Report of Activities on the RVIB N.B. Palmer Cruise 02-04

19 August 2002

 

The search for a suitable site for the Gould's second process station continued into a second day. Although we were not able to reach station 41 because of the tough pack ice conditions, the nearby site that we had reached looked good from a scientific perspective. It was not, however, thought to be a safe place to base the Gould given the pack ice's considerable drift over the past week and the potential for the open leads in the area to close tight under unfavorable wind conditions. During the late night period of 18/19 August, the Palmer and the Gould convoyed back to a location near grid station 42. There the Palmer started station work while waiting for sufficient daylight to make an assessment about the feasibility of that location as a process station. This location was also deemed unsuitable and station 28 was targeted as a third possibility. After the work was completed at station 42, the Palmer and Gould set out for station 28. Heavy ice conditions blocked the approach to station 28 and in the evening we ended up situated near station 27 instead. The site was also found wanting as a base for the Gould. During the course of the late afternoon and evening, discussions ensued between the Palmer and the Gould about other locations to assess. Near midnight on the 19th, a decision was made to backtrack to station 43, where there was a mix of open leads and ice floes that might meet the scientific and the safety criteria. By midnight, the convoy was headed in a westerly direction toward that location.

 

August 19 was the coldest day yet on the cruise. There was a nice start to the day. In the morning around 0700, the winds were out of the northwest about 15 to 20 kts and the air temperature was -5C. The ROV was just coming on deck after its under-ice survey, the ice collectors had completed their sampling, and the CTD was about to be deployed. Within 15 minutes, the wind had changed direction to southwest and sped up to between 25 and 30 kts. The temperature fell rapidly. By 1200, the wind was up to 40 kts and the temperature had dropped to -18.3C. With the high winds, the wind chill on the deck was below -50C. During the late morning and afternoon under these brutal conditions, a 1-m and a 10-m MOCNESS tow and Tucker trawl were completed. Late in the afternoon after the trawl came on board, BIOMAPER-II was deployed for about two hours. The towed body was recovered when white-out conditions (blowing snow) forced both ships to stop for a time. Visibility was so poor that the Captain and mates on the Gould could not see to follow in the Palmer's keel water. The wind remained fierce (25 to 38 kts out of the southwest) into the night and the air temperature dropped down to -23.4C.

 

Coinciding with the wind shift and temperature change was a similar change in the barometric pressure. It had been dropping during the previous day and that continued until the wind shift around 0700 where the pressure bottomed out at 966 mb. For the rest of the day, the pressure rose and reached 987 mb around midnight.

 

CTD Group report (Baris Salihoglu, Eileen Hofmann, Bob Beardsley, Chris MacKay, Francisco (Chico) Viddi, Sue Beardsley)

Throughout the late hours of 17 August and during 18 August, we did an XBT survey during the north-eastward transit to escort the Gould to the second process site. The XBT transect started at the shelf break around survey station 74 and ended on the shelf west of survey station 41.

 

During the north-eastward transit, usable temperature data were obtained from 11 XBT probes, either T-7 or T-4 depending on the depth, spaced at about 10 nm intervals. Additional probes were dropped, but interference from the extensive sea ice cover caused the wire to break prior to reaching the bottom or the maximum depth of the XBT.

 

Towards the middle of the transit (between 67 33.6'S 73 17.9'W and 67 30.6'S 72 28.3'W) we started experiencing very low ice coverage at the water surface and surface water temperatures above freezing (~-1.82). This also helped the ship to speed up in the open water.

 

After plotting the vertical temperature distribution along the transit, it was observed that the (relatively) warm surface waters coincided with an intrusion of warm (1.6-1.75C) Upper Circumpolar Deep Water around 400 m. This water mass is characterized with temperatures above 1.6C and it is generally located between 400 to 600 m deep. As we got further on-shelf we started observing surface water temperatures that are close to the freezing point of sea-water (-1.83C to -1.85C) and this coincided with the high ice coverage.

 

During the April-May 2002 U.S. SO GLOBEC survey cruise (NBP02-02), highest water temperature below 200 m was 1.4C, at the same location. The temperatures as high as 1.75C were only observed at the shelf edge. This indicates that during the two months separating this cruise and NBP02-02, Circumpolar Deep Water has moved onto the west Antarctic Peninsula continental shelf.

 

MOCNESS/ADCP/OPC Report (Phil Alatalo, Ryan Dorland, Peter Wiebe, Dicky Allison, Scott Gallager, Gareth Lawson)

Diverse assemblages of organisms continue to characterize the catch from MOCNESS nets at nearly all depths. Tow #6 was a cold mid-morning deployment on Aug. 19th, 10:30 at Station 42. In general, we found more biomass at deeper depths than on previous tows, and less biomass than expected at shallower depths. Copepods, chaetognaths, and krill predominated from 384 m to 75 m. Salps and ostracods were also common to most deep depths, with amphipods, worms, pteropods, molluscan larvae, radiolarians, larvaceans, and siphonophores present throughout. Thysanoessa sp. furcilia were observed at 300-200m, furcilia and juveniles at 200-150 m, and mostly adults at 125-75 m. Biomass above 75 m was low; a single fish larva was caught between 50-0 m.

 

BIOMAPER II group report (Gareth Lawson, Peter Wiebe, Scott Gallager, Phil Alatalo Dicky Allison, Alec Scott)

The BIOMAPER II was deployed in the early evening of August 18 for a two hour towyo between stations 42 and 27. This was by far our most exciting (and nerve-wracking) deployment to date as the wind was blowing at 40 knots, making it difficult to keep the tow-body in position on deck, and bringing the temperature with wind chill down to a bracing -51C! Scattering was low throughout most of the water column, other than a weak layer coincident with the pycnocline at 90-110 m. Images that we captured with the VPR suggest that this layer was composed of copepods. We also observed one single denser patch of backscatter at 150 m, of unknown composition. At present, only our 120 and 200 kHz transducers are operational. Since lower frequencies are able to penetrate farther through water and our lowest frequency transducers (43 kHz) are not working, we are currently limited to looking 300 m from the tow-body. The body is generally towed between the surface and 100 to 200 m, which means that we can make observations no deeper than 500 m. On this particular occasion, we sent the tow-body down to only 100 m, and with the bottom at 450 m, we were unable to determine whether the deep layer observed close to the bottom two days ago was also present in this area.

 

Current Position and Conditions

The convoy to station 43 ended around noon on 20 August and suitable conditions were found to establish the L.M. Gould's second process station. The Palmer left the site in mid-afternoon after completing station work there and headed for station 44. We are currently working at that station. Our position at 0030 on 21 August is -67 05.426′S; -73 20.631′W. The air temperature is -16.2C and the barometric pressure is 982.8 mb. Winds are around 10 kts out of the south-southwest (199). Skies are clear.

 

Cheers, Peter