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Rebecca A Woodgate Applied Physics Laboratory, University of Washington,

The Pacific Gateway to the Arctic Ocean – Changes and Implications of the Bering Strait Throughflow. Rebecca A Woodgate Applied Physics Laboratory, University of Washington, Knut Aagaard, Tom Weingartner, Terry Whitledge, Ron Lindsay With thanks to Jim Johnson, Seth Danielson, Mike Schmidt,

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Rebecca A Woodgate Applied Physics Laboratory, University of Washington,

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  1. The Pacific Gateway to the Arctic Ocean – Changes and Implications of the Bering Strait Throughflow Rebecca A Woodgate Applied Physics Laboratory, University of Washington, Knut Aagaard, Tom Weingartner, Terry Whitledge, Ron Lindsay With thanks to Jim Johnson, Seth Danielson, Mike Schmidt, and crews of the Alpha Helix, the Laurier, and the Khromov Funded by ONR and NSF-OPP, SBI Program Little Diomede Island, Bering Strait

  2. The Pacific Gateway to the Arctic Ocean – Changes and Implications of the Bering Strait Throughflow BERING STRAIT BASICS - why? how? what? NEW RESULTS - increasing heat and freshwater fluxes FUTURE WORK - intensive observing system for IPY, focused on dynamics - biogeochemical data? Little Diomede Island, Bering Strait

  3. 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 • - 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?

  4. Bering Strait good proxy of Arctic inflow Bering Strait dominates the Chukchi Sea Woodgate et al., 2005, DSR and via http://psc.apl.washington.edu/HLD

  5. FRESHWATER HEAT The Role of Pacific waters in the Arctic Significant part of Arctic Freshwater Budget ~ 1/3rd of Arctic Freshwater (Wijffels et al, 1992; Aagaard & Carmack, 1989; Woodgate & Aagaard, 2005) Important for Marine Life - most nutrient-rich waters in the Arctic (Walsh et al, 1989) Implicated in the seasonal melt-back of ice - summer (?) source of near-surface heat to the Arctic (Paquette & Bourke, 1981; Ahlnäs & Garrison, 1984) Important for Arctic Stratification - bolsters the cold halocline layer, insulating ice from underlying warm Atlantic Water (Shimada et al, 2001, Steele et al, 2004)

  6. Broecker, 1991 Global role of Bering Strait A Freshwater source for the Atlantic Ocean Pacific waters exit the through the Fram Strait and the Canadian Archipelago (Jones et al, 2003) May - slow the Atlantic Ocean overturning circulation (e.g., Wadley & Bigg, 2002) - affect the deep western boundary currents & Gulf Stream separation (Huang & Schmidt, 1993)

  7. Long-term moorings in Bering Strait From 1990 to present (many years ONR funded) 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 deployed all years 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 Grey arrow marks the Diomede Islands (Little and Big Diomede). Russian EEZ line passes between the islands.

  8. Bering Strait properties from 1990 to present

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

  10. Interannual Variability (up to 2004) Velocity (cm/s) Temperature (degC) Salinity (psu) Since 2001 - increasing flow - warming and some freshening Woodgate et al, 2006, GRL Transport (Sv, =106m3/s)

  11. - 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) 10-6sea surface slope between Pacific and Arctic Oceans? (Coachman & Aagaard, 1966; Stigebrandt, 1984) But WHY? - atmospheric freshwater transport from Atlantic? - steric height difference? - global winds?(Nof) ASSUMED constant - but why should it be? (Woodgate et al, 2005 DSR) In winter, when winds are strongest, the northward flow is weakest. Woodgate et al, 2005 AUG APRIL What Drives the Bering Strait Throughflow? Velocity = “Pacific-Arctic Pressure Head” + “Wind Effects” Mean=northwardMean=northwardMean=southward

  12. Interannual Variability (up to 2004) Annual Mean Wind (m/s) (NCEP) Transport change significant 0.7 Sv in 2001 1 Sv in 2005 (long term mean 0.8 Sv) Increasing flow since 2001 mostly attributable to changes in local wind Woodgate et al, 2006, GRL

  13. Bering Strait Heat Flux 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 2004 largest heat flux observed Increasing flow accounts for 50% of the heat flux increases The Alaskan Coastal Current is ~ 1/3 Bering Strait heat BUT is not yet properly measured Woodgate et al, 2006

  14. Bering Strait Freshwater Flux IN PERSPECTIVE: Extra freshwater since 2001 is about ¼ of annual mean river run off (Bering Strait significant source of Arctic freshwater variability?) Bering Strait freshwater flux is ~ 1/3 of Arctic freshwater input 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 Alaskan Coastal Current (ACC) carries ~ 10% of freshwater entering the Arctic! ~ 1/4 Bering Strait FW BUT is not yet properly measured Woodgate et al, 2006

  15. Bering Strait 2007-2009(IPY Program) 1) understand dynamics of the strait 2) quantify heat and freshwater fluxes (WITH Alaskan Coastal Current and stratification) 3) develop flow-proxies from wind/model/insitu/satellite data 4) design a monitoring network for the Bering Strait - NSF and NOAA funded - US, Russian, Canadian collaboration Multi-mooring array - velocity profiling (ADCP) - upper layer T-S (ice-avoidance sensors) - bottom pressure gauges - annual CTD sections - satellite altimeter and SST data

  16. Using Satellites to constrain the ACC SST in June 2004 For - sea surface temperature (SST) - width of Alaskan Coastal Current (ACC) - timing of ACC And thus heat flux, and maybe FW flux Black lines: weekly averages of eastern channel SST from MODIS

  17. But beyond the physics? North of the Diomedes, Sept 2004, large area of dead copepods A trifloat after 14 months in the water http://psc.apl.washington.edu/AlphaHelix2004.html

  18. The Pacific Arctic Gateway http://psc.apl.washington.edu/BeringStrait.html woodgate@apl.washington.edu 2001-2004 CONCLUSIONS Flow increase (from 0.7 to 1 Sv) dominantly due to weakening of local (southward) wind Extra flow explains 50% of heat flux increase and 80% of freshwater increase Extra heat enough to melt 800km x 800km of 1m thick ice (comparable to observed ice loss) Extra freshwater ~ 10% of annual arctic freshwater input FUTURE PLANS - strait dynamics (local/remote forcing) - biogeochemical measurements? - influence on the Arctic and beyond ...

  19. The Alaskan Coastal Current (ACC) Summer Observations 10km wide, 40m deep, wedge-shapeSummer 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 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 Salinity July 2003 from the Diomede Islands (left) to the Alaskan Coast (right)

  20. 5th Sept 2004 1st Sept 2004 Bering Strait 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

  21. The Bering Strait Freshwater Flux(Woodgate & Aagaard, 2005) 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 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 Arctic Rivers ~ 3300 km3/yr P-E ~ 900 km3/yr Fram Strait water & ice ~ 820 km3/yr & ~ 2790 km3/yr Canadian Archipelago ~ 920 km3/yr

  22. 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) Pacific Nutrient Max Atl Pac Woodgate et al, 2005 Influence of shelf waters Use silicate to track Pacific Water in the Chukchi Borderland

  23. 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 (from, for example, ice sheet collapse) can “vent” through the Bering Strait, allowing a speedy return to deep convection in the Atlantic. Land Bridge for migration of mammals and people? www.debbiemilleralaska.com Note: in modern times, people have swum, driven and walked across!

  24. - shallow (50 m), but covered by ice (keels to 20 m) from ~ January to April - stratified in spring/summer - split by the US-Russian border Two boundary currents - Alaskan Coastal Current (ACC) in the east present from ~ spring to mid-winter (Paquette & Bourke, 1974) - Siberian Coastal Current (SCC) present seasonally in some years in the west (Weingartner et al, 1999) Special Observational Challenges of Bering Strait Eastern Bering Strait in Winter

  25. 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

  26. CTD cruises e.g. Bering Strait & Chukchi Sea 2003 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 Photo from akbrian.net

  27. Getting the 4-dimensional pictureBering Strait and Chukchi Sea 2003 23rd June 2003 Convention line Fluorescence Chlorophyll from SeaWifs Satellite from NASA/Goddard Space Flight Center & Orbimage Sea surface temperature and altimeter satellite data too 5th – 7th July 2003

  28. 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

  29. 3 2 ICE 1 4 AUG APRIL 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) • WHY CARE? • Seasonally varying input to the Arctic Ocean • - temperature • - salinity • -volume • - equilibrium depth • (~50m in summer • ~120m in winter) • nutrient loading (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

  30. P-E Bering Strait and Arctic Freshwater Aagaard & Carmack, 1989 (AC89) BERING STRAIT ~ 0.8 Sv (moorings) ~32.5 psu (summer 1960s/70s) Freshwater Flux relative to 34.8 psu ~ 1670 km3/yr OTHER INPUTS Runoff = 3300 km3/yr P-E = 900 km3/yr + ... OTHER OUTPUTS Fram Strait water = 820 km3/yr Fram Strait ice = 2790 km3/yr Canadian Archipelago = 920 km3/yr + ...

  31. The Alaskan Coastal Current July 2002-2003 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 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) 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)

  32. Put it together (with stratification and ice) Summer Stratification Chukchi~ 2 layer system, with salinity step of ~ 1 psu Assume stratified 6 months, ~350 km3/yr Ice Transport - annual mean NORTHWARD ice flux of130 ± 90 km3/yr (despite almost 2 months of net southward ice flux) Salinity July 2003 from Little Diomede (left) to the Alaskan Coast (right) Total ~ 400 km3/yr (~20% of AC89 estimate) 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)

  33. Arctic Freshwater revisedSerreze 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%

  34. 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

  35. 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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