWS Channel fronts, just prior to the atmospheric transition .

Figure 6 Sequence from June and July illustrating the extension of the cool water from the Ballenas Channel region into the upper Gulf.

18

Figure 7 Images taken during the m onth of August , when the a/ongshore component of the 1rind has reversed directions tn h/owji·om the south , and up1re/ling has developed off Baja California.

Figure 8

SURFACE THERMAL PATIERNS IN THE GU LF OF CALIFORN IA

Enlargements of selected images from the mon th of April that illustrate anoma/ous warm water patches in the upper Gu/(

Jess for the upwelling off the California coast has been documented by Breaker and Gilliland (1981). During the month of May, when the alongshore component of the wind is weak (Fig. 2), Figure 5 shows that the Ballenas front extends southward along the west coast. On 8 May, the cool water is present in the Ballenas Channel , but the front is weak with a small extension along the west coast. By 12 May, structures appear more organized and

19

displacement offshore is apparent. On 15 May the cool water has collected in a wide band, about 30 to 40 km across, off the west coast and extends south to Santa Rosalia, where it maves offshore into the Guaymas Basin. Finally, by 18 May (see also Fig. 3) the front assumes the shape of narrow plume 10 km wide, separated from the coast and extending freely over the Guaymas basin. In contrast with its April configuration, this plume reaches a length of

A. BAOAN- DANGON. C.J . KO BLIN SKY. T. BAUMGARTNER

about 100 km and terminates in a vortex pair. The decay of this southward tongue of cool water can be seen on 21 and 23 May. The entire event occurred over twenty days. The plume took about ten days to develop toits fully defined stage on 18 May and another ten days to decay. By June, the winds have assumed a southwesterly direction, which prevailed through the first week in July (Fig. 2 and 6). On 29 June, the main frontal structure is over the Salsipuedes sill, with a narrow ( < 10 km) south ward extension along the west coast to Bahia Concepcion (Fig. 3). A small plume of cool water is found just off Santa Rosalia, but it is not clear whether this is an extension of the front from the Ballenas Channel region or whether it represents a dynamically independent feature. To the north, entrainment of cool water into the upper Gulf is present. These features persist over a period of about twenty days as illustrated in Figure 6. This may be associated with alongshore flow driven by a local southerly wind off the east coast_ Since the water is shallow in this region, it makes the local wind more effective in accelerating the water along the .coast (Brink, Allen, 1978). The configuration of the frontal system during summer conditions is exemplified by the images taken during August (Fig. 3 and 7). The wind has a strong southerly component (Fig. 2) and upwelling

20

Figure 9 Co/or mosaic summarizing the three domina/li conditions observed in the satellite imager y. Images are chosen <1.1' representatives of nortlm•esterly ll'ind, transition period, and sou­rheasterly or sou/herly wind candi­rions. respective/y . Clouds have been rin red in yellc11r.

is occurring along the west coast. The images show that cool water is bounded to the south by a sharp front over the Salsipuedes sill. There is no evidence ofits extension farther south except for a thin ribbon of cool water next to the west coast, which joins an upwelling plume north of Santa Rosalia. Cool water invades the entire Tibur6n basin, and collects into a northward jet next to the east coast. It continues northward, forming an anticyclonic vortex in the upper Gulf. Subsequent images indicate this vortex lasted on the order of three to four days.

COAST AL UPWELLING

Coastal upwelling is prominent throughout most of the observation period . During March and April, upwelling is visible predominantly off the east coast, corresponding to winds blowing from the northwest (Fig. 2). U pwelling was particularly well developed from 31 March until 12 April and appeared weak but detectable for a few days before and after that period (Table). The typical development time scale of this event is on the order of a couple of da ys, which corresponds to the time scale of major fluctuations in the wind record; its duration is from ten to twenty da ys.

The development of the coastal upwelling patterns does not occur simultaneously along the eastern coast, as is shown in Figure 4. As early as 2 April, coastal upwelling is weil developed, following early manifestations on 29 March. The coolest water in this system is found between Bahia Kino and San Pedro Nolasco Island. From there the cool water extends to the south in a well-defined plume (A) that rn oves offshore just north ofCabo Lobos and reaches across the Gulf to Bahia Concepci6n. The plume forks into two branches as it approaches the west coast and then recrosses the Gulf. During this period weak manifestations of upwelling are present south of Guaymas. On 6 April (not shown because of partial cloud cover) and 7 April, plume A has become more intense and a second plume (B) extending southward from Cabo Lobos is present, stretching offshore just north ofTopolobampo. Plume B beee­mes more intensely defined in subsequent images. Also visible on 7 April are the initial stages of a third plume (C) that stretches toward the south from Topolobampo. On 10 and 12 April, plume A begins to decay. Relaxation ofupwelling leads to a weaken­ing of the plume front and surface patterns appear less organized and more dominated by small scale features. From 10 to 12 April, plume B is well­developed and at roughly the same stage that plume A was on 2 April. It reaches across the Gulf at a point off San José Island and splits into two returning branches defined by major fronts. One front reaches back to the east coast south of Topolobampo. This behavior of the upwelling centers suggests a lag in the development of the atmospheric forcing along the Gulf, which appears to have shifted southeastward at a rate of about 300 km in 5 to 10 da ys. By 17 April, the winds have diminished and only weak traces of upwelling are visible. There is also evidence of moisture flowing into the region from the south. On 22 April, only relie patterns of upwelling can be observed and atmospheric moisture can be seen farther north of Topolobampo. Thus 17 April appears to mark the end of the upwelling event.

Coastal upwelling is not apparent on either coast during May/June. Only a small thin jet that extends offshore from Santa Rosalia could be interpreted to be Iocally wind-driven upwelling. An examination of a large scale image taken on 4 July (not shown), however, suggests a different explanation. It shows a mass of cold stable air abutting the west side of the Baja California peninsula, overlying the low desert lands of the central peninsula and partially blocked by the mountain range of 2000 to 3000 rn in elevation. The pressure gradient, presumably set up at low levels in the atmosphere across the peninsula, could then force intense winds through mountain passes and generate small clumps of cool water through vertical mixing in the Gulf. This could also provide a mechanism for driving the weil defined jet of cool water seen in these images across the Gulf, from the Ballenas Channel region into the shallow water off the east coast. The presence of

21

SURFACE THERMAL PATTERNS IN THE GULF OF CALIFORNIA

intense local winds generated in this way is suggested by Mosiiio-Alemân and Garcia (1974). In 1982, wind speeds of over 25 m/s with sharp spatial gradients off Los Angeles Bay were reported by Alvarez et al. (1984). After the atmospheric transition in mid-June, the coastal upwelling off the west coast intensifies. A major event with a time scale on the order of two weeks, can be detected in August (Table). We show only two images (Fig. 7) to illustrate the distribution of coastal upwelling on the western coast. A well­defined plume can be seen, extending offshore from Cabo Virgenes. It stretches across the Gulf and spreads into a vortex pair against the east coast. During much of this period, however, the southerly winds bring large amounts of moisture, obscuring the area to infrared remote sensing. The broad platform southeast of Tibur6n Island region is at times covered with cool water, which appears to be separated from the cold surface water in the Ballenas Channel. This feature is weil defined during the upwelling event of April (Fig. 4), but it is non-existent la ter in the year when the alongshore wind bas reversed direction, altough the cold water remains present in the Ballenas Channel (Fig. 5 and 6). We suspect that a small region of coastal upwelling is active over this shelf during periods of northerly winds. This interpretation is supported by a numerical investigation of Behrens (1983). Despite its smalllateral dimensions, the plume that originates in this area stretches over the Guaymas Basin and parallels the plume generated off Guaymas and in the Ballenas Channel.

UPPER GULF WARM WATER PATCHES

On sorne of the images (e.g., Fig. 8), the surface of the upper Gulf has a textured, cotton-like appear­ance. The small warm elements constituting this surface pattern have widths of 5-10 km and lengths of 25-50 km and, in this sequence of images, the entire group of warm elements advects slowly westward. We do not know the cause ofthese unique features, but they could be associated with spatial variations of the intense heatand moisture fluxes at the surface in this region or with the resulting near-surface convective overturning, since evapora­tion off Guaymas is maximum in April and May (Rodeo, 1958), when the images indicate the presence of these surface patterns. A direct manifestation on satellite imagery of evaporation convection has not been demonstrated, but its importance resides in that it could be linked to the formation of a water mass particular to the Gulf (Sverdrup, 1941).

SUMMARY

Satellite imagery has been used to document surface thermal patterns in the Gulf of California. Wind

A. BADAN-DANGON. C.J. KOBLINSKY. T. BAUMGARTNER

data shows that a well.defined spring to summer atmospheric transition from northerly to southerly winds took place during May. This transition is reflected in the behavior of surface thermal patterns, summarized in Figure 9. A frontal system is associat­ed with the quasi-permanent pool of cool water in the Ballenas Channel. It is in this area that the coldest surface temperatures are found in the Gulf. The shapes of the major fronts south and north of the pool depend on the wind regimes in existence over the Gulf. Northerly winds favor a southerly extension of the front and its interaction with thermal features in the Guaymas Basin. During those times when the alongshore component of the wind is generally weak, the southerly extension of the Balle­nas front takes the form of a free jet that extends over the Guaymas Basin after moving offshore somewhat north of Santa Rosalia. Southerly winds promote an extension of this cool water into the upper Gulf. The second dominant feature observed in the images is coastal upwelling. Predictably, it appears on diffe­rent coasts depending upon the direction of the wind. During our observations, it was found off the eastern coast during spring, it was almost not present during May and June, and then reappeared off the west coast during July-August, coinciding with the direc­tion and strength of the wind. The strength of the upwelling appears to be greater off the east coast than off the west coast but, in ali cases, the plume of upwelled water originates at one point, moves along the coast in the direction of the wind to the nearest major cape and then moves offshore, in a manner first suggested by Reid et al. (1958), and later corroborated by Traganza et al. (1981), for the

REFERENCES

Alvarez L.G., Badan-Dangon A., Robles J.M., 1984. Lagrangian observations of near-surface currents in Canal de Ballenas, Ca/COH Rep., 25, 35-42. Behrens A., 1983. Untersuchungen zur Simulation von Auftriebsvorgangen im Golf von Kalifomien min Hi1fe eines Baroklinen Numerischen Modells, Diplomarbeit, Inst. Meeres., Univ. Hamburg, 97 p. Breaker L.C., Gilli1and R.P., 1981. A satellite sequence on upwelling along the Ca1ifomia coast, in: Coast al upwelling, edited by F.R. Richards, American Geophysical Union, Washington, D.C. 87-94. Brink K.H., Allen J.S., 1978. On the effects of bottom friction on barotropic motion over the continental shelf, J. Phys. Ocea­nogr., 8, 919-922. Christensen N. Jr., de la Paz R., Gutierrez G., 1983. A study of sub-inertia1 waves off the west coast of Mexico, Deep-Sea Res., 30, 835-850. Enfield D.B., Allen J.S., 1983. The generation and propagation, of sea leve! variability along the Pacifie coast of Mexico, J. Ph ys. Oceanogr., 13, 1012-1033. Fu L.-L., Holt B., 1984. Internai waves in the Gulf of Califomia: observations from a spaceborne radar, J. Geophys. Res., 89, C2, 2053-2060. Gaxiola-Castro G., Alvarez-Borrego S., Schwartzlose R.A., 1978. Sistema del bi6xido de carbono en el Go1fo de California, Ci. Mar., 5, 25-40. Hastings J.R., Turner R.M., 1965. Seasonal precipitation regimes in Baja California, Mexico, Geograf. Annal., 47, 204-223. Hendershott M.C., Speranza A., 1971. Co-oscilla ting tides in long, narrow hays; the Taylor problem revisited, Deep-Sea Res., 18, 959-980.

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upwelling along the west coast of North America. In that way, the plume ofupwelling crosses the Gulf to the opposite coast, where it usually divides into two branches that extend back across the Gulf. Hence, it appears reasonable to expect motions that are coherent across the Gulf and on longshore scales comparable to its width. Less persistent features include upwelling south of Ti buron Island and warm water patches in the upper Gulf. The upwelling off Ti buron Island implies that cool water covering the mid-Gulf island region may come from various sources of different dynamical character. The warm water patches of the upper Gulf detected by this sequence of images, suggest that air-sea interaction processes may be bery active in this region and that the usefulness of this method in documenting events of water mass formation should be investigated.

Acknowledgements

We wish to acknowledge C.D. Winant, K.H. Brink and our reviewers for several helpful comments and Miguel Tenorio for processing many of the images contained in this communication. Maricela Gonza­lez, Jennifer Davis and Joan Semler patiently typed several versions of this manuscript, and Nancy Hulbirt prepared the illustrations. This research was supported by the Consejo Nacional de Ciencia y Tecnologia of Mexico, and by the US Sea Grant contract No. NOAA NA79AA-D-00117. Additional funds for image processing were also obtained from the NSF/NASA/ONR block grant to the Scripps Institution of Oceanography for remote sensing.

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