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Rebecca WoodgatePolar Science Center, Applied Physics Laboratory, University of Washington,
Tom Weingartner, Terry Whitledge, Ron Lindsay, Knut AagaardIn 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
Pacific Inflow to the Arctic Ocean – Changes in the Bering Strait throughflow
2Little Diomede Island, Bering Strait
Pacific Gateway to the Arctic Ocean
Why care about the Bering Strait?
What are we doing, and what are we finding?
What next?
3Little Diomede Island, Bering Strait
The Arctic Ocean
4
NOAA.gov
RUSSIA
NORWAY
GREENLANDUSA
CANADA
ICELAND
You are downhere somewhere
ARCTICOCEAN
5
Figure from Harry Stern, UWTOTAL Arctic Ocean area ~ 2 x lower 48 ~ 1.5 x USA
2500 miles 1500
mile
s
Smallest of world’s 5 major oceans(Pacific, Atlantic, Indian, Southern, and Arctic)
Russia
Greenland
Alaska
Canada
6
80N
Greenland
Russia
Alaska
Lomonosov Ridge
Mendeleev Ridge
Nansen-Gakkel R
Eurasian B
asin
Canadian/Amerasian BasinBering Strait
Fram Strait
Barents Sea
GreenlandSea
Canadian Archipelago
- 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!
ATLANTIC LAYER- warm, salty
- largest volume input
7
80N
Greenland
Russia
Alaska
Lomonosov Ridge
Mendeleev Ridge
Nansen-Gakkel R
Eurasian B
asin
Canadian/Amerasian BasinBering Strait
Fram Strait
Barents Sea
GreenlandSea
Canadian Archipelago
- 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
PACIFIC WATER - nutrient rich
- source of heat and freshwater
8
Bering Strait Connections
Weingartner and Danielson
Only Pacific Gateway to the Arctic Ocean
To the SOUTH – BERING SEA(over 50% of US fish catch)- Bering Strait throughflow coulddrain 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.
BeringStrait
ARCTIC
PACIFIC
9
Bering Strait BasicsThe 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?
10
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
500
50
50
350 150 350
150
15
50
50
50
150
20100
The role of Pacific waters in the Arctic
11
20%50%
The role of Pacific waters in the Arctic
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)
Chlorophyll from SeaWifs Satellite
from NASA/Goddard Space Flight Center and Orbimage
Sea ice concentration from SSMI (IABP)
12
The role of Pacific waters in the Arctic
Important for Arctic StratificationIn 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)
Pacific winter waters
13
The role of Pacific waters in the Arctic
Significant part of Arctic Freshwater
BudgetBering Strait throughflow
~ 1/3rd of Arctic Freshwater
(Wijffels et al, 1992; Aagaard & Carmack, 1989;
Woodgate & Aagaard, 2005)
ARCTIC FRESHWATER FLUXES
Bering Strait ~ 2500 km3/yr(0.08 Sv)
Arctic Rivers ~ 3300 km3/yrP-E ~ 900 km3/yr
Fram Strait water ~ 820 km3/yrFram Strait ice ~ 2790 km3/yr
Canadian Archipelago ~ 920 km3/yr
Largest Interannual
Variability
??
14
The role of Pacific waters in the Arctic
Significant part of Arctic Freshwater
BudgetBering Strait throughflow
~ 1/3rd of Arctic Freshwater
(Wijffels et al, 1992; Aagaard & Carmack, 1989;
Woodgate & Aagaard, 2005)
Important for Arctic StratificationIn 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)
Important for Marine Life
Pacific waters are the most nutrient-rich
waters entering the
Arctic(Walsh et al, 1989)
Largest Interannual
Variability
??
15
Global role of Bering StraitA Freshwater source for the Atlantic Ocean
Pacific waters exit the Arctic into Atlantic
Deep Water formationregions
(Jones et al, 2003) Broecker, 1991
- slowing the Atlantic Ocean overturning circulation (see Wadley & Bigg, 2002)
- affecting deep western boundary currents & Gulf Stream separation (Huang & Schmidt, 1993)
16
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?
Note: in modern times, people have swum, driven and walked across!
www.debbiemilleralaska.com
17
Observational Challenges of Bering Strait
Eastern Bering Strait in Winter
- shallow (50 m)
- stratified in spring/summer
- important seasonal boundary currents (e.g. Alaskan Coastal
Current)
Sea Surface Temperature 26th August 2004, from MODIS/Aqua level 1courtesy of NASA/Goddard Space Flight Center, thanks to Mike SchmidtGrey arrow marks the Diomede Islands (Little and Big Diomede).
- ice covered (keels to 20 m) from ~ January to April
- split by the US-Russian border
18
Russian-US Long-TermCensus of the Arctic
- John Calder- Kathy Crane
Joint US Russian cruises and moorings
since 2004
19
Since 19901-4 near–bottom moorings
Your instrument
here!!!!
Bering Strait Long-term Moorings
- Understand the Physics- Design a Monitoring Scheme
Since 2007 (International Polar Year)
8 moorings with upper and lower sensors
20
Seasonal cycle in water properties (Woodgate et al, 2005)
SALINITY 31.9 to 33 psu
TEMPERATURE-1.8 to 2.3 deg C
TRANSPORT0.4 to 1.2 Sv
(30 day means)
(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
ICE1
2
3
4
AUG APRIL
WHY CARE?
Seasonally varying input to the Arctic
Ocean
- temperature- salinity-volume
- equilibrium depth(~50m in summer~120m in winter)
-nutrient loading
21
Bering Strait
properties from 1990 to present
22
Interannual Variability
(up to 2004)
Warming since 2001
Increasing flow since 2001 (mostly attributable to changes in local wind)
Woodgate et al, 2006, GRL
23
Scale of variabilityExtra 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
24
Most recent change?NOT a continuation of the 2004 warmingBut what about 2007?
25
Mooring data and SST (MODIS and AVHRR)
2004 2005 2006 2007
Tem
per
atu
re (
deg
C)
2007 from Satellites?
Sea surface height and temperature data suggest 2007 heat fluxwill 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
26
20%50%
The role of Pacific waters in the Arctic
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)
Sea ice concentration from SSMI (IABP)
- SIGNIFICANT amount of heat
- TRIGGER for ice-albedo feedback
- CONDUIT for local heat (as low density)
- PAN-WESTERN-ARCTIC subsurface Temperature source
27Little Diomede Island, Bering Strait
Future for Bering Strait?
STATUS NOW? - Moorings since 1990, (EEZ, ice, stratification issues)- High resolution array 2007-2009 by NSF/NOAA.- Monitoring scheme design
NEXT? NSF/NOAA proposal for completing design
Arctic Observing Networkand operational monitoring scheme – insitu and satellite?
WHY CARE?- 1/3rd of Arctic Freshwater- large % Arctic oceanic heat- most nutrient-rich inflow
Local and global effects onice, climate and ecosystems
Significant interannual change
psc.apl.washington.edu/BeringStrait.html
28
29
Using MODIS to constrain the ACC(with Ron Lindsay)
Black lines: weekly averages of eastern channel SST from MODIS
For - sea surface temperature (SST) - width of ACC - timing of ACCAnd thus heat flux, and maybe FW flux
30
Long-term moorings in Bering Strait
From 1990 to present
T, S and velocity at 9m above bottom
A1 = western ChannelA2 = eastern Channel
A3 = combination of A1/2A3’ (up north)
A4 = Alaskan Coastal Current
Not all moorings are deployed all years!
Sea Surface Temperature 26th August 2004, from MODIS/Aqua level 1courtesy 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.
31
Moorings in Bering StraitShort (~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
32
CTD cruises e.g. Bering Strait & Chukchi Sea 2003
Photo from akbrian.net
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
33
Bering Strait 2004
1st Sept 2004 5th Sept 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
34
Getting the 4-dimensional pictureBering Strait and Chukchi Sea 2003
Convention line Fluorescence
Chlorophyll from SeaWifs Satellite from NASA/Goddard Space Flight Center & Orbimage
23rd June2003
5th – 7th July 2003
Sea surface temperature and altimeter satellite data too
35
http://psc.apl.washington.edu/BeringStrait.html
36
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)
37
What Drives the Bering Strait Throughflow?Velocity = “Pacific-Arctic Pressure Head” + “Wind Effects”Mean=northward Mean=northward Mean=southward
10-6 sea surface slope gives rise to pressure gradient
between Pacific and Arctic Oceans (Coachman & Aagaard, 1966;
Stigebrandt, 1984)
But WHY?- freshwater transport from Atlantic by atmosphere?- steric height difference?
- global winds? (Nof)
ASSUMED constant- but why should it be?
(Woodgate et al, 2005 DSR)
- 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 varianceand the seasonal cycle (Roach et al, 95)
In the mean, the winds oppose the
pressure head forcing. Thus, in
winter, when winds are strongest, the northward flow is
weakest.
Woodgate et al, 2005
AUG APRIL
38
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
39
Influence of shelf waters
Use silicate to track Pacific Water
in the Chukchi Borderland
Along the Chukchi Shelf, upwelling and diapycnal mixing of
lower halocline waters and Pacific waters
(Note ventilation by polynya waters
couldn’t give this T-S structure)
Woodgate et al, 2005
Atl
Pac
Pacific Nutrient Max
40
Bering Strait and Arctic Freshwater
BERING STRAIT~ 0.8 Sv
(moorings)
~32.5 psu (summer 1960s/70s)
Freshwater Flux relative to 34.8 psu
~ 1670 km3/yr OTHER OUTPUTSFram Strait water = 820 km3/yrFram Strait ice = 2790 km3/yr
Canadian Archipelago = 920 km3/yr+ ...
OTHER INPUTSRunoff = 3300 km3/yrP-E = 900 km3/yr
+ ...
Aagaard & Carmack, 1989 (AC89)
P-E
41
The Alaskan Coastal Current (ACC)
Salinity July 2003 from the Diomede Islands (left) to the Alaskan Coast (right)
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
Summer Observations10km 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
42
The Alaskan Coastal Current July 2002-2003
VELOCITY NORTHA2 bottom central eastern
channel 47m A4 the Alaskan Coastal Current 34m, 24m, 14m
SALINITYA2 bottom central eastern
channel 48m A4 the Alaskan Coastal
Current 39m
Black solid line = temperatures at freezing
surface currents ~ 170 cm/s (at depth 70 cm/s) across-strait salinity gradient of ~ 3 psupresent until late December 2002 (JD365), returns late April 2003 (JD480)
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)
43
Put it together (with stratification and ice)
Salinity July 2003 from Little Diomede (left) to the Alaskan Coast (right)
Summer StratificationChukchi~ 2 layer system, with salinity step
of ~ 1 psuAssume stratified 6 months,
~350 km3/yr
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)
Total ~ 400 km3/yr (~20% of AC89 estimate)
Ice Transport- annual mean NORTHWARD ice flux of
130 ± 90 km3/yr(despite almost 2 months of net southward
ice flux)
44
The Bering Strait Freshwater Flux(Woodgate & Aagaard, 2005)
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
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
Arctic Rivers ~ 3300 km3/yr P-E ~ 900 km3/yrFram Strait water & ice ~ 820 km3/yr & ~ 2790 km3/yr
Canadian Archipelago ~ 920 km3/yr
45
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%
46
The Pacific Gateway to the ArcticCOORDINATION AND INTEGRATION
http://psc.apl.washington.edu/BeringStrait.html
WITHOUT ACC/Stratification
47
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
48
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
49
The Pacific Arctic Gateway
ICE1
23
4
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
50
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
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