1 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 Pacific Inflow to the Arctic Ocean – Changes in the Bering Strait throughflow

2 Little 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?

3 Little Diomede Island, Bering Strait The Arctic Ocean

4 NOAA.gov RUSSIA NORWAY GREENLAND USA CANADA ICELAND You are down here somewhere ARCTIC OCEAN

5 Figure from Harry Stern, UW TOTAL Arctic Ocean area ~ 2 x lower 48 ~ 1.5 x USA 2500 miles 1500 miles 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 Basin Canadian/Amerasian Basin Bering Strait Fram Strait Barents Sea Greenland Sea 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 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 Basin Canadian/Amerasian Basin Bering Strait Fram Strait Barents Sea Greenland Sea 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 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. Bering Strait ARCTIC PACIFIC

9 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?

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, 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 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) Pacific winter waters

13 The role of Pacific waters in the Arctic Significant part of Arctic Freshwater Budget Bering Strait throughflow ~ 1/3 rd of Arctic Freshwater (Wijffels et al, 1992; Aagaard & Carmack, 1989; Woodgate & Aagaard, 2005) ARCTIC FRESHWATER FLUXES Bering Strait ~ 2500 km 3 /yr (0.08 Sv) Arctic Rivers ~ 3300 km 3 /yr P-E~ 900 km 3 /yr Fram Strait water ~ 820 km 3 /yr Fram Strait ice ~ 2790 km 3 /yr Canadian Archipelago ~ 920 km 3 /yr Largest Interannual Variability??

14 The role of Pacific waters in the Arctic Significant part of Arctic Freshwater Budget Bering Strait throughflow ~ 1/3 rd 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) 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 Strait A Freshwater source for the Atlantic Ocean Pacific waters exit the Arctic into Atlantic Deep Water formation regions (Jones et al, 2003) Broecker, 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!

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 26 th August 2004, from MODIS/Aqua level 1 courtesy of NASA/Goddard Space Flight Center, thanks to Mike Schmidt Grey 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-Term Census of the Arctic - John Calder - Kathy Crane Joint US Russian cruises and moorings since 2004

19 Since 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 TRANSPORT 0.4 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 ICE AUGAPRIL 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 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 Sv in 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 warming But what about 2007?

25 Mooring data and SST (MODIS and AVHRR) Temperature (degC) 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

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

27 Little Diomede Island, Bering Strait Future for Bering Strait? STATUS NOW? - Moorings since 1990, (EEZ, ice, stratification issues) - High resolution array by NSF/NOAA. - Monitoring scheme design NEXT? NSF/NOAA proposal for completing design Arctic Observing Network and operational monitoring scheme – insitu and satellite? WHY CARE? - 1/3 rd of Arctic Freshwater - large % Arctic oceanic heat - most nutrient-rich inflow Local and global effects on ice, climate and ecosystems Significant interannual change psc.apl.washington.edu/BeringStrait.html

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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 ACC And 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 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 26 th 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.

31 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

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 st Sept th 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 picture Bering Strait and Chukchi Sea 2003 Convention line Fluorescence Chlorophyll from SeaWifs Satellite from NASA/Goddard Space Flight Center & Orbimage 23 rd June th – 7 th July 2003 Sea surface temperature and altimeter satellite data too

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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 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 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 variance and 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 AUGAPRIL

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 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 km 3 /yr OTHER OUTPUTS Fram Strait water = 820 km 3 /yr Fram Strait ice = 2790 km 3 /yr Canadian Archipelago = 920 km 3 /yr +... OTHER INPUTS Runoff = 3300 km 3 /yr P-E= 900 km 3 /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 26 th August 2004, from MODIS/Aqua level 1, courtesy of Ocean Color Data Processing Archive, NASA/Goddard Space Flight Center, thanks to Mike Schmidt 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 km 3 /yr) But only present ca. April - December

42 The Alaskan Coastal Current July 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 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) ACC annual mean velocity 40 cm/s; transport 0.08 Sv, salinity 30.3 psu Annual Mean Freshwater Flux km 3 /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 Stratification Chukchi~ 2 layer system, with salinity step of ~ 1 psu Assume stratified 6 months, ~350 km 3 /yr Annual Mean Freshwater flux = Previous estimate AC km 3 /yr + ~ 400 km 3 /yr (Alaskan Coastal Current) + ~ 400 km 3 /yr (stratification and ice) ~ 2500 ± 300 km 3 /yr (Woodgate & Aagaard, GRL, 2005) Total ~ 400 km 3 /yr (~20% of AC89 estimate) Ice Transport - annual mean NORTHWARD ice flux of 130 ± 90 km 3 /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 km 3 /yr including ~ 400 km 3 /yr (Alaskan Coastal Current) ~ 400 km 3 /yr (stratification and ice) ~ 1/3 rd 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 Arctic Rivers ~ 3300 km 3 /yr P-E~ 900 km 3 /yr Fram Strait water & ice ~ 820 km 3 /yr & ~ 2790 km 3 /yr Canadian Archipelago ~ 920 km 3 /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 Arctic COORDINATION AND INTEGRATION 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

49 The Pacific Arctic Gateway ICE SEASONAL VARIABILITY – significant in T,S and volume INTERANNUAL VARIABILITY - very influenced by local wind - warming and freshening since 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