Physical Oceanography
II. Water Masses and Hydrography

The surface and intermediate depth water masses in the MBNMS are a mixture of Pacific Subarctic water having low salinity and cool temperatures and the warmer, saltier Pacific Equatorial water (Sverdrup et al. 1942). For example, water at 500 m at CalCOFI Station 3 in Monterey Bay (34.25 practical salinity units [psu], 6.3°C) is nearly a 50:50 mixture of those two water types. The proportion of the water types changes as does the strength of the northward flowing California Undercurrent (Wickham 1975). Nearshore surface temperatures vary from 8°C during winter and early spring to 17°C during fall. Nearshore surface salinities vary from 34.0 psu (practical salinity units) when upwelling is strong to 33.2 psu otherwise. Streams and rivers can have large local effects on salinity, but even during flood conditions the salinity of Monterey Bay surface waters does not fall below 31 psu (Figure 9; Broenkow and Smethie, 1978). Lynn's (1967) analysis of surface waters in the California Current 150 km offshore of Monterey Bay, showed from harmonic analysis that the seasonal variation of temperature and salinity were 12.2°C to 15.5°C and 33.1 to 33.3 psu. Both temperature and salinity maxima are reached typically in September-October, while minimum temperature occurred in February-March and minimum salinity in December-January. At a station 10 km south of Monterey Bay off Point Sur, temperature varied from 11.1°C in May to 13.8°C in November and salinity from 33.4 in January to 33.6 in July-August. Variance about the regression lines was about 1°C and 0.2 psu.

The vertical distributions of salinity, temperature, dissolved oxygen and inorganic nutrients were systematically characterized by the California Cooperative Fisheries Investigations (Bolin and collaborators 1964). Monthly or biweekly hydrographic stations were occupied at Hopkins Marine Station CalCOFI Station H3 about halfway between Point Pinos and Point Santa Cruz where the canyon depth is about 900 m. A profile typical of the offshore region of MBNMS (Figure 4) shows that salinity increases monotonically from about 33.5 psu at the surface to deep North Pacific values of 34.68 psu at depths greater than 2000 m. The near-surface halocline is accompanied by a similar thermocline. It is noted that in spring and summer, the mixed layer is often absent. Conditions similar to those offshore are found at the H3 entrance to Monterey Bay so that mid-Bay waters are only slightly altered by localized warming and nutrient assimilation (Skogsberg and Phelps 1946, Bolin and Abbott 1963, Broenkow and Smethie 1978). The oxygen minimum which is prevalent throughout the North Pacific is found near 600 m where oxygen concentrations are less than 0.5 ml/liter or 20 mmoles/kg and saturation levels are less than 10%.