489 Journal of Oceanography, Vol. 59, pp. 489 to 502, 2003 Keywords: ⋅ Coastal upwelling, ⋅ Baja California peninsula, ⋅ wind stress, ⋅ bottom topography, ⋅ tidal mixing. * Corresponding author. E-mail: ozaytsev@ipn.mx Copyright © The Oceanographic Society of Japan. Coastal Upwelling Activity on the Pacific Shelf of the Baja California Peninsula OLEG ZAYTSEV 1 *, RAFAEL CERVANTES-DUARTE 1 , ORZO MONTANTE 1 and ARTEMIO GALLEGOS-GARCIA 2 1 Interdisciplinary Marine Sciences Centre, Mexican National Polytechnic Institute, La Paz, B.C.S., 23096, Mexico 2 Institute of Marine Sciences and Limnology, Mexican National Autonomous University, D.F., Mexico (Received 31 May 2002; in revised form 7 December 2002; accepted 13 December 2002) High primary productivity on the Pacific coast of the Baja California Peninsula is usually related to coastal upwelling activity that injects nutrients into the euphotic zone in response to prevailing longshore winds (from the northwest to north). The upwelling process has maximum intensity from April to June, with the coastal upwelling index varying from 50 to 300 m 3 /s per 100 m of coastline. Along the entire coast of the peninsula, the upwelling intensity changes in accordance with local wind conditions and bottom topography. Spatial variability can also be modulated by the influence of mesoscale meanders of the California Current. We have identified the seasonal and synoptic variability of upwelling signatures on the Baja California shelf, using averaged monthly and weekly sea surface temperature (SST) distributions ob- tained from remote sensing imagery from the Advanced Very High Resolution Radi- ometer in the period from 1996 to 2001. Analysis of SST distribution and direct ex- perimental data on temperature and nutrient concentration shows that the areas with the coldest SST anomalies were closely related to the bottom slope, shelf width, and coastline orientation relating to wind direction. We also assume that the nutrient trans- port into the coastal lagoons may be forced by the coupling of coastal upwelling and tidal pumping of surface waters into the lagoon system. the Magdalena-Almejas lagoon system. Almost all of these coastal water bodies have high primary productiv- ity, and have an important influence upon fishery re- sources as excellent larval recruitment zones. It is impor- tant to understand the physical mechanisms that form the favorable environment of lagoonal community dynam- ics. It is common knowledge that the high productivity on the shelf of western North America is closely related to coastal upwelling activity. This highly fluctuating proc- ess is developed when the equatorward wind, which forces upwelling, blows persistently along the coast, forming the Ekman average offshore transport (based on wind stress and Coriolis force). Surface water, which moves away from the western continental coast, is replaced by cooler and saltier water, which comes from depths to 100 m, bringing nutrients to the surface layer. Upwelling affects all physical parameters of seawater, including the thermohaline structure and three-dimensional circulation, forming the environmental conditions favorable for eco- system development. Although vertically integrated 1. Introduction and Background The western shelf of the Baja California Peninsula is part of the California Current System region, one of the world’s most productive areas. The coast is situated close to the eastern transitional oceanic zone of the North- ern Pacific. The interchange of waters between the coastal zone and the open sea is a crucial area of research for determining abiotic conditions of coastal ecosystem evo- lution. The western shelf is part of the active geological features of the peninsula, so it is actually formed by in- tense bathymetric gradients. As a result, the shelf width varies from a narrow coastal shelf with steep gradient to gentle slopes of substantial width. The most steeply in- clined slopes are presented in the northern portion of the Peninsula. There are many coastal lagoons along the coast of the peninsula (Fig. 1), among them being Guerrero Negro, San Ignacio, Ojo de Liebre, and the biggest one—