406
Chapter 11: The Pacific Ocean
It is convenient to include the bight lying between Cape San Lucas and Cape Corrientes
within the gulf. The northern end of the gulf is desertic, whereas the southern end lies
below the boreal summer southerly monsoon rainfall conditions; total rainfall depends
on the incidence of tropical storms, or “chubascos.” Circulation is complex and dynamic
because of the interaction of the coastal wind regime with mountainous terrain of Baja
California between the gulf and the open Pacific Ocean. Exchange with the open Pacific
is minor and is largely driven by the fact that the upper gulf is an evaporative basin
(Bray, 1988); the general circulation therefore resembles that of a small Red Sea, except
there is no shallow sill across the mouth. In the northern gulf, the water column is fully
mixed by the boreal northeasterly winds, assisted by convective overturning to a depth
of 100 m (Roden, 1964). Over the shallow northern shelf, winter cooling of the highly
saline water of the evaporative estuary of the Colorado River forces a complex surface
pattern of turbidity gyres (Lepley et al., 1975). This dense, cool water is the origin of an
outflow of deep water from the gulf.
Residual, tidally forced circulation is strongly modified by local, temporary wind stress.
Land-sea breeze effects are important and northwesterly winds dominate the region in
October–May, whereas weaker southwesterlies dominate during boreal summer. This
alternation of wind direction forces strong upwelling on east and west coasts alternately,
according to season. Resultant plumes and dipoles of cool water may encompass the
entire width of the gulf.
Regional Response of the Pelagic Ecosystem
Perhaps because this region has a rather stable oceanographic regime, seasonality in
productivity appears to be driven by an integration of characteristic local processes,
rather than by changes common to the entire area of the province. Study of the SeaWiFS
and MODIS images suggest that we should discuss separately the following individual
processes: (i) possible winter upwelling on the coast from Cape Corrientes (21
N) to the
Gulf of Tehuantapec (16
N), (ii) upwelling that is especially strong in winter, but not
confined to that season, in the Gulfs of Tehuantapec (Mexico), Papagaya (Costa Rica),
and Panama, and (iii) multiple processes leading to seasonal eutrophication in the Gulf
of California.
Gulf of California: Tidal and drift currents can be strong through passes between
islands, especially in the northern gulf around Angel de la Guardia Island, where
the Ballenas Channel to the west of the island is isolated by relatively shallow sills
(Alvarez-Borrego, 1983). There is a persistent pool of cool water that is associated
with tidal fronts whose orientation depends on wind direction (Badon-Dongon
et al., 1985). Southwesterly winds in boreal autumn and northwesterly winds in
spring force coastal upwelling plumes from opposite sides of the gulf: on the west
coast in autumn and the east coast in spring. Wind-driven circulation changes are
reflected in seasonal blooms, especially in autumn and winter when sea surface
temperature cools under the influence of winter winds (from 30
down to 18
C)
and SeaWiFS chlorophyll increases (from 0.25 to 2.5 mg chl m
−3 ), with diatom
blooms initiating the sequence, followed by foraminifera and coccolithophores; the
bloom ends with a population of silicoflagellates and other diatoms (Thunell et al.,
1996). These data were obtained from sequential sampling from sediment traps and
must be presumed to integrate the individual local processes induced by topography
referred to previously.
Mexican west coast: During boreal winter, as noted earlier, equatorward winds and flow
of surface water along the west coast of Mexico, from Cape Corrientes southward,
are associated with coastal chlorophyll enhancement and the formation of offshore
filaments. This effect is strongest at Cape Corrientes, where it appears to be confluent
Chapter 11: The Pacific Ocean
It is convenient to include the bight lying between Cape San Lucas and Cape Corrientes
within the gulf. The northern end of the gulf is desertic, whereas the southern end lies
below the boreal summer southerly monsoon rainfall conditions; total rainfall depends
on the incidence of tropical storms, or “chubascos.” Circulation is complex and dynamic
because of the interaction of the coastal wind regime with mountainous terrain of Baja
California between the gulf and the open Pacific Ocean. Exchange with the open Pacific
is minor and is largely driven by the fact that the upper gulf is an evaporative basin
(Bray, 1988); the general circulation therefore resembles that of a small Red Sea, except
there is no shallow sill across the mouth. In the northern gulf, the water column is fully
mixed by the boreal northeasterly winds, assisted by convective overturning to a depth
of 100 m (Roden, 1964). Over the shallow northern shelf, winter cooling of the highly
saline water of the evaporative estuary of the Colorado River forces a complex surface
pattern of turbidity gyres (Lepley et al., 1975). This dense, cool water is the origin of an
outflow of deep water from the gulf.
Residual, tidally forced circulation is strongly modified by local, temporary wind stress.
Land-sea breeze effects are important and northwesterly winds dominate the region in
October–May, whereas weaker southwesterlies dominate during boreal summer. This
alternation of wind direction forces strong upwelling on east and west coasts alternately,
according to season. Resultant plumes and dipoles of cool water may encompass the
entire width of the gulf.
Regional Response of the Pelagic Ecosystem
Perhaps because this region has a rather stable oceanographic regime, seasonality in
productivity appears to be driven by an integration of characteristic local processes,
rather than by changes common to the entire area of the province. Study of the SeaWiFS
and MODIS images suggest that we should discuss separately the following individual
processes: (i) possible winter upwelling on the coast from Cape Corrientes (21
N) to the
Gulf of Tehuantapec (16
N), (ii) upwelling that is especially strong in winter, but not
confined to that season, in the Gulfs of Tehuantapec (Mexico), Papagaya (Costa Rica),
and Panama, and (iii) multiple processes leading to seasonal eutrophication in the Gulf
of California.
Gulf of California: Tidal and drift currents can be strong through passes between
islands, especially in the northern gulf around Angel de la Guardia Island, where
the Ballenas Channel to the west of the island is isolated by relatively shallow sills
(Alvarez-Borrego, 1983). There is a persistent pool of cool water that is associated
with tidal fronts whose orientation depends on wind direction (Badon-Dongon
et al., 1985). Southwesterly winds in boreal autumn and northwesterly winds in
spring force coastal upwelling plumes from opposite sides of the gulf: on the west
coast in autumn and the east coast in spring. Wind-driven circulation changes are
reflected in seasonal blooms, especially in autumn and winter when sea surface
temperature cools under the influence of winter winds (from 30
down to 18
C)
and SeaWiFS chlorophyll increases (from 0.25 to 2.5 mg chl m
−3 ), with diatom
blooms initiating the sequence, followed by foraminifera and coccolithophores; the
bloom ends with a population of silicoflagellates and other diatoms (Thunell et al.,
1996). These data were obtained from sequential sampling from sediment traps and
must be presumed to integrate the individual local processes induced by topography
referred to previously.
Mexican west coast: During boreal winter, as noted earlier, equatorward winds and flow
of surface water along the west coast of Mexico, from Cape Corrientes southward,
are associated with coastal chlorophyll enhancement and the formation of offshore
filaments. This effect is strongest at Cape Corrientes, where it appears to be confluent