Observations of pack ice properties in the Weddell Sea

Observations of pack ice properties in the Weddell Sea

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S. F. ACKLEY and S. J . SMITH

Nov 13 2040

U.S. Army Cold Regions Research and Engineering Laboratory Hanover, New Hampshire 03755

D. B. CLARKE*

590

Lamont-Doherty Geological Observatory of Columbia University Palisades, New York 10964

Nov 12 2030 Nov 12 330

Among the programs undertaken during the U.S.-U.S.S.R. Weddell Polynya expedition (wEPoLEx-81) (Gordon, Antarctic Journal, this issue) was observation of ice as the ship was under way. These observations culminated in a daily map of ice conditions and a narrative observation log (Ackley and Smith 1982). A section of the map prepared by the Soviet scientists who participated in the program is shown in figure 1. The ship's track as it left the ice is represented by the solid arrow line moving north and northwest. Locations of ice sampling stations taken along this line are given in Clarke and Ackley (Antarctic Journal, this issue). The narrative log included information about ice concentration, ridging, amounts of thin ice and open water, and unusual ice features. As indicated in figure 1 and confirmed by the log, a transition in ice characteristics occurred between 60°20' and 58°20'S latitude. In the southernmost regions the pack was characterized by fields of ice ("breccia") consisting mostly of medium-size floes (100-500 meters in diameter) and larger floes. Farther north, nearer the ice edge, the floes became smaller (20 meters or less in diameter) and surprisingly uniform. These northern observations coincided with observations of noticeable wave and swell penetration into the ice. Wave action apparently caused the breakup of the ice into smaller pieces. North of the area shown, ice of small floe size continued to be observed in lesser concentrations (6 tenths or less) for approximatey an additional 70 nautical miles. The photographs in figure 2, taken at 0648 on 12 November and 0610 on 13 November, show the contrast in floe sizes between these two times. These characteristics also were observed on the inbound track at similar distances from the outer pack ice edge (first ice sighting). On the basis of these observations, we divide the pack ice zone into three relatively distinct regions: • Ice edge region (within 0 to 60 nautical miles of the northern limit of pack ice). This area is characterized by uniformly small

floes (less than 30 meters in diameter), ice of less than 10 tenths concentration, and continuous wave propagation. Frontal structure in the ocean might also be associated with these ice properties, but these ice changes should be closely correlated with conductivity-temperative-depth (cTD) and expendable bathythermograph (xBT) observations to determine the extent of this relationship. Increases in bio-

*present address: U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755. 1982 REVIEW

600 Nov 11 0330 2°W

0°

LD Rotting ice

2°E

Ice breccia

concentration 1 (scale 0-5)

Ice concentration (in tenths) j I Snow-encrusted ice concentration 2 (scale 0-3) Floe diameter Ice thickness cake - 10-15 cm EIE3 Average ice thickness (cm) Q2 Ice 2-20 m

AS Ice ridging

concentration 2 (scale 0-5)

0

Icebergs concentration 1(scale 0-9)

Small floe = 15-30cm 20— 100 m Medium floe / 30-70 cm 100-500 m

Brash ice

Large floe X 70— 120 cm 0.5-2 km ____ Salt flowers //120 cm concentration 1 (scale 0-5) Example: Ice concentrations 9 tenths Wind speed and direction *979, composed of 8 tenths ice brec\ (arrow denotes direction; ciaof medium floes(30- 70 cm i each feather denotes 5 m/s o thickness) and small floes (30 and 1/2 feather 2 m/s). 70 cm thickness); and 1 tenth small flows () 5 c thickness). Ship drifting

Figure 1. Ship's track showing sea ice conditions. Symbols are explained in the table.

logical activity (birds, mammals, zooplankton, and phytoplankton) also were apparent in this region. Ship navigation was unimpeded, and it was easy to maintain speeds in excess of 5 knots. • Ice edge—pack ice transition zone (within 60 to 160 nautical miles of the outer limit of pack ice). The boundary between the ice

edge region and the transition zone is not well defined. We traversed regions of continuous small floes (9 to 10 tenths concentration) coinciding with noticeable swell propagation. As the swell became severely attenuated and discontinuous in the southern regions, the floe sizes ranged from about 30 meters in diameter at the outer edge into a jumble of very large and some smaller floes. Similar features were apparent on the inbound track. We traversed nearly 100 105

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S.

-'-

Figure 2. Photographs of sea ice conditions on 12 and 13 November 1981.

nautical miles of pack (160 to 60 nautical miles from the outer ice edge) from our first observations of swell motion in the ice until we encountered the "ice edge region." Ship navigation in this transition region was fairly easy, with only occasional difficulty encountered in crossing some of the larger floes. Figure 1 essentially brackets this region. • Deep pack (at distances greater than 160 nautical miles from the outer limit). This region had ice concentrations usually in

excess of 9 tenths (most commonly 10 tenths concentration). Narrow, parallel leads oriented according to the prevailing stress were the only open-water areas. Floes usually exceeded several kilometers in diameter. Ship navigation was exceedingly difficult, except where broad leads happened to coincide with the ship's course. These conditions were encountered between 150 and 180 nautical miles from the first (or last) ice sighting on both the inbound and outbound legs (south of the track shown in figure 1).

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In most satellite microwave images, the ice edge-pack ice transition zone (60-160 nautical miles) appears as an area of lesser concentration. Our observations did not confirm this. We usually found ice concentrations of 9 to 10 tenths. We believe that this discrepancy arises from a change in the microwave signal; this change is caused by the infiltration of seawater into the snow/ice interface as a result of wave action breaking the floes and water surging into this porous layer. This process changes the surface emission characteristics of the ice at microwave frequencies. A comparison of the images received on board with our observations indicates that the Soviet meteorological satellites ("Meteor"), which use visible and infrared imagers, depicted the ice concentration in this region more accurately (that is, obtained higher values). Future work will include using our 300 photographs of ice conditions and the ice observation log (Ackley and Smith 1982) to correlate observed ice concentrations with those obtained from U.S. satellite mapping. Also unexpected was our observation that noticeable swell propagation occurred at great distances from the outer pack limit. On both the inbound and outbound legs, we measured swell amplitudes of 0.25 meter or greater at distances greater than 120 nautical miles from the outer limit. This limit previously was thought to be about 60 nautical miles (100 kilometers) for the highly concentrated ice observed here (Wadhams 1980). A feature contributing to the deep pack propagation of swell in this region is the long wavelength (greater than 200 meters) and high amplitude (5-7 meters) of swell incident on the ice edge, characteristic of the long-fetch, high-wind regime of the southern ocean. A complete report covering the entire cruise period and including interpretations of the ice map, the narrative log, sea level photographs, and satellite photographs is available from the authors. We wish to thank Captain F. A. Pesyakov and the crew of the Somov for their support, and Edgar Andreas, Ivan Chuguy, and Ed Lysakov for their technical assistance. We also acknowledge the support of the other members of the Soviet science and meteorological support group, E. I. Sarukhanyan, chief scientist. This research was supported by National Science Foundation grants DPP 80-06922 (to the U.S. Army Cold Regions Research and Engineering Laboratory) and DPP 80-08011 (to the Lamont-Doherty Geological Observatory).

References Ackley, S. F, and Smith, S. J . 1982. Weddell Polynya Expedition ice map interpretation and daily ice observation log (USACRREL Tech. Note 1R777). Hanover, N.H.: U.S. Army Cold Regions Research and Engineering Laboratory. Clarke, D. B., and Ackley, S. F 1982. Physical, chemical, and biological properties of winter sea ice in the Weddell Sea. Antarctic Journal of the U.S., 17(5). Gordon, A. L. 1982. The U.S.-U.S.S.R. Weddell Polynya Expedition. Antarctic Journal of the U.S., 17(5). Wadhams, P. 1980. Ice characteristics in the seasonal sea ice zone. Cold Regions Science and Technology, 2, 38-87.

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