Winter water masses and nutrients in the northern Gulf of California

Abstract

Hydrographic data (salinity, temperature, O2, PO4, NO3 and SiO2) 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 30.5°N, 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 ∼15°C (±0.4°C), 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 m 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 m. In the upper ∼120 m, intermediate salinity (35.0 to 35.3) and nutrient concentrations (phosphate, 1.5–2.0 μM; nitrate, 12–16 μM; and silicate, 25–32 μM) identify the Gulf of California Water this winter. In the deeper layers, low salinity (<35.0) and high nutrients (phosphate, >2.2 μM; nitrate, >22 μM; and silicate, >35 μM) 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 defined 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 El Niño‐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.