A large intense low pressure system centered to the north continued to dominate our weather on 20 August. Most of the day we had high winds and blowing snow. For a while in the afternoon, the winds died down into the 20 kt range, but this evening as the barometer dropped again, the winds picked up and gusts were up to 50 kts. Currently we are stopped to do a plummet net tow in a small open water pond in a region where the ice is tightly compacted and there are few open water areas. Our position at 2335 is -69° 01.213S; -74° 49.214W. The air temperature is higher than it has been for several days at -5.9°C. The wind is out of the east (094) at 27 to 40 kts and the barometer is still falling and reads 944.4 mlb.
On 19 August, work was completed at stations #75 and 76. These two inshore stations were very difficult to approach and a great deal of time was spent attempting to get close to the intended locations. As it was, we only managed to get within about 5 miles of station #75 and 9 miles of station #76. There were patches of open water and some leads in the area, but they were interspersed between heavy ridges that the ship had to back and ram to get through. A contributing factor was the weather in the morning, which was cloudy with poor visibility. The very low contrast lighting and the absence of shadows made it hard to see the ice structure well from the bridge. There was some suspicion that by the time we stopped to do the work, we were in fast ice, ice attached to the coastline. That ice is often harder to break through because the ice has less give, no place to be displaced to when the ship comes through it. Both stations were in an area with quite variable bottom topography and depths ranging from 100 m to more than 500 m. BIOMAPER-II was deployed for a short distance after leaving station #76 in an open lead that ran for a mile or so, but then had to be picked up as we ran into heavier ice conditions.
John Klinck reports on 19 August the CTD group did one station today near the coast of Alexander Island on the 180 line. Heavy ice prevented occupation of the planned station but was instead about 5 miles offshore of the planned location.
Station #75 (123 m) is a shallow station with a mixed layer to about 50 m. Between 50 and 100 m, temperature, salinity, and density gradually increase. A layer of uniform temperature and salinity 10 m thick occurs at the bottom. There are no interior property minima or maxima due to the shallowness of the water.
Jay Peterson reports that on 19 August, ADCP backscatter data indicated a scattering layer from 70 to 90 m at station #75. At station #76, the highest intensity scattering appeared at 80 m and close to the bottom at 200 m.
An OPC/MOCNESS tow was attempted near station #75, but once again had to be canceled due to heavy ice conditions. In lieu of data from a tow, a brief description of the OPC and the type of information that can be gathered by it follows:
The Optical Plankton Counter (OPC) is an instrument designed to be towed through the water and detect particles. The OPC allows water to pass through a rectangular tunnel 25 cm wide and 2 cm tall. Within the tunnel particles pass through the beam of light and cast a shadow on a photo-detector. The area of the shadow is calculated and sent up a wire to a computer where it is converted to an equivalent spherical diameter (ESD), the diameter of the sphere that would block the same amount of light as the particle that passed through the beam. The instrument is capable of accurately measuring particles which have an ESD of 0.25 to 14 mm, at a rate of 200 particles per second, and are recorded with the geographic location, depth, and time of detection. Data from the OPC is typically grouped into size categories and depth intervals, ultimately providing a good estimate of the amount (in terms of numbers and volume) of plankton at specific depths and locations. The compact size of the instrument allows it to be attached to virtually any towed body and provide real-time information about the planktonic composition of the water column. During the SO GLOBEC II cruises, the OPC is attached to a MOCNESS frame where net composition analyses will help to describe the species composition of the particles detected by the OPC.
Ana Sirovic reports that she attempted to deploy 2 difar sonobuoys, one as the Palmer was doing a pass over already 'cleared' water for a MOCNESS tow attempt and the other just after the divers came back on the ship. Both failed within 1 minute. The ice conditions are proving to be impossible not just for more complicated things (such as towing or diving), but also for something as simple as a sonobuoy deployment!
Chris Ribic and Erik Chapman report that on 19 August (JD-231), they surveyed for 3 hours and 45 minutes near station #75 and on the way to station #76. The Palmer was backing and ramming and swerving the ship's path for most of the survey time, so today's survey can only be used to qualitatively assess the birds in this area. Nevertheless, low abundance of four bird species were seen, including Southern Giant Petrels, which haven't been seen for several days. A single Adélie Penguin was counted and a small number of Snow Petrels, despite traveling through areas with open water where these birds are usually seen. A summary of their observations is the following:
Primary Ice: Mostly vast floes. Some open water in cracks between floes.
|Southern Giant Petrels||3|
Scott Gallager reports on an ROV under ice survey that took place on 19 August at station #75. Considerable effort was required by the bridge to position the ship in the heavy, fast ice without washing away the krill community. After opening a small hole, the ship backed next to the opening using its forward thruster only. This allowed access to a virtually undisturbed ice edge. SeaRover went into the water at 0943 and was recovered at 1133 UTC. In an attempt to minimize the effect of the blown starboard thruster, the port thruster had been repositioned onto the dorsal surface just aft of the vertical thruster giving the only means of forward propulsion. This worked reasonably well allowing three straight transects originating from the deployment location at the starboard a frame away from the ship. Transect 1 extended 35 m towards the stern 45° relative to the ship axis. Transect 2 extended 60 m directly perpendicular to the ship, and transect 3 extended 40 m at about 130° towards the bow. Pressure ridges were observed to a height of 1 to 1.5 m above the ice surface while extending to 3 to 4 m below the surface. Under ice topography was relatively smooth to cuspate between ridges, while jagged and extremely 3 dimensional along subsurface ridges. Transect 1 ran along one ridge at a depth of 3 m followed by excursions onto the more smooth surface of the fast ice. Larval krill were observed to be scattered with observations every few seconds along the smooth surface and in relatively dense aggregations along the ridge. Aggregations were not as numerous or intense in concentration as observed at previous stations offshore, but present nonetheless. Transect 2 began under smooth ice and crossed a ridge perpendicularly about 20 m from the ship. Along the smooth fast ice, scattered furcilia were observed drifting within a meter of the under ice surface, but not at the ice interface. Aggregations were first encountered as the ROV descended 2 m to cross below the ridge. Ten to 20 furcilia per aggregation were typical. Aggregations occurred immediately below an ice feature, usually a block that had been extruded downward as a function of horizontal compression forces. Transect 3 was composed entirely of smooth ice with no ridges. Just a few furcilia were observed as the ROV traversed 2 m below the ice interface.
Conclusion: Furcilia were scattered under smooth ice and concentrated into a few aggregations under ridges. It is hopped that the ADCP record will shed some light on the under ice water currents which, potentially, could be inducing formation of eddies in association with ice features. Water motion within such eddies in combination with specific swimming behaviors, could explain why aggregations occur only under feature-rich conditions.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, and S. Gallager):
BIOMAPER-II was deployed on 19 August around 2300 for a short run between stations #76 and 77. The trackline for the towyo was back along the line leading to station #76. It started in a floe and moved into a lead with thin ice cakes distributed over the surface. The bottom along this route started out at about 100 m, and then dropped off to below 400 m as the Palmer traversed a canyon. At the beginning of the towyo, the towed body was at 40 m and the top of a strong scattering layer was about 15 to 20 m below it. BIOMAPER-II was lowered down into the depths where the layer resided, but the layer just moved below us and it was not possible to get the VPR into it and get images of the layer's residents. Because of this apparent avoidance, we strongly suspect that they were krill. The towed body was recovered about an hour after the start because heavier ice was in the offing.
No 1-m2 MOCNESS tow was done on 19 August.