Ocean-Coast Exchange Processes and
SeaSonde Mapping
Mike Kosro, COAS/Oregon State University
What processes carry water across shore?
What are their time and space scales?
How do we detect them?
•Gyre-scale circulation: West Wind Drift and its split
• Seasonality, the California Current and regional scales
• upwelling/downwelling seasons
• cross-shore currents
• alongshore currents and 3D effects
• mesoscale eddies
• Interannual variability
• Higher frequencies
• Modeling
Gyre-Scale Circulation
On the largest
scales, the
basin-scale
ocean gyres
transport
surface water
toward shore
in the West
Wind Drift.
50°N
40°N
30°N
Kirwan, et al, 1978
•“ West Wind Drift” carries surface waters toward the
west coast, where it splits to north or south.
• Latitude of split varies seasonally and interannually.
Wind-Driven Circulation
• Winds are a primary forcing mechanism for
ocean circulation. (Buoyancy forcing and tides
are others).
• Wind-driven surface transport is to the right
of the (steady) winds.
• Along the west coast, equatorward winds
produce coastal upwelling through offshore
surface flow. Poleward winds produce coastal
downwelling through onshore surface flow. On
average, these Ekman currents are weak, but
can be strong in storms.
Wind-Driven Circulation, cont
• Because cross-shore flows rearrange the
density field, geostrophic flows arise.
Equatorward (upwelling) winds produce an
equatorward current jet, and poleward winds
result in poleward currents.
• These alongshore currents can develop
instabilities, meander, form eddies, turning
strong alongshore flows into strong crossshore ones.
• The alongshore flows also respond to bottom
topography, and can be turned on/off shore.
Satellite Measured Winds: Jan Avg
Risien & Chelton
Satellite Measured Winds: July Avg.
Risien & Chelton
Time-Series Measurements
Long-term observations of
circulation and water properties
off Oregon.
Long-term Mooring (NH10): 1997-present
Newport Hydro Line: 1961-1971 & 1997-present
Surface Current Mapping: 1997-present
Newport Hydrographic Line
Summer/Winter average crosssection on Newport Line (T, S)
Summer: Upwelling.
offshore flow at surface, onshore
flow deeper. Note surfacing at
coast of properties from down to
200m. Produces an alongshore
current jet.
Winter: Downwelling.
onshore flow, pushing surface
waters down near the coast. Also
note vertical mixing. Produces a
poleward alongshore current.
Smith, Kosro, Huyer, Fleischbein,
2006.
Long-Term Midshelf Time-Series Measurements
Kosro, Hickey, Ramp, Letelier
4 GLOBEC Mooring Sites:
•
•
•
•
Newport (44.65 N, 81m depth)
Coos Bay (43.16 N, 97m)
Rogue River (42.44 N, 76m)
Gray’s Harbor (46.86 N, 25m)
Each mooring continuously recorded
measurements of:
• ADCP current profiles
• T and S at fixed depths
• chlorophyll fluorescence near 20m
Duration: 4.5 to 7 yrs
Sampling Δt: 60 mins – 3 mins
T : 4.5 yrs, 11 depths
Fall
Winter
Spring
Summer
Midshelf
Temperature
*Spring upwelling:
onshore flow at
depth, seen as
cold water builds
from depth.
*Fall downwelling:
onshore flow
initially at
surface, as warm
water builds from
surface.
v(z) : 7 yrs ADCP profiles
Fall
Winter
Spring
Summer
Midshelf
Alongshore Flow
Fall/winter: Alongshore
flow mostly poleward
(red/yellow).
Spring/summer:
Alongshore flow mostly
equatorward (blue/green)
v(z) : 7 yrs ADCP profiles
Seasonal Cycle: Alongshore Current at Midshelf
Winter
Poleward
Spring
Summer
Equatorward Jet
Fall
Poleward
Equatorward Jet (implied upwelling, offshore at surface,
onshore below) in spring/summer
Poleward Flow (implied downwelling, onshore flow at surface,
offshore below) in fall/winter.
u(z) : 7 yrs ADCP profiles
Fall
Winter
Spring
Summer
Midshelf
Cross-shore Flow
Unlike alongshore flow,
it is difficult to see the
on/offshore flow
seasonality in the dayto-day time series.
u(z) : 7 yrs ADCP profiles
Seasonal Cycle: Cross-shore current at Midshelf
Winter
Spring
Summer
Fall
Cross-shore average is an order of magnitude weaker than
alongshore avg.
Upwelling (offshore surface flow) in spring/summer.
Onshore flow is at mid-water.
Downwelling (onshore surface flow, offshore deep flow) in
fall/winter.
What about variation alongshore?
• How do conditions vary from location to
location alongshore?
– in the ocean “weather” (day-to-day)?
– in the ocean statistics (on average)?
• Use mapping tool to examine this.
Long Range Array
180 km range , 5 MHz
maps every 1-3 hrs
6 km ΔR, 5 deg. azimuth
• Cape Blanco region, 2 sites, 2001
• Winchester Bay, added 8/2001
• Yaquina Head, added 8/2002
• Manhattan Beach added 5/2004
Loomis Lake
added 5/2004
Standard Range Array
40 km range, 12MHz
hourly maps
2 km ΔR, 5 deg. azimuth
2 high-resolution regions:
• Columbia River mouth – 2 sites
• Newport/Heceta Bank – 3 sites
In Kosro(2005), showed that
• core of the average coastal jet centered
farther from shore as shelf widens from north
to south
• as the jet core moves offshore, a new jet
repeatedly spins up inshore.
Surface Currents under changing winds:
one week in June
Avg. Seasonal Cycle, 2001-2005
• 1st examine full-record monthly averages
– Watch spin-up of coastal jet
– Watch development of cross-shore flow as
season progresses.
• Then, interannual variability, comparing
monthly averages between years.
Mar
Av ST = 4/4
Apr
May Spring
June
July
Aug Summer
Sep
Oct
Nov
Fall
Dec
Jan
Feb Winter
Interannual Variability: Apr
2001
2002
2003
2004
2005
2002, stronger than usual upwelling [“subarctic invasion”]
2003, 2005: delayed upwelling
Interannual Variability: May
2001
2002
2003
2004
2005
2002, stronger than usual upwelling [“subarctic invasion”]
2005: upwelling still delayed
Interannual Variability: June
2001
2002
2003
2004
2005
2005: alongshore jet now seen, but “late”: inshore of its
usual cross-shelf location. Delayed start to upwelling
brought low productivity at coast, bird die-offs, warm
anomalies between Canada and Pt. Conception.
Eddies and Meanders:
Mesoscale Variability
Eddies can interact with the coastal flow, producing
strong, very local, cross-shore flows.
Eddies over the slope and deep sea
Eddies are large vortex
features; typically have
diameters of 30 km or more.
These are the ocean
analogue of storms in the
atmosphere.
When they lie near the
continental shelf, they can
interact with the shelf flow,
enhancing exchange between
the shelf and the deep ocean.
Eddies can form transient
closed eco-systems.
Eddies affect biological
distributions
Off British Columbia coast
Cover image, “Biological
Oceanography”, C. Miller
Satellite image (Coastal Zone Color Scanner)
enhanced to estimate chlorophyll concentrations.
Eddy effects are prominent off the west coast:
entraining, perhaps enhancing.
Eddies can form transient ecosystems
Measurements near Santa Barbara, CA, by Washburn and
Nishimoto. Red = number of fish caught per net haul.
A large spike appears near the center of the eddy
(note current arrows).
12/17/02
12/27/02
01/06/03
A
A
A
B
B
B
C
01/26
C
C
01/16
A
Eddies, Winter 2003
Satellite altimeter
measures sea surface
height.
B
Red regions are high
sea level, corresponding
to CW eddies, A,B,&C.
C
North side of eddies ->
onshore flow.
02/05
A
A?
02/15
02/25
Note persistence of
onshore flow from Eddy
C, between Cape Blanco
and Crescent City.
B
B
C
C
Strub
Winter 2003: long-lived, localized onshore flow
Jan 5
Jan 16
Jan 27
Jan 31
Kosro
Observational Summary
• West-wind drift => shoreward transport
• Ekman transport => slow on/offshore transport
at the surface (also at bottom) depending on
the wind (current)
• Alongshore currents are much stronger, and
can be turned cross-shore by topography or by
meanders/instability. Strong seasonal
component to this process at monthly time
scales.
• Eddies can produce strong cross-shore
transport, and can be long-lived. They can be
found outside of known “retention zones”.
Model Assimilation of HF Measurements
• Models provide circulation estimates at
high resolution in horizontal, vertical and
time, beyond that possible from
observations alone.
• Data assimilation: uses measurements (e.g.
HF-measured currents) to keep model
simulation “on track”.
• OSU group has developed statistical model
for estimating subsurface currents and
water properties based on surface
currents.
Measurements
help detailed
numerical models
of the coastal
circulation –
keeping the
models “on track”.
The model, in
turn, extends the
surface
measurements
into the ocean
subsurface.
Assimilation of surface data improves model
comparison w/ independent data at depth:
Correlation
Phase
Models are still
imperfect at reproducing
observations exactly, but
models informed by data
assimilation are closer to
observations, even away
from the observations.
• with assimilation of surface data
• without data assimilation
Long Range Array
180 km range , 5 MHz
maps every 1-3 hrs
6 km ΔR, 5 deg. azimuth
• Cape Blanco region, 2 sites, 2001
• Winchester Bay, added 8/2001
• Yaquina Head, added 8/2002
• Manhattan Beach added 5/2004
Loomis Lake
added 5/2004
• To North: Proposed extension
north to the Canadian border is
anticipated under NSF-ORION
California Long-Range Array funded and
spinning up
The Goal: A national system
Alongshore Current Jet and Topography
Over the shelf, there is a strong
tendency for the currents, even at
the surface, to follow the bottom
contours. Here, the alongshore
jet is being steered away from
shore by the widening of the
continental shelf south of 44.8° N.