Short communication Towards the numerical simulation of the summer circulation in Todos Santos Bay, Ensenada, B.C. Mexico E. Mateos, S.G. Marinone * , A. Parés-Sierra Department of Physical Oceanography, CICESE, Km 107 Carretera Tijuana-Ensenada, B.C., 22800 Ensenada, Mexico article info Article history: Received 19 May 2008 Received in revised form 29 October 2008 Accepted 6 November 2008 Available online 14 November 2008 Keywords: Todos Santos Bay ROMS model Circulation abstract The general circulation of Todos Santos Bay was studied using the ROMS numerical model for the sum- mer season. The model was forced with the California Current System and by synoptic winds, which are mainly towards the equator. The circulation is characterized by two systems: one at the exterior with a strong southward flow that enters to the bay but limited by the 35 m isobath and the other at the rest of the bay. The circulation in the interior oscillates between two spatial structures: one in which the general circulation is anticyclonic for two–three days overall the bay producing a large eddy and a small cyclonic eddy in front of Ensenada’s port, then the anticyclonic eddy evolves to produce the second spatial struc- ture by splitting into two counter rotating eddies making the original anticyclonic one to be limited to the northern side of the bay and last three–four days and the small cyclonic eddy reverses its circulation. Such conditions are a consequence of the transport oscillations of the southward flow on the northwest- ern boundary and the mode with two eddies appears when the associated inflow is more intense. The transition between these two modes takes place in only one–two days. The time average circulation was also analyzed and the circulation with the two eddies dominates as it is of stronger currents and last longer. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Todos Santos Bay (TSB) is a small bay (180 km 2 ) in the Baja California west coast, approximately 100 km south of the Mexi- can–U.S. border (Fig. 1). It is connected to the Pacific Ocean through the lines between Todos Santos Island (TSI) and San Mig- uel Point (SMP), which has a longitude of 10 km, and TSI and Punta Banda (PB), which is approximately 5 km. Within these lines and the coast, the basin has a depth less than 50 m in about 80% of the area; the rest corresponds to a submarine canyon located be- tween PB and TSI at the southwest of the bay. In the last few years, the development of the city of Ensenada and the use of the bay has increased in many areas, such as tour- ism, port activities (e.g. navigational channel dredging and its asso- ciated discharge of the material into the bay), marine aquaculture, and pollution due to urban discharges. These conditions have raised many questions related to the management of the bay and consequently, there is now pressure to know the bay’s circulation. As we will show below, small changes in the surrounding Cali- fornia Current System (CCS) produces important changes in the bay’s circulation. Following Lynn and Simpson (1987), the CCS is formed by: the equatorward California Current (CC), the poleward inshore countercurrent (IC), and the California Undercurrent (CU) which also flows to the north as a core of about 20 km wide at a depth of about 150 m attached to the continental shelf break. The two latter currents (IC and CU) are not always present. For example, a geostrophic study (Lynn and Simpson, 1987; Strub and James, 2000) reveals a dominant surface flow of 10 to 12 cm s 1 near the coast towards the equator. Also, these currents need the b-effect and an along the coast wind forcing in order to be generated according to numerical studies of Batteen (1997). Within the TSB the synoptic northwestern winds (summer con- ditions) are dominant and its variability is due to the high pressure system variations centered to the west of California (Pavía and Reyes, 1983; Alvarez, 1977). There is also a significant sea level breeze component (Reyes and Parés, 1983). Hydrographic data show that a thermocline develops during summer (Morales, 1977) and surface anticyclonic and cyclonic ed- dies in the northern and southern areas of the bay, respectively, have been inferred from these data (Argote et al., 1975). Direct cur- rent observations from few drifters (Alvarez et al., 1988; Durazo and Alvarez, 1988) showed tracks (with associated velocities of 15 cm s 1 ) during the months of March through August along the coast in the northern and southern areas. These tracks con- verge just north of the mouth of Punta Banda Estuary and then de- part from the coast probably towards the eddies referred by Argote et al. (1975). Direct current measurements are also scarce. Fig. 1b 1463-5003/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ocemod.2008.11.002 * Corresponding author. Tel.: +52 646 1750500. E-mail address: marinone@cicese.mx (S.G. Marinone). Ocean Modelling 27 (2009) 107–112 Contents lists available at ScienceDirect Ocean Modelling journal homepage: www.elsevier.com/locate/ocemod