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

Gulf of California Satellite Spring and summer in the Gulf Thermal pattern Upwelling of California: Fronts Golfe de Californie Satellite observations of surface thermal Structure thermique Upwelling patterns Fronts

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.

ABSTRACT 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, northwes­ terly 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.

RÉSUMÉ 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 atmosphéri­ que 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 addi­ tion 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 des­ cribes the predominant sea surface temperature fea­ tures that occur in a sequence of satellite infrared ; \ images of the Gulf between Marchand August of BAHIA DE toS 1980. Satellite infrared imagery bas emerged as an impor­ tant tool for the identification and study of oceano­ graphie processes. It provides qualitative informa­ tion 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 provin­ ces, 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 Figure 1 Confïguration and generali=ed bathymetry of the and 125 km wide, with 75% of its area covered with Gu(f'llj'Cal[fornia (in rn). Also shown is the areal less than 200 m of water. The lower Gulf consists col'erage of the satellite images presented in this of a series of silled basins which progressively deepen paper. The numbers in the brackets refer to the from about 2000 rn at the center of the Guaymas figure number. 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 The intense forcing by tides, winds, solar heating, lower Gulf have a wider shelf and a more variable and interactions with the open Pacifie Ocean creates coastline than those along Baja California (the a vigorous circulation in the Gulf. This circulation, western coast). Communication between the upper in turn, interacts with the variable bathymetry to and lower Gulf is primarily through the Ballenas create a rich space-time kinetic energy spectrum, Channel. This Channel is bounded by Baja California much of which occurs over periods less than a on the west and a long continuous submarine ridge month. These motions advect the sea surface tempe­ from which rise Angel de la Guarda, San Lorenzo rature field measured by a satellite radiometer. Tidal and smaller islands to the east. The Channel is nearly forcing, mostly by the dominant co-oscillating M2 125 km long and narrows to less than 20 km; depths component (Hendershott, Speranza, 1971), is very in the Channel exceed 1600 m. The Salsipuedes sill important. Tidal ranges as large as 7 rn at the head at the south end of the Channel lies between three of the Gulf and 4 rn in the Ballenas Channel. are islands where depths shoal to between 420 and common. This causes strong tidal flow through the 600m. Ballenas Channel, with a substantial generation of

14 SURFACE THERMAL PATTERNS IN THE GULF OF CALIFORNIA trains of internai waves over the sills (Fu, Holt, EMPALME 1984), internai tides near the continental margins, and rectified residual flow around the islands (Qui­ ros, 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 ener­ getic events with periods of a few days to a couple of weeks, with amplitudes Iarger than 10 rn/s. Over MAR APR MAY JUN JUL AUG longer time scales, the alongshore wind switches Figure 2 direction in a major transition that takes place in Time series of winds measured at a height of 1000 mb from late May or early June (Fig. 2), which is associated radiosonde launches at Empalme during 1980. The vectors have with the migration of the high pressure anticyclone been rotated so that the general orientation of the Gulf is vertical over the North Pacifie (Hastings, Turner, 1965). The on the page. unique clear dry air conditions over the Gulf make it the only evaporative basin of the Pacifie (Roden, 1958). The evaporative effect is dominant in water mass generation (Sverdrup, 1941), so fronts and warmer temperatures are associated with darker grey thermohaline circulations are important dynamical shades; bence, clouds are white. The positional processes. Externat forcing from the Pacifie Ocean accuracy of the processed images is approximately can genera te sea levet events in the Gulf with ranges ± 2 picture elements (pixels) or ± 2.2 km. The of about 20 cm at 7 to 20 day periods (Christensen brightness temperature accuracy is on the order of et al., 1983). These events may be generated by ± 0.25 K. Within the period of 150 da ys for which tropical hurricanes off the Pacifie coast south of the we examined infrared imagery, we found 61 images Gulf during the summer (Enfield, Allen, 1983). that were clear enough for use in this study, which The mountain ranges along the Baja Californ'ia indicates that there is better than 30% chance of peninsula prevent low levet clouds from the Pacifie obtaining a usable image in the Gulf on any day. from influencing the Gulf. Consequently, the Gulf By comparison, there is less than 3% chance of is cloud-free most of the time, but exceptions occur collecting a clear image to examine mesoscale varia­ primarily in the summer when moist tropical air bility in the mid-latitude North Pacifie (Van Woert, moves into the Gulffrom the south. Because ofthese pers. comm.). 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 meso­ DESCRIPTION scale processes that result from ti dai and atmospheric forcing. This application of remote sensing is an The predominant patterns in the imagery are a cold important step in understanding the dominant physi­ water pool in the Ballenas Channel region, coastal cal scales ïn the Gulf. 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 THE SATELLITE OBSERVATIONS in the upper Gulf. The available sequence of 61 cloud free images was examined subjectively for For this study, we have monitored the infrared the presence of these features and the results are radiation propagating spaceward from the Gulf of summarized in the Table. Twenty-four images, wh ose California with the Advanced Very High Resolution areal co vera ge is indicated in Figure 1, have been Radiometer (A VHRR) on the polar orbiting retained for illustration purposes. NOAA-6 and TIROS-N satellites. This instrument Figure 4 (March-April) illustra tes the temporal deve­ senses the upwelled irradiance at wavelengths of0.7, lopment of a major upwelling event off the east coast 0.9, 3.7, and 11.0 rn at a distance of 800 km above of the lower Gulf during a period of northerly the surface (Schwalb, 1978). The spatial resolution wind, as weil as sorne patterns characteristic of the of the A VHRR is 1.1 km at Nadir. The A VHRR southward excursions of the Ballenas front. Figure observations were directly received and processed at 5 (May) documents the evolution over a few days the Scripps Satellite Oceanography Facility in La of the pool of cooler water in the Ballenas-channel Jo lia. during the spring to summer wind transition. Figure The images shown here have been contrast enhanced 6 (June-July) shows the extension of cool water from separately to improve pattern recognition. Hence, , the narrow region of the mid-Gulf islands into the intercomparison of the absolute brightness tempera­ upper Gulf, after the transition in wind has occurred. ture among any of the images shown in this paper Figure 7 (August) illustrates the surface thermal is not possible. The images were mapped so that patterns observed when winds are fully developed

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

Table from the south and coastal upwelling is observed off Graphical listing of dominant patterns observed in the images. the west coast. Figure 8 (April) shows enlarged Symhols are as fol/ou•s: -- indicates features not visible, general/y because of c/ouds; x indicates region visible butfeature not present. portions of sorne images on Figure 4, illustrating '·' 0 indicates weak; • indicates strong.Jn column 2, 0 refers to east warm patches in the upper Gulf. Figure 9 (April, coast; LI refers to west coast. May and August) summarizes the patterns observed 2 3 4 5 2 3 4 5 2 3 4 5 for periods before, during, arid after the spring to MAR 25 - o - - ' MAY 8 • x 0 x JUL 2 • A x x 1 26-0- 9• x 0 x -.. 3 0 A x x 1 summer wind transition. 28 • 0 0 x ' 12 • x 0 x 1 4 • A x x .... 29 • 0 0 x 1 13 • x - 0 7 0 x x x • 14 • x 0 - \' 8 • A X )( APR 1 0 • • x ' 15 • x 0 x \ 12 • .. x - - THE BALLENAS CHANNEL REGION 2 0 • • x 16exoo• 16 • 3 - • • • 1 17 • x 0 0 ' 28 • 4 - •• - J 18exoo' 29 - x - Ali of the images we have examined show a persistent .. x 5 • . . . \ 21 • - x 0 ~ 30- pool of cooler water in the Ballenas Channel region, 7 • . ' 23oxxo~ . AUG A . 3 - 8 • ' with regard for neither direction nor intensity of the 10 • .• .• .• 1 JUN 5 o o o e \ 6 - - - x ' wind. Most of this region is deep (Fig. 1) and 12 • .. - \ 3 e A X X 7 0 Â x x "' 13 0 •• x Ill' 14 - A - - / 8 • A x o , the water column can be weil mixed throughout 14 0 0 • x .... 15 e A 0 X 9 • • x - - (Gaxiola-Castro et al., 1978). The brightness charac­ 15 0 0 0 0 \ 17 • x 0 0 - Il • .. x x 17 • 0 0 • \ 220AXX 12 0 - x x .... teristics of the region demonstrate that the coldest x ,. 18 • 0 0 0 ' 24 e A X X - 13 • .. x surface temperatures of the Gulf are found here. 22 • x - - ... 28 e A X X ~ 14 • .. x x - 29eAXX- 18 This is supported by historical data compiled by 1• BALLENAS POOL 21 0 .. x x - Robinson (1973). The mechanism that brings the 2 • COASTAL UPWELLI NG 4 •OVERTURN 22 - 3 • Tl BURON UPWELUNG 5• WINO 29 March 4 April 8 April

Figure 3 Summary of the principal positions observed of the Bal/enas Channel frontal system.

16 SUR FACE THERMAL PATIERNS IN THE GU LF 01 CA LI FORN IA

AS

29MAR

Figure 4 Sequence from Marchand April illustrating the temporal evolu­ tion of upll'elling plumes off the east coast and the southward extension of the Bal/enas Chan­ ne/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 1 m. s- •

cold water to the surface is unknown, but we suspect the Guaymas Basin. This motion is traced by two that tidal forcing is resp.onsible. There is, however, plumes, the first one representing an extension of no detectable variation of the pool of cold water the Ballenas front projecting offshore from the from spring to ueap tides. Figure 3 is a summary of western coast; the second is associated with the now the positions occupied by the front along the edge much more developed upwelling plume off Guaymas of the cold water, obtained from the IR data at and is connected to a patch of cool water offTibur6n various intervals. On 29 March a major upwelling Island. At this stage the main southward front from event appears to be in its initial stages. The main' the Ballenas Channel still follows the west coast body of cool water occupies most of the region of somewhat before moving offshore. On 8 April (Fig. the Ballenas Channel, surrounding Angel de la 3), the two fronts have coalesced next to the eastern Guarda Island, flowing over the Salsipuedes sill, and coast. The combined front has a pronounced concave following the western coastline for about 100 km to shape with the cool water extending eastward from the south. There, it abruptly moves offshore to form the west coast, following closely the shape of the a small plume with an anticyclonic configuration in upwelling plume off Guaymas. There is little lateral the middle of the Guaymas Basin. A second patch mixing in the process for it is possible to di stinguish of cool water, upwelled near San Esteban and the water that extends from the Ballenas Channel Tibur6n Islands, stretches along the east coast to and crosses the Gulf from the water south ofTibur6n join an upwelling plume off Guaymas. This surface Island. The evolution of the Ballenas front and the movement of cool water is probaoly driven by the upwelling plumes off the east coast suggest that the northerly winds that are beginning to intensify (Fig. surface circulation of the Gulf responds quickly to 2). By 4 April, the anticyclonic motion of the surface the wind, as is generally the case in other coastal layer extends across the Gulf, roughly centered over regions. For example, a response time of 9 hours or

17 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 ofth e Ba/le­ IWS 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 SURFACE THERMAL PATIERNS IN THE GU LF OF CALIFORN IA

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 Califo rnia.

Figure 8 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 displacement offshore is apparent. On 15 May the been documented by Breaker and Gilliland (1981). cool water has collected in a wide band, about 30 to During the month of May, when the alongshore 40 km across, off the west coast and extends south component of the wind is weak (Fig. 2), Figure 5 to Santa Rosalia, where it maves offshore into the shows that the Ballenas front extends southward Guaymas Basin. Finally, by 18 May (see also Fig. along the west coast. On 8 May, the cool water is 3) the front assumes the shape of narrow plume 10 present in the Ballenas Channel, but the front is km wide, separated from the coast and extending weak with a small extension along the west coast. freely over the Guaymas basin. In contrast with its By 12 May, structures appear more organized and April configuration, this plume reaches a length of

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

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

about 100 km and terminates in a vortex pair. The is occurring along the west coast. The images show decay of this southward tongue of cool water can that cool water is bounded to the south by a sharp be seen on 21 and 23 May. The entire event occurred front over the Salsipuedes sill. There is no evidence over twenty days. The plume took about ten days ofits extension farther south except for a thin ribbon to develop toits fully defined stage on 18 May and of cool water next to the west coast, which joins an another ten days to decay. upwelling plume north of Santa Rosalia. Cool water By June, the winds have assumed a southwesterly invades the entire Tibur6n basin, and collects into a direction, which prevailed through the first week in northward jet next to the east coast. It continues July (Fig. 2 and 6). On 29 June, the main frontal northward, forming an anticyclonic vortex in the structure is over the Salsipuedes sill, with a narrow upper Gulf. Subsequent images indicate this vortex ( < 10 km) south ward extension along the west coast lasted on the order of three to four days. 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 COASTAL UPWELLING a dynamically independent feature. To the north, entrainment of cool water into the upper Gulf is Coastal upwelling is prominent throughout most of present. These features persist over a period of about the observation period. During March and April, twenty days as illustrated in Figure 6. This may be upwelling is visible predominantly off the east coast, associated with alongshore flow driven by a local corresponding to winds blowing from the northwest southerly wind off the east coast_ Since the water is (Fig. 2). U pwelling was particularly well developed shallow in this region, it makes the local wind more from 31 March until 12 April and appeared weak effective in accelerating the water along the .coast but detectable for a few days before and after that (Brink, Allen, 1978). period (Table). The typical development time scale The configuration of the frontal system during of this event is on the order of a couple of days , which summer conditions is exemplified by the images corresponds to the time scale of major fluctuations in taken during August (Fig. 3 and 7). The wind has a the wind record; its duration is from ten to twenty strong southerly component (Fig. 2) and upwelling da ys .

20 SURFACE THERMAL PATTERNS IN THE GULF OF CALIFORNIA

The development of the coastal upwelling patterns intense local winds generated in this way is suggested does not occur simultaneously along the eastern by Mosiiio-Alemân and Garcia (1974). In 1982, wind coast, as is shown in Figure 4. As early as 2 April, speeds of over 25 m/s with sharp spatial gradients coastal upwelling is weil developed, following early off Los Angeles Bay were reported by Alvarez et al. manifestations on 29 March. The coolest water in (1984). this system is found between Bahia Kino and San After the atmospheric transition in mid-June, the Pedro Nolasco Island. From there the cool water coastal upwelling off the west coast intensifies. A extends to the south in a well-defined plume (A) that major event with a time scale on the order of two rn oves offshore just north ofCabo Lobos and reaches weeks, can be detected in August (Table). We show across the Gulf to Bahia Concepci6n. The plume only two images (Fig. 7) to illustrate the distribution forks into two branches as it approaches the west of coastal upwelling on the western coast. A well­ coast and then recrosses the Gulf. During this period defined plume can be seen, extending offshore from weak manifestations of upwelling are present south Cabo Virgenes. It stretches across the Gulf and of Guaymas. On 6 April (not shown because of spreads into a vortex pair against the east coast. partial cloud cover) and 7 April, plume A has become During much of this period, however, the southerly more intense and a second plume (B) extending winds bring large amounts of moisture, obscuring southward from Cabo Lobos is present, stretching the area to infrared remote sensing. offshore just north ofTopolobampo. Plume B beee­ The broad platform southeast of Tibur6n Island mes more intensely defined in subsequent images. region is at times covered with cool water, which Also visible on 7 April are the initial stages of a appears to be separated from the cold surface water third plume (C) that stretches toward the south from in the Ballenas Channel. This feature is weil defined Topolobampo. On 10 and 12 April, plume A begins during the upwelling event of April (Fig. 4), but it to decay. Relaxation ofupwelling leads to a weaken­ is non-existent la ter in the year when the alongshore ing of the plume front and surface patterns appear wind bas reversed direction, altough the cold water less organized and more dominated by small scale remains present in the Ballenas Channel (Fig. 5 features. From 10 to 12 April, plume B is well­ and 6). We suspect that a small region of coastal developed and at roughly the same stage that plume upwelling is active over this shelf during periods of A was on 2 April. It reaches across the Gulf at a northerly winds. This interpretation is supported by point off San José Island and splits into two returning a numerical investigation of Behrens (1983). Despite branches defined by major fronts. One front reaches its smalllateral dimensions, the plume that originates back to the east coast south of Topolobampo. This in this area stretches over the Guaymas Basin and behavior of the upwelling centers suggests a lag in parallels the plume generated off Guaymas and in the development of the atmospheric forcing along the the Ballenas Channel. Gulf, which appears to have shifted southeastward at a rate of about 300 km in 5 to 10 days. By 17 April, the winds have diminished and only weak traces of UPPER GULF WARM WATER PATCHES 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 On sorne of the images (e.g., Fig. 8), the surface of observed and atmospheric moisture can be seen the upper Gulf has a textured, cotton-like appear­ farther north of Topolobampo. Thus 17 April ance. The small warm elements constituting this appears to mark the end of the upwelling event. surface pattern have widths of 5-10 km and lengths of 25-50 km and, in this sequence of images, the Coastal upwelling is not apparent on either coast entire group of warm elements advects slowly during May/June. Only a small thin jet that extends westward. We do not know the cause ofthese unique offshore from Santa Rosalia could be interpreted to features, but they could be associated with spatial be Iocally wind-driven upwelling. An examination variations of the intense heatand moisture fluxes at of a large scale image taken on 4 July (not shown), the surface in this region or with the resulting however, suggests a different explanation. It shows near-surface convective overturning, since evapora­ a mass of cold stable air abutting the west side of tion off Guaymas is maximum in April and May the Baja California peninsula, overlying the low (Rodeo, 1958), when the images indicate the presence desert lands of the central peninsula and partially of these surface patterns. A direct manifestation on blocked by the mountain range of 2000 to 3000 rn satellite imagery of evaporation convection has not in elevation. The pressure gradient, presumably been demonstrated, but its importance resides in set up at low levels in the atmosphere across the that it could be linked to the formation of a water peninsula, could then force intense winds through mass particular to the Gulf (Sverdrup, 1941). 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 SUMMARY defined jet of cool water seen in these images across the Gulf, from the Ballenas Channel region into the Satellite imagery has been used to document surface shallow water off the east coast. The presence of thermal patterns in the Gulf of California. Wind

21 A. BADAN-DANGON. C.J. KOBLINSKY. T. BAUMGARTNER data shows that a well.defined spring to summer upwelling along the west coast of North America. atmospheric transition from northerly to southerly In that way, the plume ofupwelling crosses the Gulf winds took place during May. This transition is to the opposite coast, where it usually divides into reflected in the behavior of surface thermal patterns, two branches that extend back across the Gulf. summarized in Figure 9. A frontal system is associat­ Hence, it appears reasonable to expect motions that ed with the quasi-permanent pool of cool water in are coherent across the Gulf and on longshore scales the Ballenas Channel. It is in this area that the comparable to its width. coldest surface temperatures are found in the Gulf. Less persistent features include upwelling south of The shapes of the major fronts south and north of Ti buron Island and warm water patches in the upper the pool depend on the wind regimes in existence Gulf. The upwelling off Ti buron Island implies that over the Gulf. Northerly winds favor a southerly cool water covering the mid-Gulf island region may extension of the front and its interaction with thermal come from various sources of different dynamical features in the Guaymas Basin. During those times character. The warm water patches of the upper when the alongshore component of the wind is Gulf detected by this sequence of images, suggest generally weak, the southerly extension of the Balle­ that air-sea interaction processes may be bery active nas front takes the form of a free jet that extends in this region and that the usefulness of this method over the Guaymas Basin after moving offshore in documenting events of water mass formation somewhat north of Santa Rosalia. Southerly winds should be investigated. promote an extension of this cool water into the upper Gulf. Acknowledgements The second dominant feature observed in the images is coastal upwelling. Predictably, it appears on diffe­ rent coasts depending upon the direction of the wind. We wish to acknowledge C.D. Winant, K.H. Brink During our observations, it was found off the eastern and our reviewers for several helpful comments and coast during spring, it was almost not present during Miguel Tenorio for processing many of the images May and June, and then reappeared off the west contained in this communication. Maricela Gonza­ coast during July-August, coinciding with the direc­ lez, Jennifer Davis and Joan Semler patiently typed tion and strength of the wind. The strength of the several versions of this manuscript, and Nancy upwelling appears to be greater off the east coast Hulbirt prepared the illustrations. This research was than off the west coast but, in ali cases, the plume supported by the Consejo Nacional de Ciencia y of upwelled water originates at one point, moves Tecnologia of Mexico, and by the US Sea Grant along the coast in the direction of the wind to the contract No. NOAA NA79AA-D-00117. Additional nearest major cape and then moves offshore, in a funds for image processing were also obtained from manner first suggested by Reid et al. (1958), and the NSF/NASA/ONR block grant to the Scripps later corroborated by Traganza et al. (1981), for the Institution of Oceanography for remote sensing.

REFERENCES

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