Ocean Circulation Along the US West Coast: Big,
Little and In-Between
Kipp Shearman and Jack Barth
College of Oceanic & Atmospheric Sciences
Oregon State University
Thanks to B. Hickey and C. Collins
• the big picture
circulation
processes
• large scale features
California Current
Davidson Current
California Undercurrent
• seasonal changes
upwelling and downwelling
regional differences
mesoscale features
eddies, jets and squirts
• special places
The California
Current is part
of the large
scale clockwise
circulation of
the north
Pacific
Tabata (1975)
Circulation along the
US West Coast
shelf currents reverse
direction due to
winds and/or river
inputs
20
10
0 km
California Undercurrent
•
•
•
•
•
Pierce et al. (2000)
poleward
200-300 m
along continental slope
10-40 km wide
10-30 cm/s
Flow Over the Shelf: Winter vs. Summer
(from 10 years of Altimeter Data)
January
• high sea level
• poleward flow
• Davidson Current
•>50 cm/s
July
• low sea level
• equatorward flow
• shelf: >50 cm/s
Remember: Offshore flow remains weakly southward
Huyer et al. (2002)
Large Scale Features of the
California Current System
•
California Current
equatorward
surface
offshore and shelf
10-50 cm/s
•
Davidson Current
winter-time
poleward
surface
shelf and slope
50 cm/s
•
California Undercurrent
poleward
200-300 m
along continental slope
10-30 cm/s
Aleutian Low
Seasonal Changes
in the Atmosphere
North Pacific
High
•
Winter: Aleutian Low
•
Summer: North Pacific
High
•
Winds reverse direction
Huyer (1983)
Winds, Ekman Transport and Coastal Upwelling/Downwelling
Equatorward winds
Poleward winds
Offshore Ekman transport
Onshore Ekman transport
Upwelling brings deep, cold, nutrient rich
water to the surface near the coast
Downwelling traps warm, nutrient poor
surface water near the coast
Strong equatorward along-shelf flow
Strong poleward along-shelf flow
Upwelling
Downwelling
Upwelling and Downwelling along the US west coast
blue = cold
red = warm
The Oregonian
July 25, 1996
Upwelling on the Oregon Coast
downwelling
upwelling
Huyer and Smith (1978)
Historical Wind Stress Fields
(averages from 1854-1972 ship reports)
Jan
April
July
Oct
0.2 N/m2
0.05
0.1
0.05
0.1
0.15
0.05
Nelson (1977)
Huyer (1983)
Pacific
Northwest
Cape Blanco
Central
California
So. Cal.
Bight
Seasonal Changes
in Winds and Shelf
Currents
•
Pacific Northwest
– summer upwelling; currents
offshore and south
– winter downwelling; currents
onshore and north (Davidson
Current)
•
Central California
– strong winds
– upwelling year-round
– currents reverse direction
•
Southern California Bight
– complicated
Upwelling Jet and Mesoscale
eddies …
Strub and James (2000)
Upwelling Jet
separates from shelf
‘squirts’ cold shelf water
offshore
Meanders form eddies
Mesoscale Eddies
50-100 km diameter
>50 cm/s currents
Seasonal Evolution of Eddies
in the California Current
•
Spring
– Upwelling front forms
over shelf
– Intense equatorward jet
– Eddies from last season
found offshore
•
Summer
– Upwelling jet develops
instabilities; meanders
– Upwelling jet may
separate from shelf (e.g.
Cape Blanco)
– Eddies form within
meanders
•
Fall
– Offshore jet meanders
continue to grow
– More eddies
•
Winter
– Shelf currents reverse
– Eddie move offshore
Strub and James (2000)
Specific Features
• Retentive Features
– Heceta Bank
• Offshore Flow
– Cape Blanco
• River Plumes and Estuaries
– Columbia River
Chelton
Heceta Bank
•
•
•
In 1775, Don Bruno de Heceta, sailing
for the Royal Spanish Navy, set out on
a year-long mission to reach the Arctic
Circle.
He made it as far as the Columbia
River, before turning back because of
scurvy.
Offshore the shelf widens around a
complex group of shallow! submarine
banks.
30
50
100
Sea surface temperature
Heceta
Bank
phytoplankton
chlorophyll
11-day
recirculation
Circulation near Cape Blanco
•
•
•
Strong equatorward upwelling jet
separates from the coast
Send shelf water far offshore
Marks boundary between regions
Barth et al. (2000)
courtesy of Roger Samelson (OSU)
The Columbia River Plume in the coastal ocean
Upwelling moves
river water
offshore and south
summer
downwelling
keeps river
water against
the coast and
flowing north
winter
Barnes et al. (1972)
Coastal ocean signals
observed in estuaries
a. Temperatures (oC) in the
Oregon shelf off Newport
b. T* (oC) in the Columbia River
estuary
c. * (m) in the Columbia River
estuary
1997
Depth(m)
Deph
0
1998
1999
2000
2001
2002
(a)
17
15
13
11
9
7
20
40
60
(b)
(c)
1997
1998
1999
2000
2001
2002
Circulation Characteristics Along the US West Coast
Relevant to Ballast Water Exchange
• large scale features
California Current
Davidson Current
California Undercurrent
• seasonal changes
three regions
upwelling and downwelling
• mesoscale features
eddies, jets and squirts
• special places
Coastal Ocean Observing
System
Temperature
Salinity
Nose-Bag
Density
OSU Glider Lab
Chlorophyl
2 Slocum electric, 200 m gliders
New 1000 m glider (spring 2007)
Endurance Line off Newport, OR
CDOM
• began April 2006
• 100 km offshore
• 7 day repeat cycle (out+back)
Dissolved Oxygen
Gliders will play a major role in planned
ocean observing systems …
Proposed
Glider
Endurance
Lines
ORION D&I Workshop Report, 2006
Juan de Fuca
Eddy
•
•
•
•
•
•
MacFadyen et al. (2005)
Counter clockwise
(cyclonic) recirculating cold
eddy
Off NW Washington coast
Forms in spring, declines
in fall
~50 km diameter
Intense phytoplankton
blooms
Possible ‘incubator’ of
HABs
Sackmann and Perry (2005)