Physical Oceanography
I. The California Current

The oceanography of the Monterey Bay National Marine Sanctuary (MBNMS), including Monterey Bay and the coastal area between the Gulf of the Farallones and Point Piedras Blancas, is closely tied to processes of the California Current. The California Current (Wooster and Reid 1963, Hickey 1979, Chelton 1984, Lynn and Simpson 1987) is an eastern boundary current that has been characterized generally as a broad, shallow, slow southward moving current exhibiting high spatial and temporal variability. The California Current is the eastward portion of the clockwise North Pacific Gyre and transports low salinity, cool water equatorward. Associated with the coastal surface flow is a poleward undercurrent, the California Undercurrent. Even though the California Current is one of the most-studied oceanographic features in the oceans, it is difficult to predict at any particular instant the location of its velocity core, its speed, or direction. Indeed, at various locations observers might characterize the current as south-flowing (as it often is in offshore regions), westward-flowing (as is frequently observed in a coastal jet near Point Reyes), or eastward-flowing (as found in the southern regions of such jets). At times, principally in winter, the nearshore current flows northward.

Lynn and Simpson (1987) have suggested that the California Current can be divided into three regions (based on the seasonal amplitude variation and standard deviation of dynamic height): an offshore oceanic regime, a coastal regime and an intervening transition zone. This transition zone lies approximately 200-300 km west of Point Sur, as shown by closely-packed dynamic topography isopleths (Figure 1; Lynn and Simpson 1987). Where Figure 1 illustrates an average of 28 year's data, maps for any particular time will show a more complex situation with closed circulation patterns caused by eddies (as in August 1959 Figure 2). Geostrophic speeds in the core of the California Current may approach 25 cm/s, but generally are 5 to 10 cm/s (0.1 to 0.2 knots; Wooster and Reid 1963). Infra-red AVHRR (Advanced Very High Resolution Radiometer) satellite images clearly show surface effects of such eddies and the presence of coastal jets (Figure 3). The core of the California Current lies in the salinity minimum about 300 km offshore of Point Sur, within the transition zone, and is not associated generally with a thermal gradient (Lynn et al. 1982). This makes location of the California Current difficult from infra-red imagery (Figure 3). The low salinity waters derive generally from the low salinities in the Gulf of Alaska and more locally from the Columbia River discharge and outflow from the Sacramento and San Joaquin Rivers through the mouth of San Francisco Bay.

The California Current is richly populated with semi-stationary jets and eddies. Satellite imagery (Figure 3 and Figure 8) has shown cold filaments on the order of 50 km wide to extend several hundred km offshore (Strub et al. 1991, Ramp et al. 1991a). The importance of these features, which represent the highly variable oceanographic "weather" of the California Current, lies in their offshore transport of cool, nutrient-rich upwelled water. This extends the effects of nearshore upwelling which is confined to a band about 50 km wide to several hundred km. Cross-shore velocities may reach 1 m/s which is an order of magnitude greater than characteristic speeds of the California Current core (Brink et al. 1991, Ramp et al. 1991). In what are called "squirts," the flow may be directed offshore, and where the "squirt" dissipates elongated "hammerhead" features evolve (Figure 3). Between mesoscale eddies, the flow is directed offshore north of cyclonic eddies and onshore south of them. A jet may be found off Point Sur that transports cool, upwelled waters offshore 100 km (Traganza et al. 1981). Hickey (1979) describes the "San Francisco Eddy" as a semi-permanent cyclonic eddy northwest of Monterey Bay, while other observations (Tracy 1990) describe anti-cyclonic eddies in this region. The 15-year average dynamic topography maps of Wylie (1966) show a cyclonic feature northwest of MB, and Lynn and Simpson's (1987) 28-year average maps indicate onshore geostrophic flow towards Monterey Bay. Broenkow (1982) made current meter measurements and estimated geostrophic flow in an anti-cyclonic eddy south-east of MB. Between that eddy and a cyclonic eddy just north of it strong onshore geostrophic flow was observed.