Report of Activities
on the RVIB N.B. Palmer Cruise 02-04
4 August 2002
The long transit from Punta
Arenas, Chile
to the first work site in Crystal Sound
was nearly over by the end of 4 August.
During the night, the N.B. Palmer
steamed along the Bransfield Strait
and into the northern end of the Gerlache
Strait, while the physical
oceanography group continued the XBT survey of the area. A sliver of a waning
moon was setting in the northwest just as the first light of dawn, which took place
around 0800, allowed the mountain ranges on both sides of the strait to become
visible. For the most part, the transit
was ice free allowing the ship to maintain a normal cruising speed of 10 to 11 kts.
The morning was an extraordinary
opportunity to see the inner passageway (Gerlache Strait)
in all its splendor. Clear skies for most of the morning
provided superb views of the snow covered peaks and mountain glaciers that were
sparkling in the bright sunlight. Broken gray clouds were draped over some of
the peaks adding an extra dimension of texture to an already wondrous scene.
Although the sun was out, winds of around 25 kts out
of the west kept the wind chill well down into minus numbers and for the many
in the scientific party taking pictures, being out on deck was a bit
brutal.
The weather in this part of the
world is also extraordinary in that it changes rather rapidly. Early in the
morning the air temperature was around -1.9ºC and the sea temperature was
-1.268ºC. The barometer remained fairly
high at 1006.1 mb.
By early afternoon, low clouds moved in and the mountain peaks were
obscured. As we moved westward along the Bismark
Strait in the late afternoon (1800) toward the open water of the Western
Antarctic Continental Shelf, the barometric pressure, which had begun to climb
earlier in the day reached a remarkable 1017 mb,
while the air temperature dropped to -8.6ºC and the winds dropped to less than 5
kts.
Around 1830, we made the turn to
head south towards the entrance to Crystal
Sound. Water temperatures of
-1.764ºC (approaching the freezing point), were substantially colder than in
the inner passage. Within an hour of making the turn, we ran out of open water
and were back into the pack ice and the consequent slower ship's speeds (6 to 7
kts)
We should reach Crystal
Sound by mid-morning on the 5th of
August where we will rendezvous with the L.M.
Gould for a transfer of equipment and supplies, and two to three days of
joint science operations. This will provide the investigators on the Palmer their first opportunity to deploy
the over-the-side equipment and to begin collecting krill and other species for
experimental work. The Crystal Sound
area has on past cruises been a “hot” spot for krill and their associated
predators (seals, penguins, sea birds, etc.), and a part of the work will
involve a study of the hydrographic setting to provide some insight into why
this is so.
CTD Group report (Eileen
Hofmann, Bob Beardsley, Baris Salihoglu,
Chris MacKay,
Francisco (Chico)
Viddi, Sue Beardsley)
After completing the XBT section
across Drake Passage, the CTD group did two additional
XBT sections during the transit to Crystal
Sound. The first was along a
transect that extended from Drake Passage through Boyd
Strait and into Bransfield
Strait. The second extended along the axis of Gerlache Strait. The section along the Gerlache Strait
is a repeat of a section that was done in April 1993, which is described in
Smith et al., (1999, Deep-Sea Research I, Vol. 46, 925-949). Comparison of this
section with the new one will allow us to determine the consistency of the
water mass distribution in Gerlache
Strait.
The weather and sea state
conditions during both XBT transects were calm, which allowed us to deploy T-7
probes (nominal depth of 760 m) over the ship's stern from the main deck. The bottom depth at most of the XBT drop
locations was shallower or within a few 10s of meters of the maximum depth for
the XBT probes, which provided coverage of most of the water column. The XBTs were
dropped at 10-nm intervals and nine probes were dropped along each section.
Sea ice was minimal over most of
the two transects. The ship moved through
patches of sea ice that were separated by wide areas of open water. Whenever possible, we waited for open water
to deploy the XBTs. The only problems with sea ice
occurred at the first four XBT stations in Gerlache Strait
when we encountered an extensive area of slushy sea ice. At these sites, the XBT wire became entangled
in the sea ice and broke before the probe hit the bottom. The XBTs that broke
at shallow depths were repeated.
The vertical temperature
distribution along the first XBT section shows clearly the intrusion of
Circumpolar Deep Water from Drake Passage through Boyd
Strait and into Bransfield
Strait. Temperatures below 200 m were greater than
1.0ºC, and reached a maximum of about 1.23ºC at 650 m to 700 m about mid-way
through Boyd Strait. At the Drake Passage end of the section,
there is a strong temperature front that likely separates the shallower
coastal/shelf waters surrounding the South Shetland Islands from the deeper
waters of Bransfield Strait.
At the end of the section at
about -63º 45.165′S; -61º 21.505′W, a front separating the
Circumpolar Deep Water and the Bransfield
Strait water was encountered. This is the point at which the Circumpolar
Deep Water turns northeast and flows along the southern flank of the South
Shetland Islands. Our XBT
transect continued further south into Bransfield
Strait, so Circumpolar Deep Water
was not present at subsequent stations.
Rather, the temperature profiles showed vertical distributions that are
characteristic of Bransfield Strait
water.
Temperatures above 200 m were
less than 0ºC and reached a minimum of about -1.75ºC, producing a thick Winter
Water layer. It is interesting that surface temperatures were above freezing
over the entire section. This is in contrast to the surface waters in southern Drake
Passage which were all below freezing. This likely contributed to the lack of sea
ice in the southern Bransfield Strait. Why these waters remain above freezing in
winter is a topic for further investigation.
The vertical temperature distribution
constructed from the XBT section along the Gerlache Strait
showed a thick (about 100 m) Winter Water layer. However, temperatures in this
layer were all above freezing, which is consistent with the large regions of
open water encountered during the transit of Gerlache Strait.
At -64º 32.920′S; -62º
33.699′W near the opening between Brabant Island and Anvers
Island, Circumpolar Deep Water flows into the Gerlache
Strait. This appears in the vertical
temperature section as a bolus of warm water (above 0ºC) at about 200 m, which
extends north-south along the Strait for about 25 km. This feature was described in Smith et al., (1999). In the section done during NBP02-04, the
temperatures associated with the inflow water are lower than seen in April
1993, which may result from mixing of this water with the colder surface waters
that prevail in winter. It may be that
the input and subsequent mixing of this warmer water is what keeps the surface
waters of Gerlache
Strait above the freezing point in
winter. The surface waters above the
point where this water enters the Strait were the warmest encountered along the
section.
At about the mid-point of the Gerlache Strait section, the -0.2ºC isotherm intersects the
bottom and forms a temperature front between the warmer Circumpolar Deep Water
entering the Gerlache Strait from the southern end
near Anvers Island and the colder waters that flow
out of the Bransfield Strait at depth to the north. This front is similar to that described in
Smith et al. (1999).
The final XBT was dropped at the
southern end of Gerlache
Strait around 1230 on 4
August. The hydrographic group is now
preparing for the CTD casts and related activities that will take place when we
reach Crystal Sound
in about one day from now.
Marine Mammal report (Chico
Viddi)
Seven hours of marine mammal
observation effort were performed on 3 August, bringing the cumulative total
number of hours for the first four days of the cruise to 13.6 hours. Early in the morning while transiting the Drake
Passage, the mate in the bridge saw a small whale right in front
of the vessel. Unfortunately, I was just
taking the ice off of the windows, so the sighting was missed. A male elephant
seal (Mirounga leonina) was
observed on an ice floe at about 1005 at 32º to port and 450 meters from the
vessel (-61º 17.0′S; -62º 52.91′W). Another elephant seal was
observed (possible a female or juvenile) about a mile ahead, also on an ice
floe. Finally, one and half hours later, another seal (not identified) was seen
at about 0.8 nm on the port side from the vessel. We had a beautiful day, with
spectacular icebergs and weather conditions.
Antarctica
gave us an incredible day (5 August)—sunny morning with pink, golden, and blue
colors in a perfect sunrise. Today's weather conditions were excellent and the
cumulative hours of marine mammal observations to date now total 21.5, of which
7.8 hr were done today. The most frequent marine mammal species seen today was
the Antarctic fur seal (Arctocephalus gazella),
which was first sighted at 0856 (-64º 30.17′S; -62º 21.91′W). Half an hour later (0924), a few more
sightings were made bring the count up to more than 40 fur seals. The last sighting
was done at 0938 (-64º 33.45′S; 62º 36.65′W). Most of the fur seals
were on ice floes, except for one sighting in which about ten fur seals were
swimming less than 50 m from the ship. Three crabeater seals (Lobodon carcinophagus)
were sighted three different times, at 1132 (-64º 46.75′S; 63º 08.35′W),
1208 (64º 51.69′S; 63º 14.11′W) and 1259 (64º 58.28′S; 63º
25.34′W). Finally, two “like humpback whales” (Megaptera novaeangliae) were seen at 3.6 miles,
175º to starboard (-64º 53.15′S; 64º 08.37′W). No confirmation on
species identification was made.
ADCP/OPC/MOCNESS Studies (Ryan
Dorland)
The vessel-mounted Acoustic
Doppler Current Profiler (153 kHz VM-ADCP, RD Instruments) on the NB Palmer
measures currents over a depth range from 31 m to around 300 m, sometimes less
depending on weather and ice conditions.
ADCP Current data are processed through CODAS (Common Oceanographic Data
Access System) software developed by Eric Firing at the University
of Hawaii. Both the Palmer and Gould are set up with an automated
processing system, which includes heading corrections from an Ashtech ADU-2 system, and some editing based on cruise-wide
statistics.
Two data sets are produced daily,
one with 15-min averages over the previous 36 hours, and another of one-hour
averages for the entire cruise. Figures
from these sets are available on the ship intranet at http://peale/ adcp/figures/index.html. In addition to these sets and figures, a
5-min high resolution data set for the entire cruise is currently being
maintained and figures of interest will be posted daily. These data sets and figures are a first pass
and should not be considered a final product.
The ADCP measured southeastern
currents of over 150 cm/s near the South American continental shelf-break. From -56ºS to -57º 45.0′S, currents
were primarily SSE, measuring up to 50 cm/s near the surface and diminishing to
around 20 cm/s at 200 m. A 10 cm/s SW
flow was present around -58ºS. Between
-58º 30.0′S and -59º 30.0′S, the currents were predominately eastward
reaching 30 cm/s uniform with depth.
Around -59º 30.0′S, the five-minute ensemble averages were lost,
coinciding with a loss of pcode navigation data. Further processing and analysis is required
to determine the exact cause of the missing measurements. Ensemble data resumed around -61º 30.0′S,
but remains sporadic due to icy conditions. Single-ping data continued to be collected
despite the lapse in the ensemble current data.
Current Position and
conditions
We are currently steaming into
Crystal Sound in 9/10 or 10/10 pack ice and our position on 5 August at 1114 is
-66º 30.639′S; 67º 33.759′W. The air temperature is -5.3ºC and the
sea temperature is -1.833ºC. Winds are out of NE (045) at 8 to 10 kts and the barometer is at 1018.6 mb.
Skies are cloudy.
Cheers, Peter