Pacific Coastal Biome
405
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The west coast of America is an active continental margin in the geological sense, so the
continental shelf is narrow, usually a few tens of kilometers wide. Only in the Gulfs of
Panama and Guayaquil and at the head of the Gulf of California are there somewhat wider
shelf areas. Tidal stirring has been invoked as a mechanism that contributes strongly to
the productivity of the Gulf of California; here, the midbasin sill at the islands of Angel
de la Guardia and Tiburón is the most important tidal mixing region augmented by
breaking internal waves and hydraulic jumps.
Defining Characteristics of Regional Oceanography
This province is limited by the northern and southern equatorial fronts where flow
from the upwelling provinces that lie to north and south turns offshore. These fronts
are sharp features, linear near the coast but increasingly meandering out into the open
ocean (Griffiths, 1968). Circulation in CAMR is dominated by its location between
the passage across the ocean of the North and South Equatorial Currents and by the
arrival of the North Equatorial Countercurrent at the eastern margin of the ocean.
Water transported in this current forms the characteristic water mass in the entire region
between Baja California and the Gulf of Guayaquil. Seasonality is dominated by the
meridional migration of the atmospheric ITCZ that in boreal winter lies across the
southern part of the province and in boreal summer advances north to Nicaragua at
about 15
N. Forced by the consequent shift of winds in winter, the North Pacific gyre
is strengthened and the California Current extends farther south than in boreal summer
(Badan-Dangon, 1998).
Except for a few months in boreal winter along the Mexican coast, circulation is
predominantly toward the north, consistent with the poleward turn of the North Equatorial Countercurrent in the region of the Gulf of Panama: it is therefore continuous
along the entire coast of the province and may indeed be continuous with the flow
of the California Undercurrent although (as Badan-Dangon emphasizes) this is one of
the least-investigated major coastal currents anywhere. Fiedler (2002), on the contrary,
suggests that northward flow in the CAMR province eventually turns seaward to join
the flow of the NEC. Various segments have been variously named, but I shall use the
term Costa Rica Coastal Current for the whole. Recall that the eutrophic feature of the
Costa Rica Dome (see PNEC) originates on a thermocline ridge at the coast in the Gulf
of Papagaya in late winter, moving progressively offshore; the chlorophyll enhancement
associated with the nascent CRD is indistinguishable from that proper to permanent
coastal processes here.
Because drainage from the sierra mountain chains is mainly toward the east, few large
rivers discharge onto the continental shelf, so that the Gulfs of Panama and Guayaquil
are the sole recipients of significant freshwater and silt. Mixed-layer depth is generally
shallow (30–40 m) and the permanent thermocline is sharp, although a very shallow
brackish surface layer may occur during the wet season of boreal summer when southerly
monsoon conditions occur. The coastal regime is modified by the passage of strong winds
through the gaps in the mountains at head of the Gulfs of Tehuantapec, Papagayo, and
Panama; these winds cause intense cooling of the sea surface due to upwelling but also to
evaporative effects. Wind stress at such low latitude is translated into motion of offshore
current jets that have warm-core anticyclonic eddies to the right and ephemeral cold-core
cyclonic eddies to the left. Approximately five of these eddy pairs are produced annually
and, as discussed later, they have singular effects on the ecology of the region as they
propagate westward.
The Gulf of California, or Sea of Cortéz, is a long, narrow epicontinental sea that
includes deep basins and has steep-to coasts and continental slopes and numerous islands.
405
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The west coast of America is an active continental margin in the geological sense, so the
continental shelf is narrow, usually a few tens of kilometers wide. Only in the Gulfs of
Panama and Guayaquil and at the head of the Gulf of California are there somewhat wider
shelf areas. Tidal stirring has been invoked as a mechanism that contributes strongly to
the productivity of the Gulf of California; here, the midbasin sill at the islands of Angel
de la Guardia and Tiburón is the most important tidal mixing region augmented by
breaking internal waves and hydraulic jumps.
Defining Characteristics of Regional Oceanography
This province is limited by the northern and southern equatorial fronts where flow
from the upwelling provinces that lie to north and south turns offshore. These fronts
are sharp features, linear near the coast but increasingly meandering out into the open
ocean (Griffiths, 1968). Circulation in CAMR is dominated by its location between
the passage across the ocean of the North and South Equatorial Currents and by the
arrival of the North Equatorial Countercurrent at the eastern margin of the ocean.
Water transported in this current forms the characteristic water mass in the entire region
between Baja California and the Gulf of Guayaquil. Seasonality is dominated by the
meridional migration of the atmospheric ITCZ that in boreal winter lies across the
southern part of the province and in boreal summer advances north to Nicaragua at
about 15
N. Forced by the consequent shift of winds in winter, the North Pacific gyre
is strengthened and the California Current extends farther south than in boreal summer
(Badan-Dangon, 1998).
Except for a few months in boreal winter along the Mexican coast, circulation is
predominantly toward the north, consistent with the poleward turn of the North Equatorial Countercurrent in the region of the Gulf of Panama: it is therefore continuous
along the entire coast of the province and may indeed be continuous with the flow
of the California Undercurrent although (as Badan-Dangon emphasizes) this is one of
the least-investigated major coastal currents anywhere. Fiedler (2002), on the contrary,
suggests that northward flow in the CAMR province eventually turns seaward to join
the flow of the NEC. Various segments have been variously named, but I shall use the
term Costa Rica Coastal Current for the whole. Recall that the eutrophic feature of the
Costa Rica Dome (see PNEC) originates on a thermocline ridge at the coast in the Gulf
of Papagaya in late winter, moving progressively offshore; the chlorophyll enhancement
associated with the nascent CRD is indistinguishable from that proper to permanent
coastal processes here.
Because drainage from the sierra mountain chains is mainly toward the east, few large
rivers discharge onto the continental shelf, so that the Gulfs of Panama and Guayaquil
are the sole recipients of significant freshwater and silt. Mixed-layer depth is generally
shallow (30–40 m) and the permanent thermocline is sharp, although a very shallow
brackish surface layer may occur during the wet season of boreal summer when southerly
monsoon conditions occur. The coastal regime is modified by the passage of strong winds
through the gaps in the mountains at head of the Gulfs of Tehuantapec, Papagayo, and
Panama; these winds cause intense cooling of the sea surface due to upwelling but also to
evaporative effects. Wind stress at such low latitude is translated into motion of offshore
current jets that have warm-core anticyclonic eddies to the right and ephemeral cold-core
cyclonic eddies to the left. Approximately five of these eddy pairs are produced annually
and, as discussed later, they have singular effects on the ecology of the region as they
propagate westward.
The Gulf of California, or Sea of Cortéz, is a long, narrow epicontinental sea that
includes deep basins and has steep-to coasts and continental slopes and numerous islands.