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Chapter 11: The Pacific Ocean
The southward flow of the coastal current in summer is a meandering jet with
persistent eddies and cool filaments that originate at prominent capes and may extend
200 km offshore. Some of these eddies, especially those at the Straits of Juan de Fuca,
at Hecate Bank, and at Cape Blanco and Cape Mendocino, often form cold filaments
that may extend several hundred kilometers offshore. These have a dipole termination,
representing the development of a pair of counterrotating eddies that entrain upwelled
water at the coast and advect it far offshore (Mooers and Robinson, 1984); they are
now very well observed in AVHRR images. The actual development of these features is
associated with variability of coastal winds.
Coastal winds are modified by the blocking of the zonal westerlies by the coastal
mountain chain, creating a wind regime that is far from uniform: a local maximum of
cyclonic curl is associated with the Southern California Bight, and a lobe of anticyclonic
curl frequently reaches the coast at Punta Baja, where longshore equatorward wind stress
is maximal. Off Oregon and Washington, wind-stress curl is variable, with frequent brief
episodes of anticyclonic curl related to storm tracks (Bakun and Nelson, 1991) that are
reflected in event-scale circulation changes. Here, during the summer coastal upwelling
season, a front may occur 5–15 km offshore that is forced by cross-shelf circulation over
the very narrow continental shelf (Peterson et al., 1979).
Upwelling episodes generally last for periods of 1–3 weeks, including relaxation periods
of 2 or 3 days, so that individual cold filaments may transport water from more than a
single upwelling event (Lagerloef, 1992; Traganza et al., 1981). These features are forced
by along-shelf wind stress and curl, barotropic and baroclinic instabilities, and coastline
irregularities (Hickey, 1998). At about 1000 km from the coast, mesoscale eddying gives
way to less complex and slower flow to the south. This transition is often marked by
a salinity front that is continuous with the oceanic Subarctic Front and is often termed
the California Front: it forms a useful marker for the seaward boundary of this province.
The regional characeristics of upwelling processes will be dealt described later, in the
section dealing with the ecological response. One of the strengths of the 50-year CalCOFI
series of observations is that it affords a unique opportunity of analyzing the relationship
between processes over the shelf and in the Pacific Ocean as a whole. The most basic
observation is perhaps that the overall strength of the California Current responds to the
ENSO cycle. As Hickey (1998) comments, during El Niño events, flow in the California
Current is anomalously weak while the Undercurrent is unusually strong; the upper 500 m
of the water column is warmer than mean values. These effects extend to at least 200–
300 km offshore. A set of standard observations from 1950 to 1980 show how changes
in southward flow and temperature and salinity at 10 m are precisely coordinated until
1977, when the relationship changes; reduced southward flow is thereafter associated
with increased, not reduced, temperature, although the relationship with salinity and
zooplankton biomass is unchanged (Chelton et al., 1982).
This simple observation serves to introduce the problem of the large-scale climate
regime shift recently discussed by Bograd and Lynn (2003), who show that the period
1944–1976 was characterized by a weak atmospheric Aleutian Low so that surface Ekman
processes were associated with relatively cool SSTs around the entire rim of the NE Pacific
during this period. In 1976, the Aleutian Low strengthened very significantly, ushering
in a period of warmer SSTs and a relatively greater frequency of El Niño events. Off
California, after 1976, the upper 200–400 m of the water column warmed by as much as
1
C, especially nearshore, perhaps related to increased inshore penetration of subtropical
gyre water. A decrease in surface salinity was associated with increased stratification,
deeper density surfaces, and greater stability, so rendering upwelling less effective in the
induction of phytoplankton growth. These effects were associated with shifts in position
and strength of the flow structure; the California Current shifted offshore, and the eddy
behind Point Conception was strongly modified. Such conditions persisted until at least
Chapter 11: The Pacific Ocean
The southward flow of the coastal current in summer is a meandering jet with
persistent eddies and cool filaments that originate at prominent capes and may extend
200 km offshore. Some of these eddies, especially those at the Straits of Juan de Fuca,
at Hecate Bank, and at Cape Blanco and Cape Mendocino, often form cold filaments
that may extend several hundred kilometers offshore. These have a dipole termination,
representing the development of a pair of counterrotating eddies that entrain upwelled
water at the coast and advect it far offshore (Mooers and Robinson, 1984); they are
now very well observed in AVHRR images. The actual development of these features is
associated with variability of coastal winds.
Coastal winds are modified by the blocking of the zonal westerlies by the coastal
mountain chain, creating a wind regime that is far from uniform: a local maximum of
cyclonic curl is associated with the Southern California Bight, and a lobe of anticyclonic
curl frequently reaches the coast at Punta Baja, where longshore equatorward wind stress
is maximal. Off Oregon and Washington, wind-stress curl is variable, with frequent brief
episodes of anticyclonic curl related to storm tracks (Bakun and Nelson, 1991) that are
reflected in event-scale circulation changes. Here, during the summer coastal upwelling
season, a front may occur 5–15 km offshore that is forced by cross-shelf circulation over
the very narrow continental shelf (Peterson et al., 1979).
Upwelling episodes generally last for periods of 1–3 weeks, including relaxation periods
of 2 or 3 days, so that individual cold filaments may transport water from more than a
single upwelling event (Lagerloef, 1992; Traganza et al., 1981). These features are forced
by along-shelf wind stress and curl, barotropic and baroclinic instabilities, and coastline
irregularities (Hickey, 1998). At about 1000 km from the coast, mesoscale eddying gives
way to less complex and slower flow to the south. This transition is often marked by
a salinity front that is continuous with the oceanic Subarctic Front and is often termed
the California Front: it forms a useful marker for the seaward boundary of this province.
The regional characeristics of upwelling processes will be dealt described later, in the
section dealing with the ecological response. One of the strengths of the 50-year CalCOFI
series of observations is that it affords a unique opportunity of analyzing the relationship
between processes over the shelf and in the Pacific Ocean as a whole. The most basic
observation is perhaps that the overall strength of the California Current responds to the
ENSO cycle. As Hickey (1998) comments, during El Niño events, flow in the California
Current is anomalously weak while the Undercurrent is unusually strong; the upper 500 m
of the water column is warmer than mean values. These effects extend to at least 200–
300 km offshore. A set of standard observations from 1950 to 1980 show how changes
in southward flow and temperature and salinity at 10 m are precisely coordinated until
1977, when the relationship changes; reduced southward flow is thereafter associated
with increased, not reduced, temperature, although the relationship with salinity and
zooplankton biomass is unchanged (Chelton et al., 1982).
This simple observation serves to introduce the problem of the large-scale climate
regime shift recently discussed by Bograd and Lynn (2003), who show that the period
1944–1976 was characterized by a weak atmospheric Aleutian Low so that surface Ekman
processes were associated with relatively cool SSTs around the entire rim of the NE Pacific
during this period. In 1976, the Aleutian Low strengthened very significantly, ushering
in a period of warmer SSTs and a relatively greater frequency of El Niño events. Off
California, after 1976, the upper 200–400 m of the water column warmed by as much as
1
C, especially nearshore, perhaps related to increased inshore penetration of subtropical
gyre water. A decrease in surface salinity was associated with increased stratification,
deeper density surfaces, and greater stability, so rendering upwelling less effective in the
induction of phytoplankton growth. These effects were associated with shifts in position
and strength of the flow structure; the California Current shifted offshore, and the eddy
behind Point Conception was strongly modified. Such conditions persisted until at least