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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

Hydrographic Variability along the
North Eastern Mexican Pacific

Reginaldo Durazo1 Luis Soto1,2, Ana M. Ramirez1, T.R. Baumgartner3

1 UABC-Facultad de Ciencias Marinas, Apdo. Postal
453, Ensenada, B.C., México,

2 Universidad del Bío Bío, Apdo. Postal 4081112,
3 CICESE, Apdo. Postal 2832, Ensenada, B.C., México,

Systematic oceanographic observations off Baja California, Mexico (22 to 32 °N), were conducted by the California Cooperative Fisheries Investigations (CalCOFI) program in the period 1950-1978. During most of this period, quarterly cruises were carried out to study hydrographic and biological variability. Only sporadic, coastal observations exist for the period 1979-1996. In 1997, several institutions in Mexico (IMECOCAL program) restarted the quarterly sampling of physical and other parameters along a subset of coastal stations (80) of the original CalCOFI station plan. The grid was chosen to sample the ocean within 200 km of the coast along CalCOFI lines 100 to 137 (24 to 31 °N) and to cover the area of offshore flows often observed in the region of Punta Eugenia (27 °N) with lines 117 and 120 extending approximately 350 km offshore (figure 1). Off Baja California, three water masses comingle at the surface, each distinguished by characteristic temperature, salinity, dissolved oxygen and nutrient content. These are the relatively cold, fresh and highly oxygenated Subarctic Water (SAW) which forms the core of the California Current, the relatively high salinity and high temperature Subtropical Surface Water (StSW) and the also warm, less saline Equatorial Pacific Water (EPW), also known as Tropical Surface Water (TSW).

Fig. 1. CalCOFI station plan. Shaded area represents stations occupied quaterly by the IMECOCAL program.

At the time of writing, a total of 18 cruises has been conducted. Serendipitously, the cruises in this series coincided with the mature (warm and saline) El Niño in 1997-1998, its transition to (colder and fresher) La Niña in late 1998 until 2000 and the later return to normal conditions. The results presented here give emphasis to the description of the physical conditions observed during the period 1997-2002 in relation to the climatology for the period 1950-1978.

The first two cruises of the series, 9710 and 9801 (year-month) coincided with the mature phase of El Niño. During this period, large positive subsurface temperature and salinity anomalies of ~9 °C and 0.8 PSU respectively, were observed near the coast. The maximum anomalies observed were shallow (~ 50 m), although positive anomalies spread up to 200 km offshore and up to 600 m depth suggesting larger volumes of warmer and saline waters entered the region during the mature phase of El Niño at the expense of a reduction in volume of subarctic waters. Warmer and more saline waters near the surface were identified as StSW while waters at depth were Equatorial Subsurface Waters (ESsW), the California Undercurrent. The presence of StSW near the coast of Baja California is consistent with an anomalous intensification of cyclonic flow of the subtropical gyre over the southern California Current System in response to anomalous cyclonic winds over the eastern Pacific during El Niño 1997-1998. Surface and deep poleward coastal flows as well as lack of meandering structures were clearly identified in the geostrophic fields. Anomalies during La Niña from late 1998 through 2000 (~ -2 °C) were not as large as for El Niño. Furthermore, flows were predominantly equatorward with an increased mesoscale activity, consistent with the development of strong cyclonic winds over the Northeast Pacific (figure 2). Normal conditions were reached by late 2000 and continued until early 2002.

Fig. 2 Dynamic height at the sea surface relative to 500 dbar for January 1998 and 1999. Note weak dynamic height gradients and poleward flows during 9801 compared to equatorward mean flow and mesoscale features during 9901.

AVHRR weekly images from the period 1981-2002 were examined to analyze SST variability along the same region. Mean temperatures exhibited an equatorward gradient and relative minimum temperatures near the coast, mostly associated with coastal upwelling. Maximum variability was observed south of Punta Eugenia (27 °N). Energy on the annual scale was found to be an order of magnitude larger than on the semiannual and up to two orders of magnitude larger than interannual. Imagery showed four warm (El Niño) and two cold (La Niña) events but it was observed that anomalies developed spatially differently among events. Maximum SST anomalies of ~6 °C were observed during 1997-1998. In order to investigate the importance of regime shifts on computing anomalies, a new climatology is presented based upon the CalCOFI (1950-1978) and IMECOCAL (1997-2002) data sets.

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