Physical Oceanography
I. The California Current
Moss Landing Marine Laboratories
P.O. Box 450 Moss Landing, CA 95039
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.
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