Bering Strait: Pacific Gateway
to the Arctic Ocean
Rebecca A Woodgate
Applied Physics Laboratory, University of Washington,
Knut Aagaard, Tom Weingartner, Terry Whitledge
John Calder, Kathy Crane, Igor Lavrenov
With thanks to Jim Johnson, Seth Danielson, Mike Schmidt,
and crews of the Alpha Helix, the Laurier, and the Khromov
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Little Diomede Island, Bering Strait
Bering Strait: Pacific Gateway
to the Arctic Ocean
BERING STRAIT BASICS
- why? how? what?
RECENT RESULTS
- seasonal variability
- freshwater fluxes
- interannual variability
Little Diomede Island, Bering Strait
CURRENT/FUTURE WORK
- heat fluxes
- an intensive observing system
for IPY?
2
Bering Strait Basics
The only Pacific gateway to the
Arctic Ocean
~ 85 km wide
~ 50 m deep
-divided into 2
channels by the
Diomede Islands
- split by the USRussian border
-dominates the water
properties of the
Chukchi Sea
- is an integrator of the
properties of the
Bering Sea
(Coachman et al, 1975;
Woodgate et al, 2005)
-ice covered
from ~ January
to April
Why does a little Strait
matter so much?
- annual mean
northward flow
~0.8 Sv
3
Bering Strait dominates the Chukchi Sea
Woodgate et al., 2005, DSR and via http://psc.apl.washington.edu/HLD
Bering Strait good
proxy of Arctic inflow
4
The role of Pacific waters in the Arctic
Important for
Marine Life
Pacific waters
are the most
nutrient-rich
waters
entering the
Arctic
(Walsh et al, 1989)
Implicated in the seasonal
melt-back of ice
In summer, Pacific waters are a
source of near-surface heat to
the Arctic
(Paquette & Bourke, 1981; Ahlnäs & Garrison, 1984)
Significant part of
Arctic Freshwater
Budget
Bering Strait
throughflow
~ 1/3rd of Arctic
Freshwater
(Wijffels et al, 1992;
Aagaard & Carmack, 1989;
Woodgate & Aagaard, 2005)
Important for Arctic Stratification
In winter, Pacific waters (fresher than
Atlantic waters) form a cold
(halocline) layer, which insulates the
ice from the warm Atlantic water
beneath
(Shimada et al, 2001, Steele et al, 2004)
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Global role of Bering Strait
A Freshwater source for the Atlantic Ocean
Pacific waters exit the
Arctic through the
Fram Strait and
through the Canadian
Archipelago
Broecker, 1991
(Jones et al, 2003)
Freshwater inhibits deep convection, slowing the Atlantic Ocean overturning
circulation (see Wadley & Bigg, 2002, for a discussion)
Models suggest the Bering Strait throughflow also influences the deep
western boundary currents & the Gulf stream separation (Huang & Schmidt, 1993)
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Paleo role of
Bering Strait
Stabilizer for World Climate?
(DeBoer & Nof, 2004; Hu & Meehl, 2005)
- if Bering Strait is open, excess
freshwater in the Atlantic (from, for
example, ice sheet collapse) can
“vent” through the Bering Strait,
allowing a speedy return to deep
convection in the Atlantic.
Land Bridge
for migration
of mammals
and people?
www.debbiemilleralaska.com
Note: in modern times, people have
swum, driven and walked across!
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Special Observational
Challenges of Bering Strait
Two boundary currents
- Alaskan Coastal Current
(ACC) in the east present
from ~ spring to midwinter (Paquette & Bourke, 1974)
- Siberian Coastal Current
(SCC) present seasonally
in some years in the west
- shallow (50 m),
but covered by ice
(keels to 20 m) from
~ January to April
- stratified in
spring/summer
- split by the USRussian border
(Weingartner et al, 1999)
Eastern Bering Strait in Winter
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Long-term moorings in Bering Strait
From 1990 to present
T, S and velocity
at 9m above bottom
A1 = western Channel
A2 = eastern Channel
A3 = combination of A1/2
A3’ (up north)
A4 = Alaskan Coastal Current
Not all moorings are deployed all
years!
Sea Surface Temperature 26th August 2004, from MODIS/Aqua level 1
courtesy of Ocean Color Data Processing Archive, NASA/Goddard Space Flight Center, thanks to Mike Schmidt
Grey arrow marks the Diomede Islands (Little and Big Diomede). Russian EEZ line passes between the islands.
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Moorings in Bering Strait
Short (~20 m) long bottom moored
Top float at ~40 m or deeper to
avoid ice keels and barges
STANDARD MEASUREMENTS
= Temperature and salinity and velocity
at 9 m above bottom
(SBE16, and Aanderaa RCM7 and
RCM9/11 due to biofouling)
EXTRA MEASUREMENTS
= ADCP - water velocity in 2 m bins from
~15 m above bottom to near surface
- ice motion and rough ice thickness
= ULS – upward looking sonars
(good ice thickness)
= NAS – Nutrient sampler
= SBE16+ - Fluorescence, transmissivity,
and PAR
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CTD cruises
e.g. Bering Strait & Chukchi Sea 2003
5-7 day Physical Oceanography Cruise
- CTD and ship’s ADCP sections
- sampling nutrients, O18, (productivity, CDOM, ...)
- underway data and ship’s ADCP
R/V Alpha Helix
Seward. AK
Photo from akbrian.net
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Bering Strait 2004
Moorings and CTD work
show temperature, salinity
and velocity structure
changes rapidly and on
small space scales.
To resolve the physics,
we use:
- high spatial resolution
(here ~ 3km)
- high temporal resolution
(line run in ~4 hrs)
- ship’s ADCP data
1st Sept 2004
5th Sept 2004
12
Getting the 4-dimensional picture
Bering Strait and Chukchi Sea 2003
23rd June
2003
Convention line Fluorescence
Chlorophyll from SeaWifs Satellite
from NASA/Goddard Space Flight Center & Orbimage
5th – 7th July 2003
Sea surface temperature and altimeter
satellite data too
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http://psc.apl.washington.edu/BeringStrait.html
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Bering Strait
Basics
- annual mean flow ~0.8 Sv northwards, with an
annual (monthly mean) cycle of 0.3 to 1.3 Sv
- weekly flow reversals common (-2 Sv to +3 Sv)
-1 hourly flow can be over 100 cm/s
- Alaskan Coastal Current (ACC) velocities can
be 50-100 cm/s stronger than midchannel flow
- flow strongly rectilinear
- tides are weak
(Roach et al, 1995; Woodgate et al, 2005)
- away from boundary
currents, flow dominantly
barotropic (Roach et al, 1995)
- flow in east and west
channel highly correlated
(0.95, Woodgate et al, 2005, DSR)
15
What Drives the Bering Strait Throughflow?
Velocity = “Pacific-Arctic Pressure Head” + “Wind Effects”
Mean=northward
10-6
sea surface slope gives
rise to pressure gradient
between Pacific and Arctic
Oceans (Coachman & Aagaard, 1966;
Stigebrandt, 1984)
Mean=northward
Mean=southward
- across-strait atmospheric pressure gradient
(Coachman & Aagaard, 1981)
- local wind (Aagaard et al, 1985 and others)
- set-up against topography (same ref)
Wind explains ca. 60 % of the variance
and the seasonal cycle (Roach et al, 95)
But WHY?
- freshwater transport from
Atlantic by atmosphere?
- steric height difference?
- global winds? (Nof)
In the mean, the
winds oppose the
pressure head
forcing. Thus, in
winter, when winds
are strongest, the
northward flow is
weakest.
ASSUMED constant
- but why should it be?
(Woodgate et al, 2005 DSR)
AUG
APRIL
Woodgate et al, 2005
16
Reconstructing the velocity field
(e.g. Woodgate et al, 2005 GRL)
Assume
Flow = “Pressure head” + const x (Wind)
(Colours = real data; black=reconstruction)
Reconstruction generally good but tends to miss
extreme flow events, especially summer 1994.
Linear fit to the wind better than a “climatology”
But still we don’t really know the mechanism
17
Seasonal cycle in water properties (Woodgate et al, 2005)
3
2
SALINITY
31.9 to 33 psu
1
ICE
4
TEMPERATURE
-1.8 to 2.3 deg C
TRANSPORT
0.4 to 1.2 Sv
(30 day means)
AUG
APRIL
WHY CARE?
(1) Maximum
temperature in late
Seasonally varying
summer
input to the Arctic
Ocean
(2) Autumn
cooling
and freshing, as
- temperature
overlying
layers
- salinity
mixed
down
-volume
(3) Winter at
freezing point,
- equilibrium depth
salinisation due to
(~50m in summer
ice formation
~120m in winter)
(4) Spring
-nutrient loading
freshening
(due to
ice melt) and then
warming
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Influence of shelf waters
Along the Chukchi
Shelf, upwelling and
diapycnal mixing of
lower halocline waters
and Pacific waters
Use silicate to
track Pacific Water
in the Chukchi
Borderland
(Note ventilation by
polynya waters
couldn’t give this T-S
structure)
Atl
Pac
Pacific Nutrient Max
Woodgate et al, 2005
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Bering Strait and Arctic Freshwater
Aagaard & Carmack,
1989 (AC89)
BERING STRAIT
~ 0.8 Sv
(moorings)
~32.5 psu
(summer 1960s/70s)
Freshwater Flux
relative to 34.8 psu
~ 1670 km3/yr
P-E
OTHER INPUTS
Runoff = 3300 km3/yr
P-E
= 900 km3/yr
+ ...
OTHER OUTPUTS
Fram Strait water = 820 km3/yr
Fram Strait ice = 2790 km3/yr
Canadian Archipelago = 920 km3/yr
+ ...
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The Alaskan Coastal Current (ACC)
Summer Observations
10km wide, 40m deep, wedge-shape
Summer ACC Volume flux 0.2 Sv
(cf Bering Strait annual 0.8 Sv,
weekly -2 to +3 Sv)
Estimate salinity at 30 psu
Summer ACC Freshwater flux 0.03 Sv
(~900 km3/yr)
But only present ca. April - December
Sea Surface Temperature 26th August 2004, from MODIS/Aqua
level 1, courtesy of Ocean Color Data Processing Archive,
NASA/Goddard Space Flight Center, thanks to Mike Schmidt
Salinity July 2003 from the Diomede Islands (left) to
the Alaskan Coast (right)
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The Alaskan Coastal Current July 2002-2003
VELOCITY NORTH
A2 bottom central eastern
channel 47m
A4 the Alaskan Coastal
Current 34m, 24m, 14m
SALINITY
A2 bottom central eastern
channel 48m
A4 the Alaskan Coastal
Current 39m
Black solid line = temperatures at freezing
ACC annual mean velocity 40 cm/s; transport 0.08 Sv, salinity 30.3 psu
Annual Mean Freshwater Flux 220-450 km3/yr (~ 20% AC89 Bering Strait)
surface currents ~ 170 cm/s (at depth 70 cm/s) across-strait salinity gradient of ~ 3 psu
present until late December 2002 (JD365), returns late April 2003 (JD480)
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Put it together (with stratification and ice)
Summer Stratification
Chukchi~ 2 layer system, with salinity step
of ~ 1 psu
Assume stratified 6 months,
~350 km3/yr
Ice Transport
- annual mean NORTHWARD ice flux of
130 ± 90 km3/yr
(despite almost 2 months of net southward
ice flux)
Total ~ 400 km3/yr
(~20% of AC89 estimate)
Salinity July 2003 from Little Diomede (left) to
the Alaskan Coast (right)
Annual Mean Freshwater flux =
Previous estimate AC89 1670 km3/yr
+ ~ 400 km3/yr (Alaskan Coastal Current)
+ ~ 400 km3/yr (stratification and ice)
~ 2500 ± 300 km3/yr
(Woodgate & Aagaard, GRL, 2005)
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The Bering Strait Freshwater Flux
(Woodgate & Aagaard, 2005)
S = near bottom
annual mean salinity
FW = freshwater flux
assuming no
horizontal or vertical
stratification
FW+ = revised flux,
including estimate of
Alaskan Coastal
Current and
seasonal
stratification
Annual Mean Freshwater Flux
~ 2500 ± 300 km3/yr
including
~ 400 km3/yr (Alaskan Coastal Current)
~ 400 km3/yr (stratification and ice)
Interannual variability (from near
bottom measurements) smaller than
errors, although possible freshening
~ 1/3rd of Arctic Freshwater
Arctic Rivers ~ 3300 km3/yr P-E ~ 900 km3/yr
Fram Strait water & ice ~ 820 km3/yr & ~ 2790 km3/yr
Canadian Archipelago ~ 920 km3/yr
since 2003-2004
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Arctic
Freshwater
revised
Serreze et al, JGR,
in press
INFLOW
- Rivers 38%
- Bering Strait 30%
- P-E 24%
OUTFLOW
- CAA 35%
- Fram St water 26%
- Fram St ice 25%
25
Bering
Strait
properties
from 1990
to present
26
NEW from
this year’s
data?
ACC colder in
2005, but bottom
waters warmer
2006 is not
starting out colder
even though the
ice is unusually
heavy
27
Interannual
Variability
(up to 2004)
Warming since 2001
Increasing flow since 2001
(mostly attributable to
changes in local wind)
Woodgate et al, 2006, GRL
28
Scale of variability
Extra heat since 2001 could
melt an area 800km by
800km of 1m thick ice
Transport
change
significant
0.7 Sv in 2001
1 Sv in 2005
2004 largest
heat flux
observed
Increasing flow
accounts for
80% of the
freshwater and
50% of the heat
flux increases
Extra freshwater since 2001
is about ¼ of annual mean
river run off
BS heat flux is ~ 1/5 of
Fram Strait heat flux
Alaskan Coastal Current
(ACC) carries
~ 10% of all freshwater
entering the Arctic!
~ 1/4 Bering Strait FW
~ 1/3 Bering Strait heat
STILL not properly
measured
Woodgate et al, 2006
29
Using MODIS to constrain the ACC
(with Ron Lindsay)
For - sea surface temperature (SST)
- width of ACC
- timing of ACC
And thus heat flux, and maybe FW flux
Black lines: weekly averages of eastern channel SST from MODIS
30
Bering Strait Future
Plans
Moorings and CTD work
- NSF/NOAA proposal
- US, Russian and Canadian work
Altimeter and Satellite data
- continue time-series measurements
through and beyond IPY
- improve vertical and horizontal moored
resolution (especially for freshwater flux)
- develop flow-proxies from
wind/model/insitu/satellite data
- design a monitoring network for
the Bering Strait
31
IS full of
surprises
A trifloat
after 14
months
in the
water
North of the
Diomedes,
Sept 2004,
large area
of dead
copepods
http://psc.apl.washington.edu/AlphaHelix2004.html
32
The Pacific
Arctic Gateway
http://psc.apl.washington.edu/BeringStrait.html
SEASONAL VARIABILITY
– significant in T,S and volume
INTERANNUAL VARIABILITY
- very influenced by local wind
- warming and freshening since 2001
- important part of Arctic FW and
heat fluxes
3
2
1
ICE
4
FUTURE PLANS
- heat flux with satellite data
- intensive IPY array with upper layer
TS measurements
33