Pacific Coastal Biome
397
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelf is generally narrow, with the 200-m isobath occurring as little as 10 km
from the shore in southern California (or even closer, where Scripps Canyon runs into the
beach) to a distance of about 75 km off Oregon and Washington. A continental borderland
with deep (approx. 200 m) basins, shallow banks, and islands occupies the bight south of
Point Conception (345
N) that includes the Santa Barbara Basin. Tidal studies are reviewed
by Hickey (1998), and these suggest that here the stratified water column over the shelf is
rarely interrupted by tidally mixed regions; tidal currents do not appear to dominate over
current variance associated with alongshore flow of the coastal currents.
Defining Characteristics of Regional Oceanography
The California Current is the most intensively investigated of the eastern boundary
currents, because of the marathon studies of the California Cooperative Fisheries Investigations (CalCOFI) jointly by federal, state, and university research groups. Much of
what follows is drawn from reports, atlases, and research papers arising from CalCOFI;
we are especially fortunate that the CalCOFI investigations recently celebrated their 50th
anniversary of continuous study: a rich harvest of retrospective studies is therefore at
our disposal. Many comments were made at this time concerning the importance of the
CalCOFI series, and the information it presents about the manner in which long-term
environmental changes are translated into ecological change in this region. Really, it does
more than that: it should convince us that similar relationships probably occur in all
seas, at all latitudes, but are presently concealed from us for lack of serial observations.
It would be foolish to hope that similar studies can be undertaken comprehensively
elsewhere, but CalCOFI enables us to lean more confidently on causal relationships that
we infer from serial satellite observations of sea change and chlorophyll.
The California Current, which has been recently reviewed by Hickey (1998), takes
its source in the divergence of the North Pacific Current where this approaches the
American continent, within what Favorite et al. (1976) term the “dilution zone,” where
precipitation greatly exceeds evaporation. This eastward flow across the ocean diverges
northward into the Alaska Current, and southward into the California Current where it
forms an equatorward flow of cool, subarctic water. The characteristics of the present
province derive from the conjunction between the southward geostrophic current, the
coastal boundary, and an alternating wind regime—equatorward upwelling winds in
summer and poleward downwelling winds in winter. Conditions here are much modified
during El Niño events.
Equatorward, upwelling winds are maximal above the main jet of the California
Current that lies 200–400 km offshore, where consequent wind stress is greatest. This zone
coincides with the line of zero wind-stress curl, which is anticyclonic (convergent) to the
west of this line and cyclonic (divergent) to the east (Bakun and Nelson, 1991). A narrow
(10–40 km), jetlike poleward undercurrent underlies the California Current at all seasons
over the continental slope and is continuous from Baja California to Vancouver Island.
Southward flow of the California Current past the prominence of Point Conception
(35
N) creates a cyclonic eddy to the south and thus northward flow at the coast in the
bight south of the cape. Upwelling wind stress creates an upward tilt of isopycnals toward
the coast and an anomaly of the density surfaces that progressively moves offshore as
summer advances. When the eddy is most strongly developed, coastal flow may escape past
the cape and continue to the north as a short-lived California Coastal Countercurrent. In
winter, however, from Point Conception northward to Vancouver Island, some poleward
flow regularly occurs at the coast that may represent the surfacing of the undercurrent
and generally goes by the name of the Davidson Current. In exceptional winters, this
may originate as far south as northern Baja California (Wyllie, 1966).
397
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The continental shelf is generally narrow, with the 200-m isobath occurring as little as 10 km
from the shore in southern California (or even closer, where Scripps Canyon runs into the
beach) to a distance of about 75 km off Oregon and Washington. A continental borderland
with deep (approx. 200 m) basins, shallow banks, and islands occupies the bight south of
Point Conception (345
N) that includes the Santa Barbara Basin. Tidal studies are reviewed
by Hickey (1998), and these suggest that here the stratified water column over the shelf is
rarely interrupted by tidally mixed regions; tidal currents do not appear to dominate over
current variance associated with alongshore flow of the coastal currents.
Defining Characteristics of Regional Oceanography
The California Current is the most intensively investigated of the eastern boundary
currents, because of the marathon studies of the California Cooperative Fisheries Investigations (CalCOFI) jointly by federal, state, and university research groups. Much of
what follows is drawn from reports, atlases, and research papers arising from CalCOFI;
we are especially fortunate that the CalCOFI investigations recently celebrated their 50th
anniversary of continuous study: a rich harvest of retrospective studies is therefore at
our disposal. Many comments were made at this time concerning the importance of the
CalCOFI series, and the information it presents about the manner in which long-term
environmental changes are translated into ecological change in this region. Really, it does
more than that: it should convince us that similar relationships probably occur in all
seas, at all latitudes, but are presently concealed from us for lack of serial observations.
It would be foolish to hope that similar studies can be undertaken comprehensively
elsewhere, but CalCOFI enables us to lean more confidently on causal relationships that
we infer from serial satellite observations of sea change and chlorophyll.
The California Current, which has been recently reviewed by Hickey (1998), takes
its source in the divergence of the North Pacific Current where this approaches the
American continent, within what Favorite et al. (1976) term the “dilution zone,” where
precipitation greatly exceeds evaporation. This eastward flow across the ocean diverges
northward into the Alaska Current, and southward into the California Current where it
forms an equatorward flow of cool, subarctic water. The characteristics of the present
province derive from the conjunction between the southward geostrophic current, the
coastal boundary, and an alternating wind regime—equatorward upwelling winds in
summer and poleward downwelling winds in winter. Conditions here are much modified
during El Niño events.
Equatorward, upwelling winds are maximal above the main jet of the California
Current that lies 200–400 km offshore, where consequent wind stress is greatest. This zone
coincides with the line of zero wind-stress curl, which is anticyclonic (convergent) to the
west of this line and cyclonic (divergent) to the east (Bakun and Nelson, 1991). A narrow
(10–40 km), jetlike poleward undercurrent underlies the California Current at all seasons
over the continental slope and is continuous from Baja California to Vancouver Island.
Southward flow of the California Current past the prominence of Point Conception
(35
N) creates a cyclonic eddy to the south and thus northward flow at the coast in the
bight south of the cape. Upwelling wind stress creates an upward tilt of isopycnals toward
the coast and an anomaly of the density surfaces that progressively moves offshore as
summer advances. When the eddy is most strongly developed, coastal flow may escape past
the cape and continue to the north as a short-lived California Coastal Countercurrent. In
winter, however, from Point Conception northward to Vancouver Island, some poleward
flow regularly occurs at the coast that may represent the surfacing of the undercurrent
and generally goes by the name of the Davidson Current. In exceptional winters, this
may originate as far south as northern Baja California (Wyllie, 1966).