1 Rebecca Woodgate Polar Science Center, Applied Physics Laboratory, University of Washington, Tom...

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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?


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

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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

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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

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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?

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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

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20%50%


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)

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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

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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

??

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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)




Important for Marine Life

Pacific waters are the most nutrient-rich

waters entering the

Arctic(Walsh et al, 1989)

Largest Interannual

Variability

??

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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)

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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?

Note: in modern times, people have swum, driven and walked across!

www.debbiemilleralaska.com

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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

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Russian-US Long-TermCensus of the Arctic

- John Calder- Kathy Crane

Joint US Russian cruises and moorings

since 2004

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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

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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

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

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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 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

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Most recent change?NOT a continuation of the 2004 warmingBut what about 2007?

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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

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20%50%





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


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

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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

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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.

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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

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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

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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

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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

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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”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.


AUG APRIL

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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; 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

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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)


Atl

Pac

Pacific Nutrient Max

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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

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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

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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)

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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)

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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

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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 ArcticCOORDINATION AND INTEGRATION


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

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

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The Pacific Arctic Gateway

ICE1

23

4


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 year’s

data?

ACC colder in 2005, but bottom waters warmer

2006 is not starting out colder even though the ice is unusually

heavy

1 Rebecca Woodgate Polar Science Center, Applied Physics Laboratory, University of Washington, Tom...

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