Hydrographic data (salinity, temperature, O9•, PO4, NOs and SiO9•) collected in the northern Gulf of California between February 27 and March 3, 1988, reveal that bottom water formation took place that winter. North of 30.5øN, salinity increased with depth from •35.30 practical salinity units (psu) at the surface to 35.57 psu at the bottom of the 200-m deep Wagner Basin; below-•25 m, temperature was almost homogeneous, at •15øC (•:0.4øC), with some inversions. The TS diagrams and the distribution of dissolved oxygen and nutrients suggest that the most likely origin of this bottom water is the shallow coastal region (•40 ra deep) of the northern Gulf. Elsewhere, the characteristic Gulf of California decrease of salinity with depth was found, from over 35.25 in the surface to 34.90 at 200 ra. In the upper •120 ra, intermediate salinity (35.0 to 35.3) and nutrient concentrations (phosphate, 1.5-2.0 pM; nitrate, 12-16 pM; and silicate, 25-32 pM) identify the Gulf of California Water this winter. In the deeper layers, low salinity (•35.0) and high nutrients (phosphate, •2.2 pM; nitrate, •22 pM; and silicate, •35 pM) indicate the presence of oceanic water from Guaymas Basin, probably Subsurface Subtropical Water. The boundary between the two regimes was •18 km wide, with clearly defmed bottom fronts and intrusions at all depths. Of the several late-winter hydrographic data sets available, only that from March 1973 presents a similar distribution of high-salinity bottom water. Therefore interannual variability (not necessarily E1 Nifio-Southern Oscillation (ENSO) related) can have a profound effect on vertical convection, which can occur both in ENSO and non-ENSO years. An important and as yet unexplained difference between the two data sets is that there was more Gulf of California Water in the northern Gulf of California in 1973 than in 1988. Introduction One of the most striking characteristics of the Gulf of California (GC, Figure 1) is its extremely high productivity [Gilbert and Allen, 1943; Alvarez-Borrego and Lara-Lara, 1991], in contrast to other enclosed seas such as the Mediterranean and the Red Sea. This is because the euphotic zone is constantly being supplied with nutrients by the thermohaline circulation, by tidal mixing, and by coastal upwelling [Alvarez-Borrego and Lara-Lara, 1991]. The distribution of nutrients in the Gulf of California is characterized by a monotonic increase with depth, the surface waters being almost depleted by biological activity, except in the area around the large Copyright 1995 by the American Geophysical Union. Paper number 95JC00138. 0148-0227/95/95JC-00138505.00 islands, presumably due to vertical mixing induced by the strong tidal currents [Alvarez-Borrego et al., 1978]. There is some evidence that the thermohaline circulation is the reverse of that in the Mediterranean [Bray, 1988a, Badan-Dangon et al., 1991], with an input at depth of cold, less salty, nutrient-rich oceanic water. This is possible because, despite strong evaporation (-•1 m/yr), the Gulf of California gain...
