390
Chapter 11: The Pacific Ocean
many deep canyons intersect the shelf, especially along the eastern side of this province;
glaciation is the cause of the highly inflected nature of the coastline itself. About 20% of
the adjacent continental drainage areas remain covered by glaciers that serve to modify
the seasonality of the discharge of freshwater to the sea.
The shelf is also remarkable for the steepness of the topography that lies on either side:
seaward, a very steep continental slope into abyssal depths and landward the very abrupt
topography of high coastal mountain ranges. These mountains are essential elements in
the forcing of the characteristic coastal oceanography of this province.
Defining Characteristics of Regional Oceanography
This coastal boundary province comprises the rim current (the Alaska Current or Alaska
Stream as it is termed off the Aleutians) of the subarctic gyre, from its origin at the
divergence of the North Pacific Current when this encounters the American continent
near Vancouver Island right around to the end of the Aleutian chain of islands; this
province is approximately synonymous with the Alaska Current System of Favorite et al.
(1976) and the Alaska Downwelling Province of Ware and McFarlane (1989).
Circulation is carried around the periphery of the Subarctic gyre in the Alaska Current
that lies just beyond the shelf and, after it turns southwest to pass along the Aleutian
chain as a western boundary current, is spoken of as the Alaska Stream. Close inshore,
the Alaska Coastal Current is especially strongly developed west of 140
W and carries
about two-thirds of the total along-shelf flow. Circulation and water column stability
in this province are forced by wind stress and by buoyancy. As discussed recently by
Royer (1998) the presence of the coastal ranges constrains the further passage to the
east of the warm, wet, low-pressure systems that pass along the Aleutian storm track in
winter. When these encounter the coastal mountains, they rise and lose a great deal of
their moisture as rain or snow. Because the consequent precipitation rates are very high
(<800 cm y
−1 ), and the drainage basins rather small, there is a very high rate of freshwater
discharge to the coastal ocean. This is maximal in late summer because the glaciers
retain some winter precipitation until the ensuing summer; the seasonal salinity regime
over the shelf (28–31‰ at 25 m) follows the seasonal pattern of discharge. Because the
drainage area comprises high mountains, rather than forested flatlands, the river water is
nutrient-poor; supply of nutrients to the mixed layer is a complex and unusual process
(see later discussion).
The large discharge of freshwater controls shelf salinities and hence density gradients,
both horizontal and vertical, create a salinity front between the Alaskan Coastal Current
and the more variable midshelf flow; this is situated at only 10 km from the coast along
the Cape Fairfield section in the northern part of this province. A second salinity front
separates shelf water from the sluggish flow (c. 5 m min
−1 ) of the warm Alaska Current
itself beyond the shelf edge. The velocity of the flow beyond the shelf edge increases to
<50 m min
−1 in the Alaska Stream that returns southwest down along the Aleutian Chain.
The inner shelf is dominated by semidiurnal tidal streams that may be very rapid (up
to 6 m sec
−1 in the Straits of Georgia) and are associated with tidal fronts at the mouths
of bays and rivers. The heavy clouds and rainfall of this region means that irradiance has
a very strong seasonal cycle from 70 W m
−2 on a rainy winter day to 1400 W m
−2 under a
sunny sky in summer—moreover, water clarity is reduced in winter by silt effluent from
flooding rivers; Secchi depths in river plumes of 0.1 m in winter and 3.0 m in summer
are not unusual.
The persistent cyclonic winds, especially in winter, impose onshore Ekman transport
at the surface and downwelling at the coast itself. The Alaska Coastal Current is therefore
coastally trapped so that, from Vancouver Island to the Unimak Pass in the Aleutians,
it flows preferentially through the coastal passages before delivering low-salinity water to
Chapter 11: The Pacific Ocean
many deep canyons intersect the shelf, especially along the eastern side of this province;
glaciation is the cause of the highly inflected nature of the coastline itself. About 20% of
the adjacent continental drainage areas remain covered by glaciers that serve to modify
the seasonality of the discharge of freshwater to the sea.
The shelf is also remarkable for the steepness of the topography that lies on either side:
seaward, a very steep continental slope into abyssal depths and landward the very abrupt
topography of high coastal mountain ranges. These mountains are essential elements in
the forcing of the characteristic coastal oceanography of this province.
Defining Characteristics of Regional Oceanography
This coastal boundary province comprises the rim current (the Alaska Current or Alaska
Stream as it is termed off the Aleutians) of the subarctic gyre, from its origin at the
divergence of the North Pacific Current when this encounters the American continent
near Vancouver Island right around to the end of the Aleutian chain of islands; this
province is approximately synonymous with the Alaska Current System of Favorite et al.
(1976) and the Alaska Downwelling Province of Ware and McFarlane (1989).
Circulation is carried around the periphery of the Subarctic gyre in the Alaska Current
that lies just beyond the shelf and, after it turns southwest to pass along the Aleutian
chain as a western boundary current, is spoken of as the Alaska Stream. Close inshore,
the Alaska Coastal Current is especially strongly developed west of 140
W and carries
about two-thirds of the total along-shelf flow. Circulation and water column stability
in this province are forced by wind stress and by buoyancy. As discussed recently by
Royer (1998) the presence of the coastal ranges constrains the further passage to the
east of the warm, wet, low-pressure systems that pass along the Aleutian storm track in
winter. When these encounter the coastal mountains, they rise and lose a great deal of
their moisture as rain or snow. Because the consequent precipitation rates are very high
(<800 cm y
−1 ), and the drainage basins rather small, there is a very high rate of freshwater
discharge to the coastal ocean. This is maximal in late summer because the glaciers
retain some winter precipitation until the ensuing summer; the seasonal salinity regime
over the shelf (28–31‰ at 25 m) follows the seasonal pattern of discharge. Because the
drainage area comprises high mountains, rather than forested flatlands, the river water is
nutrient-poor; supply of nutrients to the mixed layer is a complex and unusual process
(see later discussion).
The large discharge of freshwater controls shelf salinities and hence density gradients,
both horizontal and vertical, create a salinity front between the Alaskan Coastal Current
and the more variable midshelf flow; this is situated at only 10 km from the coast along
the Cape Fairfield section in the northern part of this province. A second salinity front
separates shelf water from the sluggish flow (c. 5 m min
−1 ) of the warm Alaska Current
itself beyond the shelf edge. The velocity of the flow beyond the shelf edge increases to
<50 m min
−1 in the Alaska Stream that returns southwest down along the Aleutian Chain.
The inner shelf is dominated by semidiurnal tidal streams that may be very rapid (up
to 6 m sec
−1 in the Straits of Georgia) and are associated with tidal fronts at the mouths
of bays and rivers. The heavy clouds and rainfall of this region means that irradiance has
a very strong seasonal cycle from 70 W m
−2 on a rainy winter day to 1400 W m
−2 under a
sunny sky in summer—moreover, water clarity is reduced in winter by silt effluent from
flooding rivers; Secchi depths in river plumes of 0.1 m in winter and 3.0 m in summer
are not unusual.
The persistent cyclonic winds, especially in winter, impose onshore Ekman transport
at the surface and downwelling at the coast itself. The Alaska Coastal Current is therefore
coastally trapped so that, from Vancouver Island to the Unimak Pass in the Aleutians,
it flows preferentially through the coastal passages before delivering low-salinity water to