Today (May 10) is a near repeat of the day before yesterday with sustained winds in the mid- 40 knot range and gusts in the 50's. We are again in a strong gale. The cloud cover now is 100%, although daylight came this morning with the skies clear overhead. Earlier today at Station #44, a large ice-berg was drifting some three miles from where the CTD was being deployed. From the bridge, it looked like a ghost-ship out in the mists. We are now on our way to Station #45 (towyoing BIOMAPER-II as we go).
Yesterday under somewhat better working conditions, we completed work
at the shelf stations 40 and 42 and the offshore deep Station # 41. The
station work included deployment of two sonabuoys, two XBT's, two XCTD's,
two CTD's and one MOCNESS tow. BIOMAPER-II was towyo'd between the stations,
and bird and mammal observations were made along the trackline. The latter
station had depths of about 3000 m, and the CTD to the bottom and the MOCNESS
tow to 1000 m took about a quarter of the day (6 hours).
Bob Beardsley on Tidal currents at Station 26
A satellite-tracked surface drifter was deployed near Station 26 at 19:27 May 5 (yd 125.8105). For the next 2.91 days, this drifter moved in counterclockwise loops while slowing moving towards the northeast. This looping motion appears to be tidal, with the dominant constituent being the semidiurnal component M2 with a period of 12.42 hrs. To quantify this motion, a simple model consisting of a mean current plus M2 component was fit in a least-squares sense to the observed drifter position data. The results given below indicate that the drifter did experience a tidal current near Station 26 during this May 5-8 time period.
In particular, the drifter moved in an elliptical path towards the northeast with a mean speed of 3.5 cm/s. The elliptical motion has a major axis of 17.3 cm/s and a minor axis of 13.7 cm/s with the M2 period 12.42 hr. The sense of rotation, counterclockwise, is consistent with a free Sverdrup wave propagating into Marguerite Bay, however, the ratio of minor to major axis (0.79) is significantly smaller than the theoretical value of f/sigma (0.96), indicating that this not a pure free Sverdrup wave. The tidal ellipse is oriented with its major axis towards 62°, roughly ENE.
We will have to wait for more data to see if this drifter motion is persistent or episodic in this area. The other drifters deployed on NBP0103 do not show such periodic motion, suggestive that the M2 tidal current field is quite variable in space. Existing numerical tidal models do exhibit large variations in the M2 tidal currents over the west Antarctic Peninsula shelf. However, these model results depend strongly on the model topography, and the depth data collected to date on NBP0103 exhibits large differences from the ETOPO8.2A topography. Fortunately, the ARGOS satellite tracking system is obtaining on average 20 position fixes per day, so that tidal motion can be identified if present with sufficient amplitude. A detailed examination of the raw drifter position for such tidal motion is planned.
Results of tidal fit to drifter 8 position data for period 125.8105
- 128.75. Mean position: -67.4914°S, -70.5913°W. Mean u = 2.9 cm/s;
mean v = 2.0 cm/s; mean vector speed = 3.5 cm/s; mean vector direction
= 34°E; Umajor = 17.3 cm/s; Uminor = 13.7 cm/s; Inclination (CCW from
E) = 62°.
Chris Ribic and Erik Chapman surveyed today for 2 hours 56 minutes during the day as we transited to station 41, off the continental shelf. Fog and a steady mist made for low visibility, but we were still able to survey within our transect that extends 300m in front and to the port side of the ship. For the second day in a row, the Blue Petrel was the most common bird. Overall, results were similar to yesterday's offshore surveys. High winds and snow kept us from doing night surveys last night and this morning. Today's results are listed below:
Survey Period: 2 hr 56 min
|Antarctic Petrel (Thalassoica antarctica)||4|
|Cape Petrel (Daption capense)||2|
|Southern Fulmar (Fulmarus glacialoides)||3|
|Blue Petrel (Halobaena caerulea)||17|
|Southern Giant Petrel (Macronectes giganteus)||1|
|Snow Petrel (Pagrodoma nivea)||1|
Catherine Berchok deployed a difar sonabuoy on the start of the leg to station 42. She heard a humpback calling for around 30 minutes, though it was buried in ship noise at the beginning. Two hours of sound recordings were monitored. She is delighted to report a 9 nm range on the new stick antennae installed on the 7th of May. Catherine has also written up a description of what sonobuoys are and how they are used in her research.
Catherine Berchok on Sonobuoys
Sonobuoys are expendable underwater listening devices that are used primarily by the military for underwater surveillance. They are designed to be launched from aircraft - deploying upon impact. They can also be disassembled and thrown into the water by hand, which is how we are deploying them. There are four main components to a sonobuoy - the float, radio transmitter, salt water battery, and hydrophone. The hydrophone is like an underwater microphone that converts the pressure waves from underwater sounds into electrical voltages that get amplified and sent up a thin wire to the radio transmitter that is housed in the surface float. The radio signal is picked up by an antennae and radio receiver on our ship, then reviewed and simultaneously recorded onto a digital audiotape.
Most sonobuoys have a switch that controls how long the wire is between the surface float and the hydrophone. The minimum is 90 feet (27.4 meters), which is good for shallow water environments. The maximum is 1000 feet (304.6 m), which is best for very rough seas, where there is a lot of noise at the surface from the crashing waves. There is also a switch that controls how long the sonobuoy will operate for. After one, three, or eight hours, a scuttling mechanism opens a plug on the float, which then fills with water and sinks. It may seem strange to want to sink a perfectly functional piece of equipment, but remember that they are used for surveillance. The listener normally does not want to be discovered by those being listened to. The main advantage of a sonobuoy is that it allows a recording to be made far away from the noisy ship. The range of detection depends on your radio antennae, but normally ranges from 6 to 12 miles. Another advantage of the sonobuoy is that it does not have to towed through the water, which causes a lot of noise. Hydrophones are pressure sensors - they cannot differentiate between pressure waves coming from a sound source versus those caused by water motion.
There are two types of sonobuoys. Omnidirectional sonobuoys have hydrophones
that register signals the same from all arrival angles. They are nice because
they have a very wide bandwidth - up to 40 kHz. However, with a single
omnidirectional sonobuoy deployed, it is impossible to determine where
the source of the sound is. DiFaR (DIrectional Fixing And Ranging) sonobuoys
also have an omnidirectional hydrophone for recording sound, but it is
limited to a top frequency of 2.5 kHz. However, DiFARs also have two pairs
of sensors, which along with an internal compass, can determine the exact
bearing of the sound from the sonobuoy. With three or more of these sonobuoys
in the water, it is then possible to pinpoint the location of the sound
source. This can then be combined with visual observations of the species
of marine mammal in that location, along with behavior and grouping information.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager and
BIOMAPER-II continues to be towyo'd along the trackline between stations. After leaving Station #40 at the edge of the continental shelf, the acoustic volume backscattering on all three frequencies dropped of abruptly as we crossed the SACC boundary. At Station #41, it was brought back on deck to allow the MOCNESS to be deployed from the same stern A-frame. During the period on deck, electronic sub-systems in the towed body were checked and a tail fin elevator which had broken off during the tow was replaced by one crafted in the workshop on board by Mark Dennet. The BIOMAPER-II was re-deployed and the towyo to Station #42 reproduced the change in volume backscattering across the SACC boundary.
Our 10th MOCNESS tow was conducted at station 41, moving off to the northwest from the actual station location. The tow was done over the 0-1000 m depth range. Temperature and salinity indicated that the tow was conducted offshore of the boundary of the SACC and in oceanic water. Abundance and biomass were much lower here than at any other location, despite filtering high volumes of water. Highlights of the composition included a large-ish (3" diameter) purple jellyfish, very small larval krill in the upper 50 m (not E. superba?), and some pinkish, opaque krill in the 50-100 m depth interval. Also collected was another marble sized orange ball in the 600-800 m depth interval which, upon microscopic examination, was identified as an ostracod. Salps were collected in the 0-25 m depth interval. Species identifications will come when the samples are returned to the laboratory in Woods Hole, MA at the end of the cruise.