Hydrography at the North Mouth of La Paz Bay, Baja California Sur, Mexico

To describe the hydrographical variability at the North Mouth of La Paz Bay, its main point of communication with the Gulf of California, CTD data were analyzed from four sampling surveys conducted during an annual cycle (May, July and October 2001, and February 2002). The results showed an absence of the mixed layer and intense temperature and salinity stratification in spring and summer. In autumn a 30-m mixed layer was registered, with a temperature of 28.5ºC, while in winter an unusual 100-m mixed layer was detected, with a temperature of 17.9ºC. Particular noteworthy is the similarity of the structure between the surface and subsurface layers of the bay and gulf. The most significant difference was the presence of Pacific Intermediate Water (PIW) in the gulf, which does not intrude into the bay due to the barrier created by the sill at the mouth. The T-S diagrams revealed the presence of Gulf of California Water (GCW), Subtropical Subsurface Water (SSW) and PIW in May. In July, however, four water masses were identified: GCW, Equatorial Surface Water (ESW), SSW and PIW. In October, a significant amount of ESW and a considerable decrease in GCW were detected, but in February only GCW and SSW were found. This study shows important hydrographic seasonal variability in the area of the North Mouth, with marked differences between the bay and the gulf, contributing to the understanding of the regional oceanography.