Today (12 May), as yesterday, was a good day for making oceanographic observations. Both days the winds and seas were down, and today visibility was good under a mostly cloudy sky. Sea ice and Ice Bergs were always present to some degree. We are currently steaming toward Station # 54 with winds out of the North Northeast (033) at 15 kts and our current positions is -68 42.859°S; 70 59.410°W (1615 local).
10 May was another classic Antarctic stormy day with sustained winds in the 40 to 50 knot range and gusts in the 50's. The winds intensified during late afternoon and into the evening, and most of the work was centered on drops of XCTD's and the towyoing of BIOMAPER-II between 30 to 40 m and 250 m, although we were able to do a CTD at Station 44. Problems with the BIOMAPER-II towing wire (described below) around mid-night on 10 May and continued high winds and seas, caused the cancellation of the CTD work at Stations 45 to 48 and the use of XCTD's instead. By earlier morning on the 11th, however, the winds had died down and the seas were dramatically lower. In part this was because in the southern end of Marguerite Bay that we had entered, the sea surface was ice covered and many smallish icebergs were present that damped the underlying storm swell. Low clouds and a fine mist which froze to the metal surfaces of the ship made visibility limited for most of the day. Still it was a spectacular introduction to the beauty of the Antarctic winter seascape. The calm conditions made for excellent Seabeam bathymetric data acquisition and the data exposed the steep walls of the canyon that cuts into the heart of Marguerite Bay.
Eileen Hofmann reports on May 11th, that after waiting out the 10 May storm, the CTD group resumed making casts at survey station 44. However, a second storm made this a short-lived return to regular stations. At survey station 45 we again did a XCTD because of winds gusting in excess of 40 knots and 10-12 foot seas.
The temperature distribution below 200 meters continues to confirm the strong influence of Upper Circumpolar Deep Water (UCDW) on the west Antarctic Peninsula continental shelf. The temperature along survey transect seven is suggestive of a large UCDW intrusion in the southern portion of the study grid. It will be interesting to see if this is confirmed as we occupy the more southerly stations.
The nutrient distributions produced by Howard Rutherford and Rebecca Conroy show strong UCDW upwelling along survey transect 3. The nitrate distribution in particular shows downwelling associated with the anticyclonic portion of the meander in the southern ACC boundary and upwelling in the cyclonic portion of the meander. These nutrient distributions match the patterns seen in the hydrography and in the ADCP current distributions produced by Susan Howard.
At this point in our cruise, we are getting a consistent and coherent picture of the circulation and hydrography on the west Antarctic Peninsula continental shelf. The shelf is strongly influenced by episodic intrusions of UCDW and these events modify the distribution of the coastal shelf water. It will be interesting to see if the hydrographic patterns are seen in the distributions of plankton and top predators.
May 12th: The CTD group has now completed sampling along seven of the survey transects. The hydrographic structure continues to confirm the existence of a large intrusion of Upper Circumpolar Deep Water (UCDW) in the northern portion of the study grid and is suggestive of a second intrusion in the southern part of the grid.
The final CTD station along survey transect seven (station 51) was far into Marguerite Bay in the channel that leads to the George VI ice shelf. This station turned out to be located in a deep hole that may be the end point of the canyon that runs across the shelf and into Marguerite Bay. This bathymetric feature was well resolved by the Sea Beam, which showed sheer walls that dropped off rapidly as the ship steamed to the station location.
The maximum temperature below 200 m at this location was 1.26°C which suggests that this area has been recently flooded with UCDW. The cold water associated with the coastal region that was observed on the eastern side of Marguerite Bay was not found at survey station 51. This again is suggestive of a recent UCDW intrusion.
Survey transect eight, which we are just beginning, is designed to cover a large bathymetric feature just off the continental shelf. This feature may be the trigger mechanism for meanders in the southern boundary of the Antarctic Circumpolar Current that result in the intrusions.
Bob Beardsley on satellite tracked drifters:
Physical oceanographers use a variety of methods to directly measure ocean currents. With the invention of the Earth-orbiting satellite and satellite tracking systems, it is now possible to deploy a surface drifter almost anywhere in the world's oceans and track this drifter over time remotely. If this drifter has a good drogue that "sticks" to the water around the drogue, then the drifter moves with the water, and provides a direct measure of the current (velocity) of the water around the drogue. Over the last 10 years, more than 1000 surface drifters have been deployed around the world as part of international research programs. Some drifters were deployed in the Bransfield Straits in the early 1990s, but none in the Marguerite Bay area.
As part of this SO GLOBEC program, we are deploying satellite-tracked drifters over the shelf and within Marguerite Bay. The drifter features a spherical surface float (about 50 cm in diameter), attached to a smaller subsurface ball float and drogue by a thin but strong tether. The surface float contains the satellite transmitter and batteries to last 2 years. The "holey sock" drogue is a long vertical cylinder made of canvas, about 1 m in diameter and 10 m high, with some holes made in the side of the cylinder to improve the drogue's ability to "stick" to the water around it. The center of the drogue is located at a water depth of 15 m, so that the drifter is set to measure the currents at 15 m. The flow drag on the drogue is roughly 40 times the drag on the surface and subsurface floats and the tether, so that there is less than 1 cm/s slippage between the water and the drogue even in winds up to 30 knots. The satellite transmitter broadcasts a signal which is heard by several tracking satellites during each day and relayed to a ground station in France, where large computers decode the signals and compute in near-real-time the positions for all drifters working around the world at that time. This position data are then sent by email to the different investigators once a day. Over the west Antarctic Peninsula shelf, the drifter is getting located on average 20 times each day, and this information is being received at WHOI each day. Dick Limeburner checks these data and then sends processed data to the ship by email, where I receive and plot it up. Seeing the data during the cruise helps us plan where to deploy the remaining drifters, and allows us to study interesting tracks and drifter events that occur during the cruise.
Strong wind events tend to mix the surface waters of the ocean, and generally form a "surface mixed layer" in which temperature and salinity are remarkably constant through the layer. Yesterday the winds increased to gale strength (42-48 kts) and stayed strong through the night. This morning the surface mixed layer extended down to 72 m, the deepest we have found to date on this cruise. In addition to creating turbulent mixing, the wind also puts momentum into the ocean, generating currents in the surface layer which affect the surface drifters. We are continuously measuring the wind and other meteorological variables on the ship so that we will have a time series of the surface wind stress. We can use this information to investigate the response of the drifters to strong wind forcing, and thus gain insight about the near-surface currents (at 15 m) driven by strong wind forcing. As more drifter data are sent to the ship, we will start to look carefully at the wind-driven response. These drifter and shipboard met measurements have never been made together in our study area, so that the information and insight we gather over the rest of the cruise will be completely new. This makes it especially exciting to be here, experiencing the wind and sea, while we look at new data from an unexplored area.
Chris Ribic and Erik Chapman reported that on 10 May they surveyed for 5 hours 7 minutes during the day as we transited between stations 42 and 44. Winds built quickly after sunrise to 45 knots and remained heavy throughout the day. The Blue Petrel was again the most common species seen today. The big surprises of the day came with sightings of two Sooty Shearwaters and either a Gray-headed or a Black-browed Albatross. They have not observed these species on the SO GLOBEC transect and have not seen them around the ship since the Drake Passage. They were also surprised to see three immature Kelp Gulls well away from shore near station 43. Two 30-minute night surveys were conducted both this morning and last night. Blue Petrels were also the most common species seen during these surveys, though many birds were not identified.
Here is a summary of these sightings:
|Species||Number (Day)||Number (Night)|
|5hr 7min||2 hr|
|Antarctic Petrel (Thalassoica antarctica)||1||0|
|Cape Petrel (Daption capense)||8||4|
|Southern Fulmar (Fulmarus glacialoides)||17||0|
|Blue Petrel (Halobaena caerulea)||30||12|
|Southern Giant Petrel (Macronectes giganteus)||1||0|
|Snow Petrel (Pagrodoma nivea)||0||2|
|Kelp Gull (Larus dominicanus)||3||0|
|Sooty Shearwater (Puffinus grifeus)||2|
|Gray-Headed (Diomedea chrysostoma)or Black Browed (Diomedeamelanophris)||1||0|
On May 11th, they surveyed for 2 hours 3 minutes during the day as we transited to station 50 inside Marguerite Bay. We traveled through extensive sea ice (brash, pancake and ice-berg) cover throughout the day and these conditions were reflected in the bird community we observed. The Snow Petrel is known to forage in sea ice and this species dominated our observations. Results are listed below:
Survey Period: 2hr 3min
|Antarctic Petrel (Thalassoica antarctica)||2|
|Cape Petrel (Daption capense)||0|
|Southern Fulmar (Fulmarus glacialoides)||0|
|Blue Petrel (Halobaena caerulea)||0|
|Kelp Gull (Larus dominicanus)||1|
|Snow Petrel (Pagrodoma nivea)||25|
Ari Frielander reports that it was refreshing to wake up to some ice this morning, it being Antarctica. The ship was transiting through loose brash and pancake ice (4 to 6/10ths) en route to station #49 at first light. However, visibility was very poor due to fog. Therefore, an incidental watch was kept. Similar conditions persisted throughout the day as we steamed to station #50. No whales were seen today, but several seals were sighted on floes. These were: 7 Antarctic fur seals, 3 crabeater seals, and 3 Weddell seals. All of these sightings were made within 5 miles of reaching station #50. These sightings were passed on Jennifer Burns and the seal team on the L.M. Gould.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager and
BIOMAPER-II was towyo'd from station 42 to station 45 during May 10th. We were going to take it out of the water at station 44 prior to a CTD cast and MOCNESS tow, but the seas were too rough to bring BIOMAPER-II out of the water or to tow MOCNESS, so only a CTD was done under marginal conditions with the towed body "parked" about 30 m below the surface.
Towyoing continued along the trackline with two significant problems occurring. At the top of one towyo in the middle of the afternoon while approaching station 45, the slack tensioner topped out in a particularly high wave surge and the cable jumped out of one of the sheaves in the tensioner. Quick action brought the winch to a stop and the slack tensioner turned off, and then the Rathyeon Marine Techs and the BIOMAPER-II watch quickly installed Yale grips to stop off the wire. After that the cable was re-aligned in the sheaves, checked for damage (fortunately there was none), and then the towyo resumed.
The second calamity occurred near midnight during a period of particularly high winds. It was in the category "Just when you think nothing else can go wrong". As we approached Station 45 about 2315 (10 May), the bridge called down to Matt Burke, who was on the deck about to do an XCTD (it was too rough to do a CTD), saying that Matt needed to hurry because the ship was headed for a 30 m shoal. That was a wake up call for us in the van because the fish was down at 50 meters parked for the station activities. We had planned to resume towyoing after XCTD was completed. A check of the wire and slack tensioner, while BIOMAPER-II was been hauled to a shallower depth revealed the outer armor of the cable was frayed. With snow blowing horizontally across the deck and huge waves coming in against the port quarter, we examined the wire. It was heavily damaged along a 5 to 10 meter stretch. After weighing the options - bring the fish back on board or try to stabilize the situation and leave the fish in the water as we steamed towards Marguerite Bay - the decision was made to do the latter. The frayed outer armor wires were cut off and then a Yale grip was installed on the wire along with a safety lines to the slack tensioner. Data continued to be collected during the transit into Marguerite Bay in spite of the constraint imposed by the damaged wire.
Around 0900, on May 11th, just as we arrived at Station #49, BIOMAPER-II was hauled onto the deck safely. Inside the Bay, there were light winds and floating ice and ice bergs all around us. The sea was flat except for a damped swell that was running under the ice. The recovery was done nicely. In addition to the damaged wire, another tail elevator had disappeared during the tow, probably from all the pitching motion. Examination of the overboarding sheave revealed that a screw installed to hold a plastic facing onto the wood block to prevent the cable from jumping out of the groove was exposed and that caused the cable to fray. The remainder of 11 May was spent making a new cable termination, fixing the overboarding sheave, further repair of the echo sounder, and construction of a new tail fin elevator. A little after midnight, BIOMAPER-II was re-deployed for run #14 with all systems operational.
The volume backscattering data from the echosounder together with the VPR images provided evidence that some of the intense scattering in patches near the sea surface between Stations 43 and 44 were indeed composed of adult krill. These stations are situated in the middle of the continental shelf. These patches occurred within the surface mixed layer and were 30 to 40 meters in vertical extent and extended for several kilometers horizontally. Earlier in the cruise, we had seen similar acoustic patches in a similar position between the shelf edge and the coast, but could not say for certain they were composed of adult krill because our towyo?s did not go through them. Also noteworthy is the sharp contrast between relatively high volume backscattering on the shelf and the much lower backscattering in the offshore waters beyond the shelf break. The change was abrupt as we crossed over the boundary between shelf water and the SACC as we steamed out to Station 41 and again when we steamed back onto the shelf. This was corroborated by images of the plankton produced by the VPR and the MOCNESS tow collections. For example, although krill furcilia were present in the VPR images from the offshore region, they were much lower in abundance.
The strong gale force winds and seas forced cancellation of the MOCNESS tow at Station # 44. Conditions were ideal, however, for the tow at Station # 52 deep in the southeast arm of Marguerite Bay, because there was essentially no wind and no swell. The entire area was ice covered and there were a number of icebergs that past by as we steamed along a course that had we had been over earlier. The tow went down to 750 meters and back to the surface, and this could be done safely with the Seabeam depth data as assurrance that the net would not be running into a submerged shoal. In fact, the tow over a deep hole (more than 1000 m deep), that Eileen Hofmann described above. The lower part of the tow had a lot of gelatinous animals present, in particular small salps and small jelly fish. Also caught were quite a few adult E. superba (krill) and small fish in the 75 to 50 m interval and also in the 50 to 25 m interval. The surface net had a good number of furcilia.