Coastal Upwelling in the Eastern Gulf of California
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OCEANOLOGICA ACTA · VOL. 23 – N° 6 Coastal upwelling in the eastern Gulf of California Salvador-Emilio LLUCH-COTA * Centro de Investigaciones Biolo´gicas del Noroeste, S.C. PO Box 128 La Paz, BCS, Me´xico 23000 Received 11 May 1999; revised 8 December 1999; accepted 13 December 1999 Abstract – Understanding and quantifying upwelling is of great importance for marine resource management. Direct measurement of this process is extremely difficult and observed time-series do not exist. However, proxies are commonly derived from different data; most commonly wind-derived. A local wind-derived coastal upwelling index (CUI) is reported for the period 1970–1996 and is considered representative for the eastern central Gulf of California, an important fishing area where no proxies exist. The index is well related to pigment concentration distribution, surface water temperature, and population dynamics of important fish resources over the seasonal time-scale. There is a biological response to ENSO activity not reflected by this index, indicating that improvement of biological enrichment forecasting also requires water column structure input. A clearly increasing seasonal amplitude signal is detected in the coastal upwelling index and sea surface temperature since the mid 1970s. Understanding the nature of these long-term trends, the incorporation of remote tropical ocean signals into the enrichment proxy, the dynamics of atmosphere and ocean, and the biological responses are major challenges to the proper management of fish resources in the Gulf of California. upwelling / Gulf of California / seasonality / climate change Re´sume´–Upwelling coˆtier dans l’est du golfe de Californie. Comprendre et e´valuer le phe´nome`ne d’upwelling est tre`s important pour la gestion des ressources marines. Les mesures directes sont extreˆmement difficiles et aucune se´rie temporelle d’observations n’est disponible, mais des estimations sont possibles a` partir de donne´es telles que la vitesse du vent. Un indice d’upwelling coˆtier (CUI), de´termine´ dans le pre´sent travail a` partir du vent local pour la pe´riode 1970–1996, est repre´sentatif de l’upwelling dans l’est du golfe de Californie ou` se trouve une importante zone de peˆche. L’indice est en bon accord, a` l’e´chelle de la saison, avec la re´partition des concentrations en pigments, avec la tempe´rature superficielle de l’eau et la dynamique des populations des ressources halieutiques. La re´ponse biologique au phe´nome`ne ENSO n’apparaıˆt pas dans cet indice, ce qui montre que les apports dans la colonne d’eau doivent eˆtre pris en compte pour ame´liorer la pre´vision de la fertilite´ biologique. Un signal saisonnier d’amplitude nettement croissante est observe´ dans l’indice d’upwelling coˆtier et dans la tempe´rature superficielle de l’eau depuis le milieu des anne´es 70. Comprendre la nature des ces tendances a` long terme, incorporer les signaux en provenance de l’oce´an tropical dans l’e´valuation de la fertilite´, dans la dynamique de l’atmosphe`re et de l’oce´an, et dans les re´ponses biologiques sont les de´fis majeurs a` relever pour ge´rer correctement les ressources halieutiques du golfe de Californie. © 2000 Ifremer/CNRS/IRD/E´ ditions scientifiques et me´dicales Elsevier SAS upwelling / golfe de Californie / variabilite´ saisonnie`re / changement climatique 1. INTRODUCTION * Correspondence and reprints: Frictional stress of wind on the ocean surface, acting E-mail address: [email protected] (S.-E. LLUCH-COTA) together with the earth’s rotation, causes water flow © 2000 Ifremer/CNRS/IRD/E´ ditions scientifiques et me´ dicales Elsevier SAS PII: S0399-1784(00)00121-3/FLA 731 S.-E. LLUCH-COTA / Oceanologica Acta 23 (2000) 731–740 in the surface layer to deviate to the right in the fornia peninsula. Meteorologically and oceanographi- northern hemisphere and to the left in the southern. cally it is regarded as a semi-enclosed basin because it When the direction of the transported water is di- is almost completely surrounded by elevated topogra- rectly offshore, water from beneath the surface up- phy and connected to the open ocean only at its wells to replace it [4]. If pumping is strong enough to southern end [3]. These properties, together with it break the thermocline, the upwelled waters are nor- being in the tropical–subtropical transitional zone, mally nutrient-rich and, once in the photic zone, result in a complex atmospheric and oceanographic highly productive. Upwelling areas thus support large environment, and in intense intraannual variations of standing crops of plankton, massive fish stocks, and physical and biological processes. major populations of marine mammals and sea birds Atmospheric forcing is characterized by weak south- [26]. As a result, changes in upwelling over different easterly winds in summer and stronger northwesterly time-scales, linked to variations in the atmospheric winds in winter [21]. During both periods, winds are system and water column structure, are related to polarized along the gulf axis by an orographic effect variability in fish populations and other components [17]. Wind forcing, acting along with tides, solar of the coastal ocean systems [7, 11, 20]. Understand- heating and interactions with the open Pacific Ocean, ing and quantifying upwelling activity is of great creates a vigorous circulation in the gulf [2]. Recently importance for marine resource management in such modelled sea surface circulation [10] has shown an areas. incoming flux and an outgoing one intensified along Directly quantifying coastal upwelling is extremely the coasts, with anticyclonic circulation during winter difficult, and observed time-series of the phenomenon and cyclonic circulation during summer (figure 1). do not exist [26]. However, useful proxies have been This pattern is confirmed by previous geostrophic developed by processing different types of data. One calculations, drift observations, and suggested from possibility is to estimate upwelling by looking at its surface thermal patterns observed from satellites [2, consequences on common locally-measured variables 16, 19]. such as sea surface temperature (SST) and the struc- High primary productivity levels in the gulf result ture of the ocean surface as derived from satellite from an efficient nutrient transport of waters from imagery [1, 12]. under a shallow pycnocline into the euphotic zone A second approach is to look at the upwelling forcing [28], favouring the presence and reproduction of mas- agents. Bakun [4] first described and computed up- sive populations of species like the Pacific sardine welling indices for the west coast of North America. (Sardinops sagax), one of Mexico’s most important From pressure grid data he estimated geostrophic fisheries, accounting for the largest catch and provid- winds, wind stress, and Ekman transport, and pre- ing many jobs. This nutrient enrichment is mainly sumed perpendicular offshore transport to be propor- driven by two mechanisms; a permanent, vertical tional to upwelling. Other Ekman transport-derived mixing system because of strong tidal currents indices have been computed using locally-measured around the region of the larger islands (Tiburo´n and winds by coastal stations [8] and by ships at sea [9]. Angel de la Guarda) [2] and coastal, wind-driven upwelling [22]. Upwelling indices have been used in studies of ocean circulation, ENSO and climate change, linkages be- Wind is likely to produce coastal upwelling through- tween environmental and biological variability and out the year. However, only in winter, on the conti- fisheries oceanography in several areas of the world nental side, has a persistent and effective pump of ocean, especially where important marine fisheries nutrient rich waters to the surface been recognized exist [6, 26]. [16, 19]. Summer upwelling is absent or less effective because of a deeper thermocline and weaker winds, a narrower shelf on the peninsular side, and because on 1.1. The Gulf of California the western side of the gulf winds and sea surface currents flow in the opposite direction. The Gulf of California (figure 1) is a long, narrow sea Despite its importance as a major environmental between the mainland of Mexico and the Baja Cali- factor affecting biological population dynamics, there 732 S.-E. LLUCH-COTA / Oceanologica Acta 23 (2000) 731–740 are no proxies of upwelling activity to allow the study data in time and space are available from research of its variability and relationship to marine popula- programs. Data for the gulf included in world-wide tions. In this contribution, an upwelling index for the coverage databases, such as the Comprehensive eastern coast of the central Gulf of California is Ocean–Atmosphere Data Set (COADS) [23] that computed and compared to pigment concentration summarizes over 108 surface meteorological observa- distribution derived from satellite data. Some prelimi- tions collected by ships of opportunity and other nary observations on the variability of the upwelling platforms [24], are sometimes enough to draw up an are also provided. idea of the seasonal behaviour, but are almost useless for building time-series. An additional problem is that synoptic estimates such as mean pressure fields 2. MATERIALS AND METHODS (such as those provided by the U.S. Navy Fleet Numerical Meteorology and Oceanography Center) Sparsity of adequate quality data is a major problem are commonly available in grid scales larger than when analyzing environmental properties and vari- convenient for the width of the gulf. Locally-mea- ability of the Gulf of California; monitoring is in sured wind data seems to be an alternative for com- early stages of development and only a few sparse puting an upwelling index for this region considering the topographically polarized wind circulation. How- ever, caution should be used when looking at precise short-term readings. Daily wind data generated at 1900 hours at the Empalme station of the National Meteorological Ser- vice (SMN) near Guaymas, Son., on the continental side of the central Gulf of California (figure 1), were digitized.