Little DiomedeLittle Diomede Island, middle of the Bering Strait

BERING STRAIT: Pacific Gateway to the Arctic

Rebecca Woodgate, Knut Aagaard
University of Washington, USA
 Tom Weingartner, Terry Whitledge
University of Alaska, Fairbanks, USA
Igor Lavrenov
Arctic and Antarctic Research Institute, Russia

Corresponding author: Rebecca Woodgate (woodgate@apl.washington.edu)

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BERING STRAIT BASICS

Bering Strait Basic Facts

- WHY is the Bering Strait throughflow important?
- WHAT are we doing?
- WHAT are we learning?

2007-2009 International Polar Year in the Bering Strait

A Decade in the Bering Strait
Bering Strait Ice Flux

Other Publications
RECENT PUBLICATIONS

Observed increase in Bering Strait oceanic fluxes from the Pacific to the Arctic from 2001 to 2011 and their impact on the Arctic Ocean water column Woodgate et al, accepted 2012 GRL

Quantifying Sea-Ice Volume Flux using Moored Instrumentation in the Bering Strait Travers 2012 MSc Thesis

A Synthesis of Exchanges Through The Main Oceanic Gateways to the Arctic Ocean Beszczynska-Moeller et al., Oceanography, 2011.

The 2007 Bering Strait Oceanic Heat Flux and  anomalous Arctic Sea-ice Retreat Woodgate et al, GRL, 2010
 

Interannual changes in the Bering Strait Fluxes of Volume, Heat and Freshwater between 1991 and 2004 Woodgate, et al, GRL, 2006

Some Controls on Flow and Salinity in Bering Strait Aagaard, et al, GRL, 2006

Monthly Temperature, Salinity and Transport Variability of the Bering Strait Throughflow. Woodgate, et al, GRL, 2005

Revising the Bering Strait Freshwater Flux to the Arctic Ocean. Woodgate & Aagaard, GRL, 2005

Technical Report - Correction of Teledyne Acoustic Doppler Current Profiler (ADCP) Bottom-Track Range Measurements for Instrument Pitch and Roll, Woodgate and Holroyd, October 2011

Technical Report - Using A3 as a climate station for the Bering Strait Throughflow, Woodgate et al, August 2007


BERING STRAIT BASICS
  - The only ocean gateway between the Pacific and Arctic, and ~85km wide, ~55m deep
- Divided into 2 channels by the two Diomede Islands
- Covered by sea-ice from ~January to April
- ANNUAL mean flow northward, but can flow southward for a week or more
- Water speeds highly variable, from ~2.5 knots (120 cm/s) northward to ~1 knot (50 cm/s) southward depending on local wind
- flow driven (supposedly) by a pressure gradient between the Pacific and the Arctic oceans, opposed by the local winds
- Water surface temperature freezing in winter, max around 12 deg C in summer
- Annual mean flow ~0.8 Sv (800,000m3/s)


(Right is a schematic of the flows in the region from Danielson and Weingartner, click on the image to enlarge it)
schematic of
                  oceanic flows in Bering Strait region

WHY IS THE BERING STRAIT THROUGHFLOW IMPORTANT?
For the Chukchi Sea
- primary source of nutrients for the Chukchi (
one of the most biologically productive regions of the world oceans)
- dominates the water properties of the Chukchi
For the Arctic
- primary source of nutrients for Arctic ecosystems
- melts back sea-ice in summer,
- provides cold waters in winter to
stabilize the upper Arctic ocean (thus influencing ice thickness and upper ocean mixing)
- provides ~40% of the freshwater input to the Arctic
- water properties determine ventilation depth of the Arctic
For the World Ocean
- an important part of the global freshwater cycle
- believed to influence the Atlantic overturning circulation
Pacific waters can be traced through the Arctic and out through Fram Strait and the Canadian Archipelago



Sea-surface temperature for 26th August 2004  in  the Bering  Strait  region.  In the east, red indicates the warm Alaskan Coastal Current in  the  east. Black dots mark our mooring sites, A1, A2,  A3  and  A3' (black  dots).  White  areas indicate clouds.
(MODIS/Aqua level 1 image courtesy of Ocean  Color Data  Processing Archive, NASA/Goddard Space Flight Center.)

 
WHAT ARE WE DOING?

- measuring the long-term variability of volume and water properties in the Bering Strait region

We measure temperature, salinity, water velocity and sometimes ice thickness and motion, nutrients, fluorescence, transmissivity with subsurface oceanographic moorings (similar to the picture on the left)

In summer/autumn, we take oceanographic sections in the Strait and the southern Chukchi Sea (see cruise reports)

Since 1990, moorings have been deployed with only a 1 year break.
We place moorings to monitor flow through the western (A1) and eastern (A2) channels of Bering Strait.  The western channel is in the Russian EEZ, an area to which we have only had occasional access.  In other years the western channel has been monitored by a proxy site at A3

Left: Schematic of a typical mooring.
ULS = Upward-looking sonar to measure ice thickness
Steel Float  and Trifloat = for buoyancy
RCM9=Aanderaa Acoustic Current meter to measure water velocity and temperature
SBE=Seabird sensor measuring temperature, salinity, fluorescence, transmissivity NAS=nutrient measuring instrument
RT=Acoustic releases for recovering mooring

WHAT ARE WE LEARNING?

That Bering Strait flows vary from year to year .. and that the 2004 heat flux is the highest recorded since 1990.
Changes in the Bering Strait Fluxes of Volume, Heat and Freshwater between 1991 and 2004 Woodgate, et al, submitted GRL, 2006

That Bering Strait provides ~40% of the freshwater input to the Arctic Ocean.
Revising the Bering Strait Freshwater Flux to the Arctic Ocean. Woodgate & Aagaard, GRL, 2005

Quantifying the seasonal variability of the Bering Strait throughflow, with implications for significant seasonal variation in how the Pacific waters ventilate the Arctic.  
Monthly Temperature, Salinity and Transport Variability of the Bering Strait Throughflow. Woodgate, Aagaard, Weingartner, GRL, 2005


A DECADE IN THE BERING STRAIT
(for data to 2003, click here)


Monthly mean temperature and salinity in Bering Strait during 1990-2000.  The western channel of the strait (mooring A1) lies in the Russian EEZ and has often not been accessible.  The northern site (mooring A3), which lies just east of the Russia-U.S. convention line, is influenced by the flows through both channels.  It therefore can serve as a partial surrogate for the western channel site.  Note the pronounced annual cycle in both temperature and salinity.  The average peak-to-peak amplitude of the salinity variation is about 1.5 psu and is driven by freezing, which expels salt into the water.  In addition to the annual cycle of temperature and salinity, there are important longer-term variations.  In particular, the record of salinity through the past decade shows that the Bering Strait inflow to the Arctic freshened about 1 psu during 1991-92 and then remained relatively fresh until 1999-2000, when about one-half of the earlier freshening effect was reversed.  The temperature of the inflow has also changed significantly during the past decade, with a dramatic long-term warming that peaked during 1996-97.  This was followed by rapid cooling, so that water as cold as observed in summer 2000 was last seen in 1990-91.


BERING STRAIT ICE FLUX
A combination of ice thickness data (e.g. from Upward Looking Sonars) and ice motion (e.g. from Acoustic Doppler Current Meters) can yield estimates of ice flux.
For 1990 to 1991, these data suggest a mean annual northward ice flux, even though the ice flux at the start of the season is southward.
  a) Time series of the northward velocity of water at 25m depth (red) and ice (blue) show a very strong correlation between the ice and the water motion.
  b) Timeseries of ice thickness (assuming thickness = 1.13 x keel depth) shows 
- the thickening of ice over the winter
- that in the early winter (i.e. before day 352) northflowing water carries no ice.
  c) From this, we can estimate the northward transport of water (red) in 106 m3/s and ice (blue) in 105 m3/s, (assuming the strait has a width of 65km and a triangular cross-section of maximum depth 55m).
  d) Cumulative transports of water (red) in 2x1012 m3 and ice (blue) in 1011 m3 show that the cumulative ice transport is southward for the first part of the winter,  even though the cumulative water transport is always northward.
Timeseries of ice and velocity Bering Strait


CRUISE REPORTS
Bering Strait 2000  - RV Alpha Helix  - HX235

Bering Strait 2001  - RV Alpha Helix  - HX250
Bering Strait 2002  - RV Alpha Helix  - HX260
Bering Strait 2003  - RV Alpha Helix  - HX274
Bering Strait 2004  - RV Alpha Helix  - HX290

Bering Strait 2005 - CCGS Sir Wilfrid Laurier - SWL2005

REFEREED PUBLICATIONS (back to top)

Woodgate, R.A.,  K. Aagaard, and T.J. Weingartner, Monthly temperature, salinity, and transport variability of the Bering Strait throughflow. Geophys. Res. Lett., Vol. 32, No. 4, L04601 10.1029/2004GL021880, 2005

Woodgate, R. A., and K. Aagaard, Revising the Bering Strait freshwater flux into the Arctic Ocean, Geophys. Res. Lett., 32, L02602, doi:10.1029/2004GL021747, 2005
 
Walsh, J.J., D.A. Dieterle, F.E. Muller-Karger, K. Aagaard, A.T. Roach, T.E.  Whitledge, and D. Stockwell, CO2 cycling in the coastal ocean. II. Seasonal organic loading of the Arctic Ocean from source waters in the Bering Sea, Continental Shelf Res., 17,1-36, 1997.

Roach, A.T., K. Aagaard, C. H. Pease, S.A. Salo, T. Weingartner, V. Pavlov, and M. Kulakov, Direct measurements of transport and water properties  through Bering Strait, J. Geophys. Res., 100, 18,443-18,457, 1995.

Coachman, L.K., and K. Aagaard, Transports through Bering Strait: Annual and interannual variability, J. Geophys. Res., 93, 15535-15539, 1988.

Aagaard, K., A.T. Roach, and J.D. Schumacher, On the wind-driven variability of the flow through Bering Strait, J. Geophys. Res., 90, 7213-7221, 1985.

Schumacher, J.D., K. Aagaard, C.H. Pease, and R.B. Tripp, Effects of a shelf  polynya on flow and water properties in the northern Bering Sea, J. Geophys. Res., 88, 2723-2732, 1983.

Coachman, L.K., and K. Aagaard, Re-evaluation of water transports in the vicinity of Bering Strait, in The Eastern Bering Sea Shelf: Oceanography and Resources, vol. 1, edited by D.W. Hood and J.A. Calder, pp. 95-110, National Oceanic and Atmospheric Administration, Washington, D.C., 1981.

Coachman, L.K., K. Aagaard, and R.B. Tripp, Bering Strait: The Regional  Physical Oceanography, 172 pp., University of Washington Press, Seattle, 1975.

Coachman, L.K., and K. Aagaard, On the water exchange through Bering Strait, Limnol. Oceanogr., 11, 44-59, 1966.

For use of any of these figures, please contact Rebecca Woodgate (woodgate@apl.washington.edu)
© Polar Science Center, University of Washington, 2004

We gratefully acknowledge financial support for this work from  the Office of Naval Research (ONR), High Latitude Dynamics program, the National Science Foundation (NSF), and the National Oceanic and Atmospheric Administration (NOAA).
 
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