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Arctic Change 2009 – Woodgate – Wk 8 - Mon 1

Arctic Change 2009 - Woodgate

Bering Sea Bathymetry

BeringStrait

Nunivak

St. Lawrence

St.Matthew

Pribilof

A l e u t i a n I

s l a n d sAmukta

Pass

UnimakPass

UnAlaskaDutch HarborBowers

Ridge

BowersBasin

Kamch

atka S

trait

AleutianBasin

ShirshovRidgeKamchatka

Basin

Norton SoundGulf of Anadyr

AmchitkaPass

NearStrait

Shpanberg

StraitAnadyr S

trait

Map courtesy of NASA Arctic Change 2009 - Woodgate

Ice coverage on the Southeast Bering Sea shelf, 1972-2006

• Decreased ice cover since late 1970s

• Very little or no ice in recent years (increase in 2006)

Courtesy of Jim Overland, PMEL

Cold/CoolPeriod

Regime shift

MoreVariable

See Stabenoet al, 2007


Regime shift?

Grebmeier et al


Sea Ice Area Anomaly for Bering Sea




2006


From J.Zhang, UW


Trends?? Causes??

Stabeno et al, 2007

- Air temperatures?? (Arctic air masses)

- Wind advection of Ice??

- ocean inflow from the south

- warmer the preceeding summer- careful quantification of terms

- modeling (if coupled)


Aleutian Passes – “a porous boundary for fluxes of water, heat, salt and nutrients” Stabeno et al, 2005

KAMCHATKA~ 12 Sv (5-15) South

but northwarddeep flow toBering SeaDeep Water

NEAR> 10 Sv (6-12)

North

BULDIR????

AMCHITKANet ~ 2-3 Sv

North(-4 to + 4 Sv)

TOO DEEP TO MIX NUTRIENTSGOOD TO MIX NUTRIENTS

TOO SHALLOWTO MIX NUTRIENTS

ACC~ 2 SvShallowFresh

SeasonalALASKAN STREAM

~ 20 Sv

SEGUAM~0.4 SvNorth

TANGA????

AMUKTA~ 4 SvNorth

SAMALGA????

AKUTAN~ 0.1 Sv

North

UNIMAK~ 0.3 Sv

North

ALEUTIAN NORTH SLOPE CURRENT

~ 10 Sv (4Sv > 1000m), ~ 20km wide,

stable but seasonal variability?

If wider than R,can have 2-way flow


Aleutian Passes – “a porous boundary for fluxes of water, heat, salt and nutrients” Stabeno et al, 2005

= (at least ) 10 passes= deeper in the west

= medium depth channelssource for bottom nutrients

= Flow can be bidirectional within a Pass= Barotropic important (geostrophic estimates too low)= Maybe seasonal, wind-driven?, = Meanders of Alaskan Stream= Tidal mixing very strong

FRESHNUTRIENTS??



The Pacific Arctic Gateway

ICE1

23

4

Bering and Chukchi- indicator of

hydrography and change

Arctic- Most nutrient-rich, - 1/3rd of freshwater, - source of heat and

halocline waters

Globally- influences Atlantic

overturning circulation

Why is Bering Strait important?

Northward flowing, daily/weekly flow reversals

Large seasonal cycles in input to Arctic Ocean,

including equilibrium depth

Interannual variability, suggesting freshening and

warming in recent years

Bering Strait Basics

http://psc.apl.washington.edu/BeringStrait.html


The Bering Sea/Bering Strait RelationshipANSF= Aleutian

North Slope Current

BSC = Bering Slope Current

From Stabeno,

Schumacher & Ohtani,

1999

Alaska(n) Coastal Current (warm, fresh,

seasonal)

Anadyr waters (colder, saltier,

nutrient-rich)

Bering Shelf

Waters (in

between!)

Exit route!

By providing an exit, Bering Strait influences flow over the Bering Sea Shelf – in volume, BS throughflow can drain BS shelf in ~ 1 year

(although the deep Bering Sea Basin may not care)


Anadyr and Shpanberg Straits

et al., JGR, 1988

CSR, 1993

Generally Northward?

Must be comparable to Bering Strait

Anadyr Stronger

Summer and Winter “modes” different

Strongly related to wind- thus highly variable

NOT WELL MEASURED– what about models??


Compared to observations?

(not many observations)

Overland et al, 1996, JGR - drogue-less buoy (thus wind and current driven)- 2 months from Cape Navarin to Bering Strait

Weingartner et al, Website



St Lawrence Island (and its polynya)

Bering Sea Brine NOT mostly from polynyasBUT important habitat??

Mean westward flow of freshwater (drifters)

Interannual variability?Danielson et al, JGR, 2006


Bering Slope Current

Stabeno and Reed, 1994, JPO

Johnson et al, JPO, 2004

Herman et al, 2002 adapted from Stabeno et al, 2001

= fed from Aleutian North Slope Current= 3-5 Sv??= initially follows slope, then appears to leave it= meandering with eddies= upwelled waters/eddies for productivity

Evidence from drifter tracks

= could have 2 modes,stable and unstable?


, JPO, 1975

Very tricky to get something meaningful if- eddies are important (THEY ARE!)- barotropic is important (IT IS!)


Bering Sea Greenbelt

Shumacher and Stabeno, http://www.pmel.noaa.gov/np/pages/wnew_bioprod.html



Regimes and Fronts of the Bering Sea Shelf

Best Science Plan, from Kachel

SLOPE WATERS- in water depths > 200m over the slope

**Schumacher and Stabeno 1998**Stabeno et al, 2001Kachel et al, 2002Hunt et al, 2002

OUTER SHELF DOMAIN- between 100m and 200m depth contours

summer = 3 layers- top – well mixed (wind)- middle – increasing density- bottom – well mixed (tides)

--------------------Slope Front-------------------------------

MIDDLE SHELF DOMAIN- between 50m and 100m depth contours

summer = strongly stratified 2 layer- top – wind mixed- bottom – tidal mixed

T diff > 8 deg, density set by Twinter = fully mixed to 100m

------Middle Transition Zone (~ 50 km wide)---------

COASTAL DOMAIN- inshore of 50m depth contour

summer = well mixed/weakly stratified

------Inner or Structural Front (5-30km wide)-------


Tides important!!e.g. Kowalik and Stabeno, 1999, JGR

Mean Circulation~ 5 cm/s

Tidal circulation~ 30 cm/s

Rectification andtrapping

= Mix bottom 40m= x-slope transport?


Tides over the whole shelf


Many SST fronts Belkin and Cornillon, 2005

= complex= fronts align with topography

= mostly over shelf



Best Science Plan, from Kachel

The Cold PoolStabeno et al, 2002

WINTER- ice melt- ice formation

SPRING/SUMMER- surface warming- stratification

COLD POOL

=Cold (< 2 deg C) Water at depth,covered by warmer surface waters

= Variable year to year

= Important for fish!!

M4M2


Depth Averaged Temperature (°C) at M2: 2 deg C increase in winter after 2000: above freezing point. Summer 2005 warmest temperature

StabenoData from Stabeno, courtesy of Overland


PDOPacific

Decadal OscillationMantua et al, 1997

Typical wintertime Sea Surface Temperature (colors),Sea Level Pressure (contours) and surface windstress (arrows) anomaly

patterns during warm and cool phases of PDO

WARM PHASE COLD PHASE

leading principal component of North Pacific monthly sea surface temperature variability (poleward of 20N for the 1900-93 period)


.. recently also need Victoria

PatternBond et al, 2003

= now second EOF more important

PDO important

VP important

= different pattern of SST variability

SST = PC1(t)* EOF1(x,y) + PC2(t)* EOF2(x,y) + ..

PDO Victoria Pattern



Bering Sea MODELS

Herman et al, 2002, DSR- regional model- telescoped grid

(-good overview paper)NOW DOING BIOLOGY

Clement et al, 2005, DSR- northern Hemisphere model

(30N) (with US “channel”)- ?? Bstrait comparison(but at least they tried!)

Zhang et al (BESTMAS)- global, but grid focused

in Bering Wang et al, JGR, 2007- regional


UNRESOLVED ISSUES

= Cross-shelf exchange

= Seasonal and interannual variability

= Cold Pool, shelf processes and advection pathways

BASICS OF THE PHYSICAL BERING SEA SYSTEM

ALSO= Tides= Eddies= Atmos forcing= Different shelfregimes= upwelling


Stabeno et al, 1999

ALASKA COASTAL CURRENT

- fresh!!!, but ~ 2 Sv

(Alaska ..not Alaskan ..)

BERING SLOPE CURRENT

- turns into meanders and eddies

Aleutian North Slope Current

BERING STRAITTHROUGHFLOW

ALASKAN STREAM

~ 20 Sv

Main Currents in the Bering Sea

BERINGSEA

SHELFKAMCHATKA CURRENT

~ 10 Sv, variable (7-15 Sv)

- may recirculate


M2 M4

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