typical arctic profiles - polar science...
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Typical Arctic profiles
Potential Temperature= cold at surface= small subsurface Tmax(only in western Arctic)= deep subsurface Tmax(in all the Arctic)= cooler below
Salinity= fresh at surface (fresher in western Arctic)= S always increases with depth= most salinity change in top few 100ms
Density (Sigma-0)= looks like salinity= most stratification is top few 100ms(i.e., away from surface, ~ barotropic)
Typical Arctic profiles
Bottom Water“the rest”
Western Arctic warmer
ATLANTIC WATERT>0ºC, deeper than 200m
Tmax and layer belowHigher Salinities
Radionuclide tracersEastern Arctic warmer
PACIFIC WATERHigh nutrients
Shallow (<200m) TmaxComparatively fresh (<33psu)Mostly only in Western Arctic
MIXED LAYERUsually thin (no wind stirring)
Pacific and
Atlantic in T-S Space
The watermass
zoo
From Ekwurzel etal, 2001, JGR
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
- is an integrator of the properties of the
Bering Sea
-dominates the water properties of the
Chukchi Sea(Coachman et al, 1975;Woodgate et al, 2005)
Why does a little Strait matter so much?
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
(although the deep Bering Sea Basin may not care)
Bering Strait and the Chukchi SeaNutrient-rich
Anadyr waters
Bering Shelf waters
Alaskan Coastal Current
(warm, fresh, seasonal)
Siberian Coastal Current
(cold, fresh, seasonal)
Stagnation Zones over Herald and
Hanna Shoals
To first order, except for- cooling
- input from coastal
polynyas,
Chukchi dominated by input through Bering Strait
Export to Arctic ~
Input through Bering Strait
Woodgate et al, DSR, 2005, http://psc.apl.washington.edu/Chukchi.html
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
20 100
The role of Pacific waters in the Arctic
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; Woodgate et al, 2006)
Chlorophyll from SeaWifs Satellite
from NASA/Goddard Space Flight Center and Orbimage
Sea ice concentration from SSMI (IABP)
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
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;
Serreze et al, 2006)
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
Bering Strait Basics
- annual mean flow ~0.8 Sv northwards, with an annual 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)
Getting the 4-dimensional pictureBering Strait and Chukchi Sea 2003
Convention line Fluorescence
Chlorophyll from SeaWifs Satellitefrom NASA/Goddard Space Flight Center & Orbimage
23rd June2003
5th – 7th July 2003
Sea surface temperature and altimeter satellite data too
MOST NUTRIENTS
IN THE WEST!!!
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, GRL
AUG APRIL
Seasonal cycle in water properties (Woodgate et al, 2005)
SALINITY31.9 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
Interannual variability?
InterannualVariability
(up to 2004)
Since 2001
- increasing flow
- warming and some freshening
Woodgate et al, 2006, GRL
Velocity(cm/s)
Transport(Sv,
=106m3/s)
Salinity(psu)
Temperature(degC)
InterannualVariability
(up to 2004)
Increasing flow since 2001 mostly attributable to changes in local wind
Woodgate et al, 2006, GRL
Transport change significant
0.7 Sv in 2001 1 Sv in 2005
(long term mean 0.8 Sv)
AnnualMean Wind
(m/s)(NCEP)
Bering Strait Heat Flux
2004 largest heat flux observed
Increasing flow accounts for
50% of the heat flux increases
Woodgate et al, 2006
The Alaskan Coastal Current is~ 1/3 Bering Strait heat
BUT is not yet properly measured
IN PERSPECTIVE:
Extra heat since 2001 could melt an area 800km by 800km of 1m thick ice(Coincidentally, this is
about the same as the ice area lost in this time)
Bering Strait heat flux is ~ 1/5 of Fram Strait heat flux
IN PERSPECTIVE:
Extra freshwater since 2001 is about ¼ of annual mean
river run off(Bering Strait significant
source of Arctic freshwatervariability?)
Bering Strait freshwater flux is ~ 1/3 of Arctic
freshwater input
Bering Strait Freshwater Flux
Woodgate et al, 2006
Alaskan Coastal Current (ACC) carries ~ 10% of freshwater entering the Arctic!
~ 1/4 Bering Strait FWBUT is not yet properly measured
Increasing freshwater flux 2001-2004, but not record high
Increasing flow accounts for 80% of the
freshwater flux increases
Due to more sea water, not more
river water
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
Fram Strait ~ 350km wide~ 2700m deep
Barents Sea- broad shelf sea with topographic
banks- deepest Arctic
shelf sea (average ~ 230m deep)
Barents Sea
St. Anna Trough
YP
YP= Yermak Plateau
Fram Strait
Spitsbergen
Scandinavia
Greenland
Fram StraitVEINS (Variability of Exchanges in the Nordic
Seas) 1997 data cf Schauer et al., 2002
Schauer et al, 2002
INFLOW TO ARCTICSVB (svalbard branch)YPB (Yermak Plateau branch)KB+ (Knipovich Branch)MAW+ (Modified Atlantic Water)DW+ (Deep Water)
OUTFLOW FROM ARCTICPW (Polar Water)MAW- (Modified Atlantic Water)KB- (Knipovich Branch) ??DW- (Deep Water)
Schauer etal, 2002
Nordic Seas (European VEINS project)
Transports in Sv for period 1997-2000
FRAM STRAIT= NORTHWARDWest Spitsbergen Current- warm, salty- estimated 0-7 Sv (Rudels, 1989)- large barotropic component- lots of eddies- varies seasonally- T min in winter, Vol max in winter- splits into several branches- topographically steered- wind driven?
= SOUTHWARDEast Greenland Current- significant velocity shear- lower kinetic energy- eddies more dominant than wind- NOT seasonal
NB East Greenland Current inGreenland Sea MUCH stronger (~ 20Sv),recirculating, wind driven, seasonal
~ 20Sv
Details of temperature change depend on which branch?
(Saloranta and Haugen, 2001)
Max winter SST
South endof Svalbard
1980s and 1990s – agree with Fram Strait- coherent with NAO
1970s – not so good agreement
Warming not as extreme assuggested. - need increase in volume too??
Schauer etal 2002,- large change in WSC Heat
Flux 1997 – 199916±12 TW to 41 ± 5 TW (ref -0.1ºC)
- about half due to warming, half due to more volume
INFLOW VARIES- seasonally
- interannually
Barents Sea Outflow(Schauer et al, 2002, DSR)
Inflow from Greenland Sea ~ 2 SvBlindheim, 1989; Ingvaldsen et al., 2002
Outflow St Anna ~ 2 Sv (varies seasonally)(1Sv summer, 3 Sv winter)
Loeng et al, 1993Cooled and freshened from AWDeep flow little seasonal variability, butinterannual variability, esp in salinity?
Schauer et al, 2002 (and others)
Atlantic “Branch Waters” (BW) Fram Strait (FSBW) and
Barents Sea (BSBW)
FSBW – temperature max
BSBW - inflexion pt
Schauer et al, 2002
BSBW- below or mixing
into Tmax- different in
different years?
~ 2 Sv,likely seasonal
CAA and NaresStrait ChallengesGreenlandEllesmere
Island
Davis Strait
Nares Strait
Lancaster Sound
BaffinBay
Hudson Strait
Fury and Hecla Strait
Bellot Strait
Cardigan Strait(and Hell Gate)
WCBS
Lancaster Sound- BS=Barrow Strait
- WC=WellingtonChannel
Nares Strait
LancasterSound
Cardigan Straitand Hell Gate
Davis Strait ~ 360km wide ~ 1000m deep ~ 2Sv NETalso northward inflow from Atlantic (~ 2Sv northward, 4Sv southward)
Lancaster Sound ~ 68km wide ~ 285m deep ~ 0.7 Sv(seasonal 0.4Sv winter, 1.0 Sv summer) ice covered ~ 10 months
Nares Strait ~ 38km wide ~380m deep ~ 0.8Sv (SNAPSHOT)ice bridges, very strong winds
Cardigan Strait and Hells Gate ~12km wide ~180m deep ~ 0.3 Sv very variable, seasonal not clear, VERY strong (2m/s) tides
Bering Strait ~ 80km wide ~ 50m deep ~ 0.8 Sv highly seasonal e.g. Melling et al, ASOF, in press
Measurements Issues:Remote,ice covered,
small scale variability, strong tides,strong winds, compass issues,simultaneous measurements
Science Issues:Friction (ice, tides)Hydraulic Control
Pressure Head forcingMediated by wind
Rivers!!1989
Units = km3/yr
1 Sv ~ 30,000 km3/yr500 km3/yr ~ 0.02 Sv
- only care about rivers as they are freshwater.
ALL Rivers ~ 3200 km3/yr
Bering Strait ~ 2500 ± 300 km3/yr(Woodgate and Aagaard, 2005)
P-ET ~ 2000 km3/yr
Science, 2002
Woodgate et al, 2006
Bering Strait Freshwater Flux
~ 60 year river change ~= uncertainty of Bering Strait FW
<< Bering Strait interannual change
??????pathways (as fresh, not mixed)
USA
Greenland
Russia
FramStrait
Barents Sea
Bering Strait
Eurasian Basin
Lomonosov R
Canadian Basin
CanadianArchipelago
From theAtlantic
From thePacific
To theAtlantic
To theAtlantic
You arehere
PACIFIC INFLOW~ 0.8 Sv
large seasonal variabilityinterannual variability
high in nutrientsfreshwater, heat fluxes
arctic stratificationpressure head/wind forced
CANADIAN ARCHIPELAGO/NARES STRAIT~ 2 Sv, seasonal variabilitypressure head/wind forced
tides, ice friction, hydraulic control?
BARENTS SEA~ 2 Sv
seasonal variabilityinterannual variability
forcing??In and deeper than T Max
FRAM STRAIT INFLOW(West Spitsbergen Current)
~ 8-10Sv??barotropic
seasonal variabilityinterannual warming/strengthening
much recirculationT Max
FRAM STRAIT OUTFLOW(East Greenland Current)
~ 10Svmore Baroclinic, with eddies
Recirculation????