400
Chapter 11: The Pacific Ocean
Point Conception to Cape Mendocino (33–41
N ): Upwelling is strongest here but primary production is markedly seasonal. Satellite images show persistent offshore
meanders, shed eddies (anticyclonic to the north and cyclonic to the south), and
offshore cool filaments entraining coastally upwelled water. These most frequently
occur in summer and south of Cape Mendocino, Point Arena, and Point Sur.
Upwelling also occurs at the shallow banks off Point Reyes. Off Monterey Bay
(36
N), Wilkerson et al. (2000) showed clearly the formation of a plume of upwelled
water and the active utilization of NO 3 within 20 km of the shore, which was transformed into a plume of NO 3 -deplete water 90 km seaward. Above a threshold of
10–12 M NO 3 , the larger (>5 − m) fraction of the autotrophs dominated as they
did at higher biomass concentrations, supporting the idea that to reach maximal
biomass, the contribution of larger cells is essential.
Southern California Bight (32–33
N ): The upwelling plume from Point Conception is
frequently observed in summer to pass to the south around the outer limb of the
eddy that occupies this bight. This plume has consistently lower surface chlorophyll
values than those north of Point Conception or off Baja California. Off Southern
California, the winter wind regime is established only relatively briefly although,
because the bight is wholly occupied by a quasipermanent cyclonic eddy, poleward
flow along the coast is more continuous here than elsewhere. Upwelling occurs as
small coastal cells in summer (Point Dume and Del Mar) and at offshore islands
and banks during all seasons. The inshore flow around the southern limb of the
eddy may be a prominent chlorophyll feature.
Baja California (22–31
N ): The coastal wind regime is weaker but apparently favorable
for upwelling almost year-round, though some seasonality in upwelling is evident
in the chlorophyll field. Upwelling cells south of prominent capes are persistent:
Cape Colonet, Punta Baja, Cape San Quintin, Punta Eugenia, Punta Abreojos, and
Cape Falso all may generate such cells. The seasonal surface pigment fields suggest
that jets and filaments of upwelled water pass farther offshore from late summer
(August) through early winter (November) and that south of Cape San Lazaro
coastal upwelling ceases in autumn and winter (September–January).
Because of the large number of published studies of the California Current, in reviewing
this province it is easier to concentrate on the individual processes rather than to see the
whole. For instance, it is easy to lose sight of the fact that there is a seasonal cycle in
the depth of the pycnocline that obtains over the whole area of the province (summer,
20–25 m; winter, ∼75 m). At all seasons, if one ignores the effects of mesoscale features,
the thermocline slopes downward to the west, offshore, as it must. In upwelling cells, the
density profile may be relatively featureless, but wherever a significant mixed layer exists,
a DCM occurs on the density gradient, usually with the depth of primary production a
few meters shoaler. The offshore region has a seasonal cycle typical of subtropical oceans:
chlorophyll accumulation begins as soon as the mixed layer begins to shoal in spring
to peak in May–June, closely matching primary production rate. Analysis of chlorophyll
fields, integrated for the entire province (see later discussion), show that this process—
not summer upwelling at the coast of California—dominates the seasonal cycle of this
province. This observation recalls earlier suggestions that between-year variability in
biological properties was forced primarily by changes in the advection of nutrients from
the source of the California Current rather than by variation in the nutrients brought to
the surface by coastal upwelling (Chelton, 1982).
Although coastal upwelling cells—and other places where nutrients are brought into
the euphotic zone intermittently—are generally thought of as diatom-dominated, recent
studies have shown that growth of phycoerythrin-rich cyanobacteria may also be enhanced
in such places. Collier and Palenik (2003) have shown that in the California Current,
Synechococcus takes higher abundance on the coastal side of the offshore jet than beyond
Chapter 11: The Pacific Ocean
Point Conception to Cape Mendocino (33–41
N ): Upwelling is strongest here but primary production is markedly seasonal. Satellite images show persistent offshore
meanders, shed eddies (anticyclonic to the north and cyclonic to the south), and
offshore cool filaments entraining coastally upwelled water. These most frequently
occur in summer and south of Cape Mendocino, Point Arena, and Point Sur.
Upwelling also occurs at the shallow banks off Point Reyes. Off Monterey Bay
(36
N), Wilkerson et al. (2000) showed clearly the formation of a plume of upwelled
water and the active utilization of NO 3 within 20 km of the shore, which was transformed into a plume of NO 3 -deplete water 90 km seaward. Above a threshold of
10–12 M NO 3 , the larger (>5 − m) fraction of the autotrophs dominated as they
did at higher biomass concentrations, supporting the idea that to reach maximal
biomass, the contribution of larger cells is essential.
Southern California Bight (32–33
N ): The upwelling plume from Point Conception is
frequently observed in summer to pass to the south around the outer limb of the
eddy that occupies this bight. This plume has consistently lower surface chlorophyll
values than those north of Point Conception or off Baja California. Off Southern
California, the winter wind regime is established only relatively briefly although,
because the bight is wholly occupied by a quasipermanent cyclonic eddy, poleward
flow along the coast is more continuous here than elsewhere. Upwelling occurs as
small coastal cells in summer (Point Dume and Del Mar) and at offshore islands
and banks during all seasons. The inshore flow around the southern limb of the
eddy may be a prominent chlorophyll feature.
Baja California (22–31
N ): The coastal wind regime is weaker but apparently favorable
for upwelling almost year-round, though some seasonality in upwelling is evident
in the chlorophyll field. Upwelling cells south of prominent capes are persistent:
Cape Colonet, Punta Baja, Cape San Quintin, Punta Eugenia, Punta Abreojos, and
Cape Falso all may generate such cells. The seasonal surface pigment fields suggest
that jets and filaments of upwelled water pass farther offshore from late summer
(August) through early winter (November) and that south of Cape San Lazaro
coastal upwelling ceases in autumn and winter (September–January).
Because of the large number of published studies of the California Current, in reviewing
this province it is easier to concentrate on the individual processes rather than to see the
whole. For instance, it is easy to lose sight of the fact that there is a seasonal cycle in
the depth of the pycnocline that obtains over the whole area of the province (summer,
20–25 m; winter, ∼75 m). At all seasons, if one ignores the effects of mesoscale features,
the thermocline slopes downward to the west, offshore, as it must. In upwelling cells, the
density profile may be relatively featureless, but wherever a significant mixed layer exists,
a DCM occurs on the density gradient, usually with the depth of primary production a
few meters shoaler. The offshore region has a seasonal cycle typical of subtropical oceans:
chlorophyll accumulation begins as soon as the mixed layer begins to shoal in spring
to peak in May–June, closely matching primary production rate. Analysis of chlorophyll
fields, integrated for the entire province (see later discussion), show that this process—
not summer upwelling at the coast of California—dominates the seasonal cycle of this
province. This observation recalls earlier suggestions that between-year variability in
biological properties was forced primarily by changes in the advection of nutrients from
the source of the California Current rather than by variation in the nutrients brought to
the surface by coastal upwelling (Chelton, 1982).
Although coastal upwelling cells—and other places where nutrients are brought into
the euphotic zone intermittently—are generally thought of as diatom-dominated, recent
studies have shown that growth of phycoerythrin-rich cyanobacteria may also be enhanced
in such places. Collier and Palenik (2003) have shown that in the California Current,
Synechococcus takes higher abundance on the coastal side of the offshore jet than beyond