‘Santa Ana’ winds and upwelling filaments off Northern Baja California

Dynamics of Atmospheres and Oceans 37 (2003) 113–129

‘Santa Ana’ winds and upwelling filaments off Northern Baja California A. Trasviña a,∗ , M. Ortiz-Figueroa b , H. Herrera c , M.A. Cos´ıo c , E. González c a

Departamento de Oceanograf´ıa F´ısica, CICESE en BCS, Miraflores #334 e/ Mulegé y La Paz, Fracc. Bella Vista, La Paz 23050, BCS, Mexico b Departamento de Oceanograf´ıa F´ısica, CICESE, Km 107 Carret, Tijuana-Ensenada, Ensenada, BC, Mexico c CICESE en BCS, Miraflores #334 e/ Mulegé y La Paz, Fracc. Bella Vista, La Paz 23050, BCS, Mexico Received 26 March 2002; accepted 24 March 2003

Abstract An atmospheric condition known as a ‘Santa Ana’ wind occurred from 9 to 11 February 2002. Its effect was felt over a large portion of southern California and the northern half of the Baja California Peninsula. Santa Ana winds are dry, strong northwesterly through easterly mountain downslope winds, most common in winter. Satellite data from Quickscat show two large wind jets crossing the mountains of the peninsula and extending 300 km offshore. Data from a coastal station reveal that the event lasted over 52 h with average speeds of 11 m s−1 and gusts of 25 m s−1 . The southernmost jet crosses the mountains at the San Matias mountain pass and generates a cold filament off Point Colonet. Satellite imagery shows this feature lasting at least two inertial periods (Ti = 22 h) and extending 100 km offshore during the observation period. Estimates of the stationary Ekman pumping produced vertical speeds of 20 m per day, consistent in time and location with the observed structures. The ocean off Point Colonet is well known for the existence of upwelling episodes. They occur mostly in the spring or early summer when persistent winds blow towards the equator and parallel to the coast. The events described here present a different phenomenon: upwelling filaments induced by short-lived, offshore winter winds. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Upwelling; Cold filaments; Offshore winds; Air–sea interaction; Santa Ana winds

∗ Corresponding author. Tel.: +52-612-12-1-3031x109; fax: +52-612-12-1-3031x110. E-mail address: [email protected] (A. Trasviña).

0377-0265/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0377-0265(03)00018-6

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1. Introduction The west coast of Northern Baja California is well known for the persistence of equatorward winds and the intensity of its upwelling episodes (see, for instance, Bakun, 1975; Barton and Argote, 1980). Conditions for typical coastal upwelling may occur throughout the year although the months of May–June are most favorable. This work describes a different mechanism capable of inducing upwelling: winter, offshore ‘Santa Ana’ winds. Sommers (1978) describes these winds as strong northwesterly to easterly mountain downslope winds. Cold front passages produce the intensification of the Great Basin high pressure over North America. If the front moves westward enough a strong northwest-tosoutheast surface pressure gradient is established over southern California and Northern Baja California. The winds interact with the topography of the coastal mountains to produce the jets described here. Santa Ana winds are dry (less than 10% relative humidity) and can reach up to 50 m s−1 . They occur from October to May and may last for several days. Intense offshore wind bursts occur in many coastal areas around the world. The Mistral winds of the Mediterranean Sea are a well-known example. In Mexico, the ‘Nortes’ of the Gulf of Tehuantepec have been studied since the 1990s (Schultz et al., 1997; Trasviña et al., 1995) in contrast with the lesser known ‘Santa Ana’ winds described here. The theoretical response of a coastal ocean to an offshore wind jet was found by Crepon and Richez (1989). Such a wind field is characterised by regions of intense wind-stress curl at both sides of the jet. The coastal ocean reacts with offshore bands of upwelling (and downwelling) to the left (right) of the wind, looking downwind. Winds such as these are important both because of their impact on the ocean dynamics and because of its contribution to the biogeochemical cycle of the coastal ocean. The deposition of dust affects sedimentary processes and it may even produce iron enrichment of the upper ocean (Guerzoni et al., 1997). The region of interest includes southern California and the northern half of the Baja California Peninsula (Fig. 1). This is an abrupt terrain with a number of mountain ranges running parallel to the coast. In the south, centered at 31◦ N, the San Pedro Martir Mountain range reaches over 2500 m and ends abruptly at the San Mat´ıas Mountain pass. Altitudes here diminish to less than 500 m (not visible in this map). To the north the mountains rise to heights greater than 2000 m in the Sierra Juarez Mountains (centered at 32◦ N), diminish to 1500 m or less while crossing the border with the US, and rise again in the Cleveland National Forest in southern California. In this paper, we discuss satellite and in situ wind observations, as well as thermal satellite imagery gathered during the event of 9–11 February 2002. Our goal is to characterize the response of the coastal ocean to the wind. 2. Data and methods Infrared imagery from NOAA satellites is routinely collected twice a day at CICESE’s land station in La Paz, Mexico. Only NOAA 12 imagery was captured for the period anal-

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Fig. 1. Elevation contours in meters for northwestern Mexico and the southwestern US. Relevant mountain ranges are labelled and location of cities (Tijuana: TIJ and Ensenada: ENS) and coastal features (Point Colonet: COL and San Quintin coastal lagoon: SQN) are indicated for reference in the text. The inset shows the region of interest in North America.

ysed in this work. All these satellites are equipped with advanced very high resolution radiometers (AVHRR/2) and are received using high resolution picture transmission (HRPT) format. Further processing includes data calibration and georeferencing. Images of sea surface temperature (SST) are presented as false color images on a Mercator grid, using the M MAP package developed by Rich Pawlowicz. To estimate the accuracy of the sea surface temperature satellite measurements, the variability was analysed in a cloud-free area

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of 2◦ × 2◦ . We used eight images dating from 8 to 12 February 2002. The standard deviation of the series of mean SST values, for this period, is found to be 0.65 ◦ C. Temporal SST variations greater than this value are therefore considered significant throughout the discussion. Satellite wind data were obtained from NASA/JPL’s SeaWinds Scatterometer aboard the QuikSCAT. Data presented here are level 3 daily, gridded ocean wind vectors with a horizontal resolution of 0.25◦ . The data provided by JPL comes divided into a.m. and p.m. data. The former corresponds to data with equatorial crossing at 6:00 a.m. UTC and the latter to 6:00 p.m. UTC. Although exact times for the wind were not obtained, we estimated the local time of the satellite pass to make the correct match with the AVHRR images. First, we assume the time for the satellite to reach the region of interest is small. And second, we correct the equatorial crossing times to Pacific Standard Time (PST) and label the data accordingly as ‘evening’ or ‘morning’ wind fields. This way all the a.m. wind data corresponds to evening data of the previous day (subtracting 6 h from UTC we get 23:00 PST) and p.m. wind data to morning data of the same day (12:00 PST). In situ wind data comes from automatic stations of the National Water Commission (CNA, Mexico). Synoptic data was obtained from a number of sources such as the NOAA Air Resources Laboratory web site (http://www.arl.noaa.gov/ready/cmet.html) and the NOAA-Server (http://www.epic.noaa.gov/cgi-bin/NOAAServer). 3. Results 3.1. The wind event of 9–11 February 2002 The evening of 8 February the wind field off Northern Baja California is typical for the season (Fig. 2). Northwesterly winds blow along the coast with speeds increasing from 4 to 6 m s−1 near the coast to 10 m s−1 offshore. Synoptic maps from the NOAA Air Resources laboratory illustrate the development of the event. In the mean sea-level pressure map for 9 February (12:00 PST; Fig. 3) the Great Basin high shows values of 1047 HPa and an intense pressure gradient over northern California. At the same time, the associated cold front (maps not shown) advances towards southern California. Along the coast, the pressure field steadily decreases southward from the southern California Bight to Baja California. The relative humidity map (Fig. 4) shows normal conditions (60–80%) for the coast and the typical low values (20% or less) for the deserts of the southern US and northern Mexico. By the evening of 9 February, the Santa Ana winds started blowing over the coastal region of Northern Baja California. Over the ocean, the wind field (Fig. 5) turned towards the west over a large region including the northern half of the Baja California Peninsula and as far offshore as 130◦ W (figure not shown). The main coastal features are the two wind jets, both similar in intensity and shape, which extend off the Baja California coast. They blow from the east-northeast at speeds exceeding 10 m s−1 near the coast, fading gradually offshore. The area covered by winds exceeding 6 m s−1 reaches some 300 km offshore. These jets

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Fig. 2. Wind field from Quickscat for the evening (PST) of 8 February 2002. Contours of wind speed (m s−1 ) are shown superimposed on the vector field. The location of cities (Tijuana: TIJ and Ensenada: ENS) and coastal features (Point Colonet: COL and San Quintin coastal lagoon: SQN) are indicated on the map for reference.

are funnelled through mountain passes. The one in the south flows across the San Matias mountain pass, north of the San Pedro Martir Mountain range and the one in the north across the low area north of the Sierra Juarez Mountains (see Fig. 1). Synoptic charts for 10 February show the peak of the event. The high pressure has intensified to 1048 HPa (Fig. 6) displacing the cold front southwards. The pressure gradient is more intense now over Northern Baja California forcing the wind to interact with the mountain chains and passes of the region, and producing the intense winds previously described. The relative humidity along the coast (Fig. 7) decreases dramatically to values below 20%. Low humidity values over the ocean can be detected several hundred kilometres offshore. We examined data from a number of automatic meteorological stations of the National Water Commission (CNA, Mexico). Two stations (Tijuana and Ensenada) roughly 100 km apart, recorded the full force of the event. Next, we discuss raw data sampled at 10 min intervals, from both stations. The Ensenada station (Fig. 8, for approximate location see Fig. 1) is not truly representative of the wind characteristics over the ocean. It is an inland station located in a canyon, adjacent to a water reservoir. It does however capture the full intensity of the event. On 8 February, at 21:00 PST, the wind direction changed abruptly from its diurnal pattern to a persistent east-northeast direction (70–80◦ , azimuthal) lasting until 12:00 PST on 11 February. The wind speed behaves in a different way. At about 12:00 PST on 9 February it suddenly increases from 5 to 10 m s−1 in less than 3 h. At the same time, and in less than 1 h, the relative humidity drops from near saturation to 10%, and the air temperature rises almost 10 ◦ C (to 20 ◦ C). This signals the start of the event. Altered values of wind

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Fig. 3. Mean sea-level pressure map for 12:00 PST, 9 February 2002 (NOAA Air Resources Laboratory).

speed, temperature and relative humidity continue until 12:00 on 11 February when also the wind direction recovers its usual variation pattern. The event lasts 48 h at his particular location. During this period, the mean wind speed varied between 10 and 12 m s−1 , reaching a maximum of 15 with gusts of 25 m s−1 . The Tijuana station (Fig. 9, for approximate location see Fig. 1) is perhaps more representative of the winds over the ocean. It is located in a more open site and closer to the sea. It is also near the center of the northernmost jet (Fig. 5). At the onset of the event all five variables reflect drastic changes. The wind speed starts to increase at 5:00 PST on 9 February and at about 12:00 PST it has already attained a mean of 10 m s−1 with gusts of 20 m s−1 . Also at 5:00 PST the wind quickly turns to an east-northeast direction (70–80◦ , azimuthal), the air temperature rises 10 ◦ C (to about 20 ◦ C) and the relative humidity drops to less than 10%. During the event, the mean speed reaches similar values (from 10 to

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Fig. 4. Surface relative humidity map for 12:00 PST, 9 February 2002 (NOAA Air Resources Laboratory).

12 m s−1 , gusting to 20 m s−1 ) to those observed in Ensenada, without the extremes found in that station. The air temperature varies between 18 and 21 ◦ C, except at the end when it surges to 28 ◦ C. The event lasts here until 11 February at 17:00 PST, a total of 52 h. Its end is indicated by the wind speed dropping to less than 5 m s−1 , the tendency of the relative humidity to increase, a clear cooling trend and finally, by the wind turning to a more typical direction (from the north, near to 360◦ in the figure). Summarising, the Tijuana station reveals the phenomenon lasted 52 h over the coastal region of Northern Baja California. It started at 5:00 a.m. on 9 February and continued until 17:00 p.m. on 11 February. Close to the coast, in close agreement with the satellite observations, the mean wind speed averaged 11 m s−1 . During the more intense phase, the gusts reached 20 m s−1 near the coast (Tijuana) and 25 m s−1 at the inland locations (Ensenada). The direction of the wind was persistent from the east-northeast

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Fig. 5. Same as Fig. 2 for the evening (PST) of 9 February 2002.

for the whole period, the relative humidity dropped to less than 10% and temperatures rose up to 28 ◦ C. Following Sommers’ (1978) classification, this was a moderate event. 3.2. Response of the coastal ocean The thermal (AVHRR) image of 8 February (Fig. 10) is used to describe the situation prior to the wind event. Clouds cover part of the image but a 100 km wide coastal band is clearly visible. Most of the sea surface temperature field off Northern Baja California (from Tijuana, 32.5◦ N, to San Quintin, 30.5◦ N) maintains values above 15.5 ◦ C (orange to red tones). Warmer waters are clearly separated from a cool (SSTs below 15 ◦ C, greenish tones) coastal strip by a meandering front. Colder areas in the image arise due to interference by clouds and are not real oceanic features. The wind field corresponds to later in the evening of that day. Winds blow from the northwest along the coast with typical speeds around 5 m s−1 . Small filaments extend offshore and intrude the warmer water north and south of Point Colonet. Another one can be seen off San Quintin. They have a length scale of about 30 km. The wind event started the evening of 9 February. The coastal wind field is shown superimposed on the SST field for the morning of 10 February, about 8 h into the event (Fig. 11). After the wind starts the imagery improves since the dry winds move the clouds away from the coastal region. The wind jets observed in Fig. 5 are seen here at a closer range and for clarity the speed contours are omitted. The southernmost jet blows between Point Colonet and Ensenada, and the northernmost between Ensenada and Tijuana. Cool offshore filaments with temperatures below 14.5 ◦ C (in the blue range) start to develop at several places along the coast. Their characteristic cool signature is still somewhat short (about 50 km) but

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Fig. 6. Mean sea-level pressure map for 12:00 PST, 10 February 2002 (NOAA Air Resources Laboratory).

already noticeable between Tijuana and Point Colonet. The coastal strip is up to 1 ◦ C cooler than on 8 February (compare with Fig. 10), and unbroken along the coast. The filaments off Tijuana and Point Colonet have faint but clear offshore extensions giving the image a ‘banded’ appearance. The next image corresponds to the morning of 11 February (Fig. 12), some 40 h after the onset of the event. The winds are for the evening of 10 February. They had been blowing for 36 h and were beginning to subside. Two large cold filaments are clearly seen in this image. Inside the one to the north, off Tijuana, temperatures near the coast reach 13 ◦ C (deep blue) and an elongated area below 14 ◦ C (blue range) extends some 120 km offshore. The filament to the south, off Point Colonet, has an even larger 13 ◦ C area near the coast. Its offshore extension is also larger although slightly warmer. In this filament, an area below 14.5 ◦ C extends about 200 km offshore.

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Fig. 7. Surface relative humidity map for 12:00 PST, 10 February 2002 (NOAA Air Resources Laboratory).

4. Discussion The thermal imagery describes significant SST changes that can be attributed to different processes. First of all, SST in the region as a whole is lowered during the event. Maximum SSTs vary from 16 ◦ C the afternoon of 8 February, to 15.5 ◦ C on the morning of 10 February and to 15 ◦ C on the morning of the 11 February. At least two processes are likely to produce this effect. Advection of cool water from the coast evidently plays an important role. The whole evolution of the event and the generation of filaments show this. However, the effect of a dry, warm wind blowing over the ocean also plays an important part due to its effect on the flux of latent heat. A simple calculation serves to illustrate its significance. Latent heat flux depends on the atmosphere–ocean temperature difference, the atmospheric pressure,

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Fig. 8. Time series of atmospheric data at the Ensenada automatic station of the National Water Commission (CNA, Mexico): (a) 10 min mean and maximum (gusts) wind speed; (b) wind direction in azimuthal degrees (90◦ : east, 0◦ : north); (c) air temperature and relative humidity (divided by 10 to use the same scale).

124 A. Trasviña et al. / Dynamics of Atmospheres and Oceans 37 (2003) 113–129 Fig. 9. Time series of atmospheric data at the Tijuana automatic station of the National Water Commission (CNA, Mexico): (a) 10 min mean and maximum (gusts) wind speed; (b) wind direction in azimuthal degrees (90◦ : east, 0◦ : north); (c) air temperature and relative humidity (divided by 10 to use the same scale).

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Fig. 10. Thermal (AVHRR) image of 15:47 PST on 8 February, with Quickscat winds for the evening of that day shown superimposed. The color scale corresponds to sea surface temperatures as in the vertical bar. Clouds and land are masked in white. The location of cities (Tijuana: TIJ and Ensenada: ENS) and coastal features (Point Colonet: COL and San Quintin coastal lagoon: SQN) are indicated on the map for reference.

relative humidity and wind speed. It can be estimated using bulk formulae such as those described in Gill (1982). Typical parameters for this wind event are an SST of 15 ◦ C, air temperature of 25 ◦ C, atmospheric pressure of 1025 db, relative humidity of 10% and wind speed of 10 m s−1 . Using these values, the latent heat flux from the ocean into the lower atmosphere would be about 300 W m−2 . Knowing that the specific heat at constant pressure of a surface water mass is close to 4000 J kg−1 ◦ C−1 (at 15 ◦ C and 34.5 psu), we can estimate the average surface cooling. For a 20 m deep mixed layer and during the peak (40 h) of the event this would be around 0.5 ◦ C. Minimum SSTs diminish even more than maximum SSTs. The more drastic changes occur at well-defined locations at the coast and below both wind jets. There the SSTs change from 14.5 ◦ C the afternoon of 8 February, to about 14 ◦ C the morning of 10 February, to 13 ◦ C the morning of the 11 February. In the Levitus climatology, this temperature range belongs to depths between 30 and 50 m for this region, this month of the year. This phenomenon has been observed, for instance, at the head of the Gulf of Tehuantepec during similar offshore wind events. The cause is the baroclinic elevation of the thermocline in response to the lowering of the sea level induced by the wind (Trasviña et al., 1995; Filonov and Trasviña,

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Fig. 11. Same as Fig. 10. The thermal image corresponds to 6:30 PST on 10 February. Quickscat winds are previous to the image, for the evening of 9 February (see also Fig. 5).

2000). This produces upwelling confined to a small coastal area beneath the region where the winds blow strongest. The more important feature of the phenomenon is produced by yet another type of upwelling. The distribution of the Ekman pumping velocity readily explains the existence of the cool filaments. A first approximation to this velocity can be estimated for the stationary case, based on the curl of the wind stress (Gill, 1982): wE =

∂x Ys − ∂y Xs ρf

(1)

where ρ is the density of the air, f the Coriolis parameter, Xs and Ys the wind-stress components at the surface and wE the Ekman pumping velocity, positive upwards. The wind-stress components at the surface were estimated from the Quickscat wind data using the formula proposed by Wu (1982). τ = ρC10 U10

(2)

where τ is the wind stress acting on the sea surface, U10 the wind velocity component (m s−1 ) measured at 10 m above the mean sea surface, C10 the wind-stress drag coefficient given by Eq. (3) and again ρ the density of the air.

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Fig. 12. Same as Fig. 10. The thermal image corresponds to 6:06 PST on 11 February. Quickscat winds are previous to the image, for the evening of 10 February.

C10 = (0.8 + 0.065U10 ) × 10−3

(3)

To estimate the effect of the Ekman transport on the coastal ocean, we assume that the wind field of the evening of 9 February is representative of the whole event. Therefore, we compare the Ekman pumping velocity at the beginning of the event with the temperature field near the end of the event, after almost two inertial periods (Ti = 22.9 h at 31.5◦ N). The aim is to discuss the response of the ocean after allowing enough time for the Ekman adjustment to develop. Contours of Ekman pumping velocity calculated with data from the beginning of the event on 9 February are shown superimposed on the image of the morning of 11 February (Fig. 13). Warm and cool areas alternate along the coast from north to south. Positive (upwelling) contours of Ekman pumping correspond very closely to areas with cool filaments. The agreement is particularly good on the southern filament, off Point Colonet. Warm areas largely correspond to regions with negative (downwelling) pumping velocities. The agreement is not perfect due to limitations of our basic assumptions (the real process is not stationary), to the low horizontal resolution of the winds compared to the scale of the upwelling, and to the coastal shadow where wind data from the satellite are lost. It is however good enough to identify the basic balance of forces responsible for generating

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Fig. 13. The thermal image corresponds to 6:06 PST on 11 February. Ekman pumping contours (in m per day) from data of the evening of 9 February are shown superimposed. The ocean adjusts to the forcing through mechanisms explained by stationary Ekman theory. Positive/negative Ekman pumping velocities correspond to colder/warmer areas.

these filaments. This is in good agreement with the theoretical response of a coastal ocean to offshore forcing, as explained by Crepon and Richez (1982). Summarising, Santa Ana winds are capable of producing intense air–sea flux of latent heat. Evaporation is promoted by dry conditions and the moderately high wind speeds. This in turn lowers SSTs at distances off the coast of the order of hundreds of kilometres. Forcing by the wind lowers SSTs dramatically at the coast, below areas of maximum intensity. The likely cause is the lowering of the sea level at the coast, and the baroclinic response of the thermocline, thus producing coastal upwelling. Unfortunately, no in situ data are available to confirm these observations. Data in the Levitus climatology is consistent with the coastal upwelling coming from depths of 30–50 m. Intense positive/negative wind-stress curl to the left/right of the wind jets (looking downwind) is capable of producing offshore bands of upwelling/downwelling (Crepon and Richez, 1982). Upwelling is responsible for the generation of cool filaments. Their influence on the coastal ocean is not negligible; one of the filaments observed here extends up to 200 km offshore. On an additional note, perhaps the transient effect of winter upwelling on the organic primary productivity may not be significant in comparison with the continuous effect of spring upwelling. However, consecutive Santa Ana winds (5–10 per season) are also responsible

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for large amounts of dust transport that may significantly fertilize the ocean surface during the winter season. Illustrative images of dust plumes over southern California and Baja California on 9 and 10 February 2002 were captured by the sea-viewing wide field-of-view sensor (SeaWiFS). They are posted on two separate web sites: 1. http://eob.gsfc.nasa.gov/NaturalHazards/natural hazards v2.php3?img id=1620. 2. http://www.jpl.nasa.gov/images/earth/usa/california.html. This, along with the offshore transport induced by the wind, can have a significant impact on the overall productivity of the coastal ocean. This is particularly true in an area where river sediment discharges are not significant.

Acknowledgements We wish to acknowledge the work of Ricardo Torres who kindly provided the matlab programs to read the Quickscat data. Rich Pawlowicz created the M MAT toolbox for matlab. Meteorological data was kindly provided by officials of the National Water Commission (CNA, Mexico). We are particularly grateful to Ing. José Francisco Tellez Gámez and Ing. Manuel Colima Sánchez. Guillermo Gutierrez and Dale Haidvogel reviewed the manuscript and contributed helpful comments. This manuscript was produced with support from the Oceanology Division of CICESE; the Satellite Oceanography Program, Phase 1 (CONACyT DAJ/J002/750/00); CICESE’s campus at La Paz, BCS Mexico and from the Northwest Center for Biological Research (CIBnor). Armando Trasviña and Modesto Ortiz are SNI grant holders. References Bakun, A., 1975. Daily and weekly upwelling indices. West coast of North America, 1967–73. NOAA technical report, NMFS SSRF-693. Barton, E.D., Argote, M.L., 1980. Hydrographic variability in an upwelling area off Northern Baja California in June 1976. J. Mar. Res. 38, 4. Crepon, M., Richez, C., 1982. Transient upwelling generated by two-dimensional atmospheric forcing and variability in the coastline. J. Phys. Oceanogr. 12, 1437–1457. Filonov, A.E., Trasviña, A., 2000. Internal waves on the continental shelf of the Gulf of Tehuantepec, Mexico. Estuarine, Coastal Shelf Sci. 50, 531–548. Gill, A., 1982. Atmosphere–ocean Dynamics. Academic Press, New York, p. 662. Guerzoni, S., Molinaroli, E., Chester, R., 1997. Saharan dusts inputs to the western Mediterranean Sea: depositional patterns, geochemistry and sedimentological implications. Deep-Sea Res. II, 44 (3–4), 631–654. Schultz, D.M., Bracken, W.E., Bosart, L.F., Hakim, G.J., Bedrick, M.A., Dickinson, M.J., Tyle, K.R., 1997. The 1993 Superstorm Cold Surge: Frontal Structure, Gap Flow, and Tropical Impact. Monthly Weather Rev. 125 (1), 5–39. Sommers, W.T., 1978. LFM Forecast variables related to Santa Ana wind occurrences. Monthly Weather Rev. 106, 1307–1316. Trasviña, A., Barton, E.D., Brown, J., Vélez, H.S., Kosro, M., Smith, R.L., 1995. Offshore wind forcing in the Gulf of Tehuantepec, Mexico: the asymmetric circulation. J. Geophys. Res., OCEANS 100 (C10), 20649–20663. Wu, J., 1982. Wind-stress coefficients over sea surface from breeze to hurricane. J. Geophys. Res. 87 (C12), 9704–9706.