Today we have been working in Lazarev Bay bounded on the southwest by Rothschild Island, on the southeast by a small section of the Wilkins Ice Shelf, and on the northeast by Alexander Island. It has been the work site for the R/V Lawrence M. Gould for the past several days. Based on information from scientists on the Gould, the Bay provided us with the possible opportunity for conducting bird, especially AdJ lie penguin, and mammal studies, and ROV studies of under ice krill abundance and behavior. Our current position at 1650 local time on 23 May is -69 19.727°S; -72 20.892°W. Winds are out of the west southwest (145) at about 12 kts and the air temperature is -2.6°C.
Yesterday (22 May), was another ideal period for looking for krill bird and mammal predators to study. Winds were light and the visibility was very good. In the early morning, we had reached the northern end of Charcot Island and at a pre-dawn (0845 local) meeting on the bridge of the Palmer, a consensus was reached that we should head towards the Wilkins Ice Shelf between the northern part of Charcot Island and the Southern end of Rothschild Island in search of penguins and whales to study from the Zodiacs. In spite, of the good meteorological conditions, we spent most of the day in unconsolidated pack ice that was too thick to effectively operate the Zodiacs, yet too unstable to allow a person to walk on. We did go deep into the ice pack and got to a place where there were few if any flying birds around the ship and only some occasional seals (leopards and crabeaters) lying on ice chunks. So around noon, we headed towards Rothschild Island and the site where the Gould was seeing quite a few penguins and seals. Only a couple of penguins were sighted during the daylight period and also a pair of minke whales.
In the mid-afternoon, the ROV was again ready for deployment after its electronics had been repaired from the damage caused by a seawater leak. The ship's track was altered to put us back into the heavier pack ice. At a location just off some very large ice bergs, we stopped and after clearing a hole in the ice pack, the ROV went into the water. The ROV deployment went very well for an hour or so, and krill larval forms and adolescents were observed moving around on the underside of the ice pack. The ROV was brought back on the deck in order to adjust the stereographic video cameras to provide a better view of the underside of the pack ice, but on deck, it was observed that the ROV again had leaked water in the electronics housing. This put an end to the deployment. We steamed at modest speed during the night through varying degrees of pack ice cover to a rendezvous point with the R/V LM Gould in Lazarev Bay, arriving early in the morning on 23 May.
Eileen Hofman reports that during the transit from the front of the Wilkins Ice Shelf back around the tip of Charcot Island, the CTD group started making XBT drops once we reached the inshore portion of survey transect thirteen. XBT drops were continued until we rounded the eastern end of Charcot Island. Once past Charcot Island, the ship stopped in the western end of the embayment between this island and Rothschild Island to the east. One CTD station was done in the western portion of the embayment following deployment and retrieval of the ROV. We continued XBT drops until reaching the rendezvous point with the Gould in Lazarev Bay, to the east of Rothschild Island.
The data from the XBTs and CTD cast filled in the inner portion of shelf that was not included in the basic survey grid. These additional data make the point that nothing is fixed. Adding these few data points to the distribution of the temperature maximum below 200 m resulted in a revision our thoughts on the pattern of flow on the inner shelf. It appears that the southwesterly flow out of Marguerite Bay turns into the embayment between Rothschild and Charcot Island and then flows back out around Charcot Island. This flow pattern removes what was previously thought to be a small easterly current along the inner shelf inside of the southwesterly flowing current. The southwesterly flowing current appears to be continuous around the western tip of Charcot Island and southward as far as the Wilkins Ice Shelf.
The ADCP-derived current patterns produced by Susan Howard show south-southwesterly
flow along the western side of Charcot Island. The current vectors also
show the southwesterly current along the inner shelf inside of survey transects
eleven, twelve, and thirteen. An interesting feature in the ADCP maps is
north-northwesterly flow in front of the Wilkins Ice Shelf, which is opposite
to the flow further offshore. The current velocities associated with this
flow are on the order of 15 to 20 cm/sec. The dynamics resulting in this
flow pattern are unclear and need further investigation. Overall, the ADCP
data appear to be of high quality and are providing a coherent and consistent
picture of the circulation in the area encompassed in the survey.
Bob Beardsley's MET notes update on the past couple of days:
Charcot Bay: After we completed the large-scale CTD/biology survey at Station 84, the NBP make a transit around the western end of Charcot Island and into Charcot Bay, the large bay just south of the island, on May 21. This area seemed a good place to look for whales, birds, thick enough ice for ice sampling and the deployment of a ROV to sample the zooplankton under the ice. The ship steamed slowly eastward towards the ice shelf, collecting some samples along the way, and eventually launched the Zodiacs to look for whales and birds. After sunset, the ROV was tested, and the ship then left the bay and returned to the last CTD station (84) and headed eastward towards Lazarev Bay where the LMG was working. During the roughly 12 hours spent deep in Charcot Bay, the surface forcing conditions showed some of the strongest surface cooling during the entire cruise. This note summarizes that period.
The NBP was deep into Charcot Bay during the period yd 141.4 - 142.05. Winds during this period were relatively weak, dropping from about 20 to 6 kts, primarily from the north. The air temperature dropped as we entered the bay from roughly -2°C to a minimum of -5°C. The sea surface temperature also decreased as the ship got further into the ice and closer to the Wilkins ice shelf, from -1.40°C to a minimum of -1.73°C at our closest approach. The surface salinity decreased into the bay, with the minimum salinities being below 33 psu. The ice was thickest there, and the ship stopped for take ice samples and deploy the ROV.
This bay appears to be protected from the strong southward winds over the continental shelf that generally carry relatively warm and moist air from over the ocean. Instead, the air reaching the ship was quite cold and drier, suggesting it came from Charcot Island and the ice shelf. The mean wind speed was 6.1 m/s, and the mean air-sea temperature difference was Ta - SST = -2.2°C. This negative air-sea temperature difference, lower mean relative humidity (81%), and modest winds combine to produce sensible and latent heat losses of on average 19-24 W/m2. The sky was clear as the NBP entered the bay, resulting in a relatively large mean longwave heat loss of 111 W/m2. With essentially zero shortwave heating, the mean net heat flux was Qnet = -153 W/m2 during the period the NBP was in the bay.
A simple interpretation follows. Charcot Island and the Wilkins ice sheet seem to shelter Charcot Bay from the southward flow of warm, moist air that generally occurs over the continental shelf. The air over the bay is colder and drier (presumably from over land), leading to surface cooling by the sensible and latent components in addition to the longwave cooling. These three terms contribute to make the surface heat loss a regional maximum, causing more ice formation here. Whether this leads to a feedback process whereby more ice means colder air leading to increased sensible and latent cooling, thus more ice is not clear.
After leaving Charcot Bay, the NBP has moved around to the North of
Charcot Island and then eastward to Rothschild Island and Lazarev Bay.
During this run, as the ship has gotten into continuous ice, the air temperature
has continued to drop with larger sensible and latent cooling. The pattern
of larger net heat loss within these more protected bays where ice is being
formed seems robust, but needs to be tested with the surface data being
collected today (23 May).
Chris Ribic and Erik Chapman reported that on 22 May, they began the day in search of a good location to deploy a zodiac to make a second attempt at capturing petrels for diet sampling while we were looking for ice suitable for ROV operations. As noted above, we traveled into the large pancake and brash ice in a large bay between Charcot and Rothschild Islands. However, they saw few petrels at the edge of and within the ice in this area, and they decided to forego any attempt to capture birds. Outside of an Antarctic Petrel and an Adélie Penguin, they saw only Snow Petrels today.
Ari Freidlander reported on 22 May that at first light the ship was
entering the bay on the north and east side of Charcot Island. Conditions
were good, with moderate swell running through pancake and new ice. As
we headed east towards the Wilkins Ice Shelf and the scores of bergs to
the east, the ice became more dense. Eventually made a turn to the north.
Zodiacs were not deployed due to a lack of whale activity. Around 1300,
two minke whales were sighted. At days end, two Adélie penguins,
twenty-two crabeater, and four leopard seals had also been sighted.
Catherine Berchock has summarized her sonabuoy deployments for the
past several days:
May 19th: Only one sonobuoy (#50) was deployed today - around 5 nm from station 81. She listened for 5 hours and heard a couple of yet to be identified sounds which could be seals. It is also possible that she heard some extremely faint humpback calls.
May 20th: The first difar sonabuoy was deployed about 2 miles from station 84. Humpbacks were heard and also some possible minke calls. At 13:00 local time the bearing of one of the calls was determined to be off the starboard beam of the ship. About 5 hours were recorded.
The second difar was thrown out approximately 7 miles from station 84. Catherine was hoping to get a cross bearing with the first buoy, but the signal from the first buoy was lost shortly after deploying the second partly because of our rapid steaming (11-12 kts). She listened to this second buoy for one hour before signal was lost. She deployed a third difar 19 nm from station 84 and listened for an hour. Faint humpback calls were recorded. The calls were too weak, however, to get a directional fix on.
May 21st: The first buoy deployed failed to transmit. The second buoy was deployed 14 nm from station 87 (in the embayment south of Charcot Island), right before we turned around and went back out of the bay. She heard a few grunts and moans, and possibly seal vocalizations. About 3.5 hours of recordings were made.
Catherine went out in the inflatable with Ari in hopes of putting sonobuoys in next to some whales, or at least away from the boat. Engine trouble brought us back to the ship without any deployments. Bob Beardsley was taking care of the receiver end of things here in the dry lab.
May 22nd: Six sonobuoys were deployed today in hopes of getting exact
locations of calling whales. Twelve hours of recordings were made. They
were being deployed about every 5-7 nm, in order to have at least two with
a chance of hearing the same call. Unfortunately she had trouble having
two sonobuoys within range and without excessive ship noise. She did manage
at one point to hear the same loud call on two of the buoys, but one was
defective and impossible to get a bearing on even though the call was loud.
From the bearings, she was able to make with one hydrophone , there were
at least 2 humpbacks around 1000 local time somewhere off the port beam.
At 1400 local, she was able to get a bearing on another humpback that was
in line somewhere between us and the Gould (some 70 nm away). A total of
61 sonobuoys have been deployed to date.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager and
BIOMAPER-II and the MOCNESS were not used during the 22nd of May. Processing of the data continued in between stints on the bridge to take in the awesome Antarctic setting.