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Satellite Observations of Mesoscale Eddies in the Gulfs of Tehuantepec and Papagayo (Eastern Tropical Pacific)

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Satellite Observations of Mesoscale Eddies in the Gulfs of Tehuantepec and Papagayo (Eastern Tropical Pacific) ARTICLE IN PRESS Deep-Sea Research II 51 (2004) 587–600 www.elsevier.com/locate/dsr2 Satellite observations of mesoscale eddies in the Gulfs of Tehuantepec and Papagayo (Eastern Tropical Pacific) Adriana Gonzalez-SilveraÃ, Eduardo Santamaria-del-Angel, Roberto Milla´ n-Nun˜ ez, He´ ctor Manzo-Monroy Facultad de Ciencias Marinas, Universidad Auto´noma de Baja California, AP. 453, Ensenada, BC - CP 22800 Mexico Accepted 3 May 2004 Abstract SeaWiFS high-resolution ocean-color images and AVHRR sea-surface temperature were obtained for the period from November 1998 to March 1999. Generation of mesoscale eddies was observed for the region between the Gulf of Tehuantepec and the Gulf of Papagayo (Tropical Pacific Ocean). Eighteen eddies with diameters ranging between 100 and 450 kmwere identified; 14 originated in the Gulf of Tehuantepec and mostwere cyclonic. Our results show that the frequency of cyclonic eddy formation is higher than it has been reported and their lifetime can be longer. Using both ocean color images and daily temporal resolution, instead of sea-surface temperature alone, improved our ability to identify and follow the propagation of these eddies. The generation and fate of smaller cyclonic eddies around the periphery of the anticyclonic eddy indicate the importance of the onshore–offshore exchange of energy and biological material, which has not been considered previously. Three anticyclonic eddies were generated in the Gulf of Papagayo. These eddies have a common origin, and they travel along the main flux of the Costa Rica Coastal Current (CRCC), which later turns offshore and joins the North Equatorial Current. This behavior can be seen clearly on SeaWiFS images and suggests that the CRCC influences eddy propagation. Time series of chlorophyll-a concentration and SST were obtained fromtwo points at Tehuantepec and two at Papagayo. Cross-correlation analysis confirms an inverse relationship between these variables, showing that a decrease in SST is followed by an increase in chlorophyll-a concentration values, indicating high growth and primary production rates. r 2004 Elsevier Ltd. All rights reserved. 1. Introduction ÃCorresponding author. Tel.: +52-646-1744570x109; fax: +52-646-1744103. The eastern border of the tropical Pacific Ocean, E-mail addresses: [email protected] (A. Gonzalez-Silvera), between 51N and 211N is influenced by intense [email protected] (E. Santamaria-del-Angel), [email protected] (R. Milla´ n-Nun˜ ez), [email protected] wind events that produce upwelling at three sites (H. Manzo-Monroy). of Northern and Central America (Lavı´ n et al., 0967-0645/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2004.05.019 ARTICLE IN PRESS 588 A. Gonzalez-Silvera et al. / Deep-Sea Research II 51 (2004) 587–600 1992; Barton et al., 1993; Trasvin˜ a et al., 1995; sphere, a cyclonic eddy means that the sense of Chelton et al., 2000a,b; Mu¨ ller-Karger and rotation of such eddy is anti-clockwise, and when a Fuentes-Yaco, 2000; McClain et al., 2002): the vortex is developed it causes the upwelling of Gulf of Tehuantepec in Mexico, the Gulf of subsurface waters in its center and a downwelling Papagayo in the Costa Rica–Nicaragua border, at its extremities. The upwelling of subsurface and the Gulf of Panama´ . These strong, focused waters brings colder and nutrient-reach waters winds can last from1 to 15 days and blow offshore to the photic zone of the water column, promoting over the Pacific Ocean, generating filamentous phytoplankton growth. For this reason a cyclonic ocean jets and high waves along their trajectory. eddy appears as a cold and high biomass- Their effect favors strong near-shore oceanic core eddy. Anticyclonic eddies have an opposite mixing and intense lowering of sea-surface tem- behavior, appearing as a warm-core and low peratures (Barton et al., 1993; Trasvin˜ a et al., biomass-core eddy. These facts have proved the 1995). This phenomenon usually occurs in fall, use of temperature and ocean color satellite images winter and early spring when polar air masses to be efficient in visualizing and following eddy move south into the Gulf of Mexico and the motion in this area (Lluch-Cota et al., 1997; Intertropical Convergence Zone (ITCZ) is at its Mu¨ ller-Karger and Fuentes-Yaco, 2000; McClain southernmost position (Chelton et al., 2000a,b). et al., 2002). Another feature observed in this zone is the Costa Mu¨ ller-Karger and Fuentes-Yaco (2000) used Rica Dome (CRD), centered at 901W and 91N, satellite images from both the ocean-color sensor which is a shoaling of the strong and shallow Coastal Zone Color Scanner (CZCS) and ad- thermocline of the eastern Pacific Ocean. Surface vanced very high-resolution radiometer (AVHRR) currents flow cyclonically around it and its to study these eddies and their effect on pigment seasonal evolution is affected by the same large- concentration in the region of Tehuantepec, scale wind patterns. The upwelling associated with Papagayo and Panama´ . From1979 to 1986, the dome is about 0.5 m dayÀ1, yielding cool sea- the formation of anticyclonic and cyclonic eddies surface temperatures (SST) (Fiedler, 2002). Anti- that moved distances greater than 1500 km from cyclonic and cyclonic eddies are generated in this their point of origin were reported (Mu¨ ller-Karger zone as a result of the effect of wind stress over the and Fuentes-Yaco, 2000); this emphasizes their ocean (Hansen and Maul, 1991; Barton et al., importance as source of energy and biological 1993; Mu¨ ller-Karger and Fuentes-Yaco, 2000; constituents fromthe continental marginto McClain et al., 2002). The result is that surface the offshore tropical Pacific. In a more recent waters are fertilized by the introduction of paper, McClain et al. (2002) confirmed the nutrient-rich waters fromsubsurface layers or importance of coastal winds and Ekman diver- fromthe coast; both sources increase phytoplank- gence for the formation of phytoplankton ton abundance, chlorophyll-a concentration (Tras- blooms along the coast of Central America vin˜ a et al., 1995; Lluch-Cota et al., 1997; Mu¨ ller- and the Costa Rica Dome (CRD). Considering Karger and Fuentes-Yaco, 2000; McClain et al., the knowledge of the region, the period from 2002) and modify trophic conditions as shown November 1998 to March 1999 was chosen, when analyzing zooplankton productivity data for to study the type (anticyclonic or cyclonic), the Gulf of Tehuantepec (Fa¨ rber-Lorda et al., number and frequency of eddy generation, propa- 2003). The overall effect of these strong winds can gation, and the relationship between chlorophyll-a be inferred on fisheries and the fact that interna- and SST for the region between the Gulf of tional pelagic fishing fleets heavily exploit this Tehuantepec and the Gulf of Papagayo. High- area. Indeed, the eastern tropical Pacific contains resolution images were used (1.1-km spatial some of the most productive waters of the world’s resolution in daily composites) from the ocean- oceans (Fiedler et al., 1991). color sensor Sea Viewing Wide Field of View An eddy is a deviation in the steady-state flow of Sensor (SeaWiFS) and complemented with SST a fluid, causing a vortex. In the northern hemi- data fromAVHRR. ARTICLE IN PRESS A. Gonzalez-Silvera et al. / Deep-Sea Research II 51 (2004) 587–600 589 2. Methods DAS 4.1), where chlorophyll a concentration (Chla) was estimated using the OC4 algorithm The conventional method to obtain a synoptic (O’Reilly et al., 1998), making a total of 146 view of a cloudy region is to calculate the average images. The study region was set between 102 and value (e.g., chlorophyll) for a specific location 811W and 0 and 211N, which includes both the during a certain time period (e.g., 1 week). This is Gulfs of Tehuantepec and Papagayo (Fig. 1). not ideal for studying dynamic processes, which These images were examined to obtain informa- will be blurred or not be seen at all. Daily images tion on eddy formation, size and range of Chla were used to avoid this problem, although cloudy concentration, and eddy duration. areas were obtained occasionally. The period of Sea-surface temperature (SST) images were November 1998–March 1999 was chosen, consid- obtained fromthe NOAA/NASA AVHRR Path- ering that this period has the best sequence of finder subsetting system( http://podaac.jpl.nasa. SeaWiFS images in comparison with other years, gov). These images are daily records of the and it is also the period of the year when wind jets ascending pass of the sensor at 9 kmspatial are strongest and eddy generation frequency is resolution, and they correspond to PO.DAAC’s higher. V4.1 data. SST data were used for comparison with Each image was processed using the SeaWiFS Chla maps in order to confirm sense of rotation of processing package distributed by NASA (Sea- eddies, range of temperature, and also as a way to Fig. 1. Study region and location of points where time series of chlorophyll-a and sea-surface temperature data were taken (stars). Gray arrows indicate the location of wind jets entering the Pacific Ocean over the continent. Black arrows represent surface currents for boreal winter (NEC=North Equatorial Current, CRCC=Costa Rica Coastal Current, SEC=South Equatorial Current). ARTICLE IN PRESS 590 A. Gonzalez-Silvera et al. / Deep-Sea Research II 51 (2004) 587–600 complement information on eddy translation when In order to ascertain the relationship between no SeaWiFS images were available. Chla concentration and SST, time series were Eddies were first identified by looking for eddy- extracted fromthe locations indicated in Fig. 1: shaped features on ocean-color images. Next, the Gulf of Tehuantepec, the Gulf of Tehuantepec profiles of Chla concentration and SST for each and 430 kmoffshore; the Gulf of Papagayo and eddy were compared to confirm the sense of 450 km offshore.
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