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> July 12 - 2005
post Jul 13 2005, 03:37 PM
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July 12, 2005

At the end of the last line (a series of closely set stations) we moved through Unimak Pass last night around 10 PM. We're now steaming toward our next line which will be in the Bering Sea. While underway, I took the opportunity to speak with Dr. Rebecca Woodgate. Rebecca is the senior oceanographer at the Polar Science Center of the University of Washington in Seattle. Although she usually does her work from the Alpha Helix (a former Amazon basin river boat, now at the University of Alaska, Fairbanks), the Helix is tied up for a year, and she is doing her work on this cruise in collaboration with Jackie Grebmeier. While on board, she will be retrieving three moorings and setting out three new ones. Each mooring is out for approximately one year, and this particular series of moorings has been on-going since 1990, set out in the same place each year. If you're interested, you can check out the website at: http://psc.apl.washington.edu/HLD/Bstrait/bstrait.html.

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Each mooring contains instruments to measure water flow/velocity, temperature and salinity, and ice thickness and motion. One has an instrument to measure fluorescence (used to assess phytoplankton). To measure the water flow/velocity, the ADCP (Acoustic Doppler Current Profile) sends an acoustic signal that reflects off organisms and inanimate objects drifting in the water column. Because it comes back with a slightly different frequency, you can tell how fast the water is moving. It picks up a signal every two meters, thus showing differences in the currents. For example, while there is a one knot current at the east side of the Bering Strait, the Alaska Coastal Current, above the mooring closest to the coast, flows at two - three knots. Five percent of the fresh water entering the Arctic Ocean comes through this current; the Bering Strait, as a whole, brings in one third of the fresh water entering the Arctic Ocean. The Arctic Ocean flows into the Atlantic Ocean, and scientists are concerned about the amount of fresh water entering the Atlantic. It's possible that an increase in fresh water could disturb global ocean circulation. It's this circulation pattern that brings the warm Gulf Stream water along the northeastern shores of North America and on to parts of Europe. A change in the circulation might mean harsher winters in some parts of Europe. Global warming doesn't mean that temperatures increase everywhere!

I was interested to find out how a mooring instrument could measure ice thickness and motion. An acoustic sonar device on the top of the mooring sends a signal up to the ice above it. From the return signal, you can tell how deep the mooring is under the ice. If you know the actual depth of the mooring instrument, it's possible to calculate how much ice is above it.

There are really two channels running through the Bering Strait, one in U.S. waters, and one in Russian waters. Rebecca will retrieve and set out two moorings in the U.S. side of the Strait and one just north of the two channels. The northern moorings will give integrated data about flow in both channels. Once the moorings have been retrieved, Rebecca will download all but the ice thickness data. With instruments that collect data every 30 - 60 minutes, a year's worth of data is extremely valuable.

Rebecca is hopeful that she will be able to retrieve all three moorings. Each mooring is anchored by train wheels to the bottom. Two releases (one for back up) are just above the anchor, followed by large glass spheres for flotation, and then the series of instruments. A large, round, steel float sits at the top of the mooring. Once we're in the area of the mooring, Rebecca will lower a hydrophone into the water to send a coded acoustic signal to 'wake up" the instruments which will then send a signal back. The hydrophone can pick up a signal from up to ten miles. When we're close enough to see the mooring, Rebecca will trigger the release and retrieve it.

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I asked Rebecca what changes she's seen since 1990 when this series began. >From 1990-1997, there was a warming trend, from 1997-2000, scientists saw a period of cooling, and from 2000 on, we are once again into a warming period. In a few days, she'll have data from this past year.
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