Rebecca Woodgate

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Pacific Inflow to the Arctic Ocean Changes in
the Bering Strait throughflow
Rebecca Woodgate Polar Science Center, Applied
Physics Laboratory, University of Washington,
Tom Weingartner, Terry Whitledge, Ron Lindsay,
Knut Aagaard In collaboration with AARI (Arctic
and Antarctic Research Institute), Russia. With
thanks to Jim Johnson, Seth Danielson, Mike
Schmidt, and crews of the Alpha Helix, the
Laurier, the Khromov and the Sever Funded by
NSF-OPP, NOAA-RUSALCA, and in the past ONR, and
NSF-SBI
Little Diomede Island, Bering Strait
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Pacific Gateway to the Arctic Ocean
Why care about the Bering Strait? What are we
doing, and what are we finding? What next?
Little Diomede Island, Bering Strait
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Little Diomede Island, Bering Strait
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ARCTIC OCEAN
NOAA.gov
You are down here somewhere
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Figure from Harry Stern, UW
TOTAL Arctic Ocean area 2 x lower 48

1.5 x USA
Russia
Alaska
Canada
Greenland
Smallest of worlds 5 major oceans (Pacific,
Atlantic, Indian, Southern, and Arctic)
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ATLANTIC LAYER - warm, salty - largest volume
input
- follows slopes and ridges ... SLOWLY
(a few cm/s 8hrs for 1 mile) - strong eddies
within that flow - likes to stick to a depth
contour .. MOSTLY - takes more than 10-20-30 ..
years to get back out!
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PACIFIC WATER - nutrient rich - source of heat
and freshwater
- follows slopes a bit (and faster!), but also
crosses basins ... in response
to wind?? - eddies important again - keeps
nutrients high in the water - takes 10 years or
less to cross Arctic
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Bering Strait Connections
ARCTIC
Bering Strait
Only Pacific Gateway to the Arctic Ocean To the
SOUTH BERING SEA (over 50 of US fish catch) -
Bering Strait throughflow could drain Bering Sea
Shelf in about 1 year To the NORTH CHUKCHI
SEA (future US fish catch????) - Bering Strait
throughflow drives properties and throughput of
the Chukchi Sea.
PACIFIC
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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 US-Russian border
• ice covered from January to April
• - annual mean northward flow 0.8 Sv

• 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)

Why does a little Strait matter so much?
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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)
Primary Productivity gC m-2 yr-1 Courtesy of
(and adapted from) Codispoti, Stein, Macdonald,
and others, 2005
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The role of Pacific waters in the Arctic
Chlorophyll from SeaWifs Satellite from
NASA/Goddard Space Flight Center and Orbimage
Sea ice concentration from SSMI (IABP)
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)
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The role of Pacific waters in the Arctic
Pacific winter waters
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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The role of Pacific waters in the Arctic
Largest Interannual Variability??
ARCTIC FRESHWATER FLUXES Bering Strait 2500
km3/yr (0.08 Sv) Arctic Rivers 3300
km3/yr P-E 900 km3/yr Fram Strait water 820
km3/yr Fram Strait ice 2790 km3/yr Canadian
Archipelago 920 km3/yr
Significant part of Arctic Freshwater
Budget Bering Strait throughflow 1/3rd of
Arctic Freshwater (Wijffels et al, 1992 Aagaard
Carmack, 1989 Woodgate Aagaard, 2005)
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The role of Pacific waters in the Arctic
Largest Interannual Variability??
Important for Marine Life Pacific waters are the
most nutrient-rich waters entering the
Arctic (Walsh et al, 1989)
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)
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)
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Global role of Bering Strait
A Freshwater source for the Atlantic Ocean
Pacific waters exit the Arctic into Atlantic Deep
Water formation regions (Jones et al, 2003)
- slowing the Atlantic Ocean overturning
circulation (see Wadley Bigg, 2002) - affecting
deep western boundary currents 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 vents
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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Observational Challenges of Bering Strait
- shallow (50 m) - stratified in spring/summer -
important seasonal boundary currents (e.g.
Alaskan Coastal Current)
- ice covered (keels to 20 m) from January to
April - split by the US-Russian border
Eastern Bering Strait in Winter
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Russian-US Long-Term Census of the Arctic - John
Calder - Kathy Crane Joint US Russian cruises
and moorings since 2004
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Bering Strait Long-term Moorings
Since 1990 1-4 nearbottom moorings
Since 2007 (International Polar Year) 8 moorings
with upper and lower sensors
- Understand the Physics - Design a Monitoring
Scheme
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Seasonal cycle in water properties (Woodgate et
al, 2005)
SALINITY 31.9 to 33 psu TEMPERATURE -1.8 to
2.3 deg C TRANSPORT 0.4 to 1.2 Sv (30 day
means)

• WHY CARE?
• Seasonally varying input to the Arctic Ocean
• - temperature
• - salinity
• -volume
• - equilibrium depth
• (50m in summer
• 120m in winter)
• nutrient loading

(1) Maximum temperature in late summer (2) Autumn
cooling and freshing, as overlying layers mixed
down (3) Winter at freezing point, salinisation
due to ice formation (4) Spring freshening (due
to ice melt) and then warming
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Bering Strait properties from 1990 to present
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Interannual Variability (up to 2004)
Warming since 2001 Increasing flow since 2001
(mostly attributable to changes in local
wind) Woodgate et al, 2006, GRL
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Scale of variability
Extra heat since 2001 could melt an area 800km by
800km of 1m thick ice 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
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
Woodgate et al, 2006
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Most recent change?
NOT a continuation of the 2004 warming But what
about 2007?
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2007 from Satellites?
Sea surface height and temperature data suggest
2007 heat flux will be very high (Mizobata et al,
submitted) Modeling results suggest increased
volume and heat fluxes in 2007, with impact on
Arctic sea-ice (Zhang et al, submitted)
Moorings to be recovered in early October 2008
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The role of Pacific waters in the Arctic
- SIGNIFICANT amount of heat - TRIGGER for
ice-albedo feedback - CONDUIT for local heat (as
low density) - PAN-WESTERN-ARCTIC subsurface
Temperature source
Sea ice concentration from SSMI (IABP)
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)
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Future for Bering Strait?
NEXT? NSF/NOAA proposal for completing
design Arctic Observing Network and operational
monitoring scheme insitu and satellite?
psc.apl.washington.edu/BeringStrait.html
Little Diomede Island, Bering Strait
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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
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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
ships ADCP sections - sampling nutrients, O18,
(productivity, CDOM, ...) - underway data and
ships 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) - ships ADCP data
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Getting the 4-dimensional pictureBering Strait
and Chukchi Sea 2003
23rd June 2003
Convention line Fluorescence
Chlorophyll from SeaWifs Satellite from
NASA/Goddard Space Flight Center Orbimage
Sea surface temperature and altimeter satellite
data too
5th 7th July 2003
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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)
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What Drives the Bering Strait Throughflow?
Velocity Pacific-Arctic Pressure Head
Wind Effects Meannorthward
Meannorthward
Meansouthward
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Reconstructing the velocity field(e.g. Woodgate
et al, 2005 GRL)
Assume Flow Pressure head const
x (Wind) (Colours real data blackreconstructio
n)
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 dont really know the
mechanism
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Influence of shelf waters
Use silicate to track Pacific Water in the
Chukchi Borderland
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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
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)
Salinity July 2003 from Little Diomede (left) to
the Alaskan Coast (right)
Total 400 km3/yr (20 of AC89 estimate)
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
Interannual variability (from near bottom
measurements) smaller than errors, although
possible freshening since 2003-2004
Annual Mean Freshwater Flux 2500 300
km3/yr including 400 km3/yr (Alaskan Coastal
Current) 400 km3/yr (stratification and ice)
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
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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
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The Pacific Gateway to the Arctic
COORDINATION AND INTEGRATION
http//psc.apl.washington.edu/BeringStrait.html
WITHOUT ACC/Stratification
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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
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IS full of surprises
North of the Diomedes, Sept 2004, large area of
dead copepods
A trifloat after 14 months in the water
http//psc.apl.washington.edu/AlphaHelix2004.html
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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 FUTURE PLANS - heat flux with satellite
data - intensive IPY array with upper layer TS
measurements
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NEW from this years data?
ACC colder in 2005, but bottom waters warmer 2006
is not starting out colder even though the ice is
unusually heavy