Report of Activities on the RVIB N.B.
Palmer Cruise 02-04
On 18 August we continued the
transit started around 1800 on 17 August between stations 65 and 41. The
purpose of the transit was to assist the L.M.
Gould move from their first time-series pack ice station to the second. The
straight line distance between the two stations was 84 nm, but the route that
had to be taken was circuitous. We had
learned from experience over the past several days that trying to traverse the central
portion of the continental shelf off
During 18 August, there was no
over-the-side station work done from the Palmer.
Seabird and Marine Mammal observations were made during the daylight periods
and an XBT survey was conducted along the route. As described below, the XBTs provided insight into why we were able to travel so
far toward the inshore destination in open water or thin ice afforded by the
leads.
Weather conditions for the convoy
toward station 41 were very nice with little wind (mostly < 10 kts) and relatively warm temperatures. Air temperatures
varied between -6.7ºC at night and -3.6ºC during the mid-afternoon. Visibility, however, varied dramatically. There were high clouds during the morning and
good visibility. Off to the northeast in
the direction we intended to go, the darkness of “water sky” areas contrasted
sharply with the lightness of clouds over the pack ice. In the distance, there were also areas where
a layer of white fog-like clouds pressed low against the pack ice. During a portion of the afternoon, an ice fog
similar to the one we experienced a couple of days ago set in and it was
probably related to the large expanses of open water we traveled through. At
other times in the afternoon, the viewing was really excellent. We passed by a series of gigantic icebergs
with a couple that were several times the height of the vessels. With the Gould following close behind us, the
stage was set for some good picture-taking opportunities of that ship passing
the icebergs. Fortunately, we passed the bergs when the visibility was good. During the day, the barometer began a steady decline
from around 1000 mb at midnight of 17/18 August to
977 mb around midnight of 18/19 August, a sign that
the fine weather was about to end.
CTD Group report (Baris Salihoglu, Eileen Hofmann,
Bob Beardsley, Chris MacKay, Francisco (
In the evening of 17 August, we
began a transit to escort the Gould
to process site two, which is in the vicinity of survey station 41 off the southern
end of
Like many things, the plan for
the XBT survey worked better in theory than in practice. The considerable sea ice encountered soon
after beginning the transit resulted in deviations from the proposed transit line
to take advantage of leads and areas of thin sea ice, where available. Also, the inability to obtain consistent and
accurate bathymetry readings from the various depth sounders (due to
interference from sea ice) made it nearly impossible to target specific
bathymetric features. As a result the
XBT survey was done at 10-nm intervals and along a rather unusual survey line.
A total of 15 XBTs,
either T-7 (depth of 760 m) or T-4 (depth of 460 m), were dropped along the
transit line. Of these, 10 returned data
for the entire, or nearly entire, operational depth of the XBT probe. The remaining
casts covered only the upper portion of the water column because the wire broke
on sea ice before the probe hit the bottom or reached its maximum depth. However, the partial casts do provide temperature
observations for the upper water column.
A benefit of our unusual route to
a possible process site two was that we went over a part of the continental
shelf that would have otherwise been missed.
The XBT-derived temperature profile from this site (-67º 33.604′S;
-73º 17.926′W) showed a maximum of 1.75ºC at 350 m. Surface waters at this site were about -1.7ºC,
which is above the freezing point. Also,
the well mixed surface layer was reduced to the upper 50 m, rather than the 80
m to 100 m observed at nearby locations.
An interesting coincidental observation is that sea ice was reduced
around the region where the warm water at depth and the warmer surface waters were
observed. Also, there were large regions
of open water and areas of newly forming sea ice. The transfer of heat from depth to the upper waters
and the subsequent effect on sea ice and biological properties is a topic of
considerable interest to many components of SO GLOBEC and is an area of active
study.
The maximum water temperature
near this same site observed during NBP02-02 (April-May 2002) was about 1.4ºC. The temperature increase observed during this
cruise suggests that Upper Circumpolar Deep Water has moved into the central
portion of the survey region. We are now
analyzing the entire XBT survey and hope to supplement these observations with
those from CTD casts in the next few days.
The results will be presented in subsequent reports.
Report on CTD performance
during NBP02-04 (Bob Beardsley, Eileen Hofmann, Baris
Salihoglu, Chris MacKay, Francisco (
The NBP SeaBird
911-plus CTD is equipped with a primary and secondary set of temperature (T)
and conductivity (C) sensors, a new SBE dissolved oxygen (DO) sensor, plus
sensors for pressure P, PAR, beam transmission, and fluorescence. The primary and secondary T/C sensor sets are
attached to their own pumps, and the DO sensor is placed in line between the
primary T/C set and its pump. The
effects of the inclusion of the DO sensor and the extra tubing between the T/C set
and pump were not known prior to the cruise, since the DO sensor is of a new
design used for the first time in SO GLOBEC on NBP0202 cruise. At the minimum,
the extra tubing increases the lag time between DO and pressure readings. Preliminary estimates of this lag time were
during CTD casts 25-30, assuming that the DO hysteresis
between DO and pressure during those parts of the down and up casts made at 40
m/min was due entirely to pressure.
These tests suggest that the DO value corresponds to the pressure value
taken about 15 sec earlier.
The CTD-mountable Microstructure
Profiling System (CMiPS) has been mounted on the CTD
rosette frame during most CTD casts during this cruise. CMiPS is a
self-contained instrument that records temperature, conductivity, and pressure
measured with two fast-response thermistors, a microconductivity probe, and precision pressure sensor all
sampled at 512 Hz. The data, recorded
internally to disk, are downloaded to a PC after the CTD is back on deck after
a cast. The data are then processed to
obtain two high-resolution temperature (HRT), one
conductivity (HRC), and a pressure (HRP) time series. To help synchronize the CMiPS
and CTD time series, the CMiPS raw temperature
voltage is digitized inside the CTD fish and included in its scan data stream
received and recorded by the CTD deck unit.
The purpose of CMiPS is to collect microstructure TC data to help identify and quantify turbulent mixing within the water column sampled by the CTD. To calibrate the CMiPS temperature and pressure scaling (i.e., the conversion from A/D counts to scientific units), the CMiPS HRT/HRP data were compared with the 1m-averaged CTD T/P data from several casts, resulting in approximate linear scaling rules for the HRT and HRP data. These comparisons suggested that the CTD primary T sensor did not response as quickly to sharp changes in HRT as the secondary T sensor. To investigate this further, the raw (un-averaged) 24 Hz CTD data were examined, and salinity spiking first minimized when lags of 2 and -1 scans were used in processing the primary and secondary T/C data. After these lag corrections were made, the primary sigma-t profile became everywhere stable while the secondary sigma-t profile remained stable. While difficult to quantify, these comparisons of the 24 Hz lagged CTD data with the 512 Hz CMiPS data suggested that the primary T/C set has a longer response time than the secondary T/C set. For this reason, the lagged secondary T/C data should be used forfurther scientific analysis.
Water samples are drawn from
10-liter Niskin bottles closed at various depths
during each cast for use in checking the CTD conductivity cell performance
during the cruise. The conductivity of
the bottle samples is measured at 24ºC using a Guildline
AutoSal (model 8400B) salinometer
located in a new temperature-controlled Salinity Lab on the NBP. These conductivity values are then converted
to bottle salinities (Sb) using MatLab
Sea-Water Toolbox codes. These bottle
salinities are compared with the CTD primary and secondary salinities (S0 and
S1) computed using the primary and secondary T/C/P data recorded when the bottles
were fired and the same MatLab codes. To monitor the CTD conductivity cell, the
bottle salinity Sb and CTD temperature T are used to
compute the equivalent bottle conductivity Cb at the in-situ primary and secondary
temperatures T0 and T1.
Plots of differences between
primary and secondary temperature (T1-T0), conductivity (C0-C1), salinity
(S0-S1), bottle and primary salinity (Sb-S0), and bottle and secondary salinity
(Sb-S1) were made on August 18 using data collected during the first 28 CTD
casts. The CTD difference data show very small, but consistent offsets between
the primary and secondary measurements (Table 1) that appear constant in time. The CTD-bottle salinity differences show
offsets that appear to change in time after sample 47. The first 47 samples had been run as the
batch on the AutoSal, while the remaining samples had
been run in two separate batches by different operators. It appears that the AutoSal
was not properly standardized during the first batch. Using the later samples
starting with 48, the CTD-bottle salinity and corresponding conductivity
offsets were small and appear constant in time (see table below). These offsets should be used to correct the
raw 24 Hz CTD conductivity data before computing depth averages and further analysis.
In summary, over the first 28 CTD
casts made during NBP02-04, the CTD has worked quite well, exhibiting very
small DC offsets in primary and secondary temperature and conductivity values
with water samples taken in well-mixed waters. At this stage, final processing
of the CTD data should follow the following steps:
1. Set conductivity lags for raw 24 Hz CTD
data (2 scans for C0, -1 scan for C1)
2. Add CTD-bottle mean conductivity offsets
to C0 and C1
3. Compute salinity and other derived
variables (i.e., σt) for each scan
using MatLab code
4. Lag
DO 15 sec with respect to pressure to account for the delay time introduced by
the tubing between T/C and DO cells (see DO sensor section)
5. Construct depth-bin averaged data if
desired.
6. Use secondary sensor T/C/S data for final
scientific analysis.
It seems clear for the limited
analysis to date that the DO sensor being inline between the primary T/C
sensors and their pump does not adversely affect their accuracy in regions of
very weak T and S vertical stratification.
However, the apparently slower response of the primary system to rapid
changes seen in the secondary and CMiPS data suggests
that the flow rate in the primary system may be reduced by the added flow
friction caused by the additional tubing and the DO cell.
Mean and 95% confidence limits of
the mean difference between CTD primary (0) and secondary (1) temperatures,
conductivities, and salinities with the bottle sample salinities and
conductivities at the CTD temperatures. The number of samples in each group is
given in the right column.
|
Difference |
Mean |
95% Confidence Limit |
Sample Number |
|
T0-T1 |
-0.0007 |
+/- 0.0003 |
107 |
|
C0-C1 |
-0.0005 |
+/- 0.0003 |
107 |
|
S0-S1 |
+0.0001 |
+/- 0.0003 |
107 |
|
S0-Sb |
-0.0019 |
+/- 0.0005 |
76 |
|
S1-Sb |
-0.0019 |
+/- 0.0005 |
76 |
|
C0-Cb0 |
-0.0014 |
+/- 0.0004 |
76 |
|
C1-Cb1 |
-0.0014 |
+/- 0.0003 |
76 |
Sea Birds (Chris Ribic and Erik Chapman)
On August 18, seabird and
Crabeater seal surveying was conducted for 8 hours as the ship traveled with
the L.M. Gould from station 65 to 41. The first few hours of surveying were
conducted in very thick ice-cover and the ship was forced to back and ram its
way forward. We soon found an opportune
lead that extended in the direction we were heading, so during most of the day
we found ourselves in open water and 7 to 8-tenths ice-coverage. This was a relatively large lead and we saw more
open water today than any other day in the study area.
Crabeater Seals were common in
the morning along one particular section of the lead, but through most of the
day seals were not abundant. Snow Petrels
were more abundant here than any other area in the study grid that we have
surveyed to this point. This is probably
a result of the relatively large amount of open water and ice-edge near the
ship, a habitat in which Snow Petrels typically forage. In areas of the lead that were covered in new
gray ice, we observed Snow Petrels landing on the ice and doing what seemed to
be picking through small holes in the ice in search of food. In
late afternoon, a single Adélie Penguin was observed hauled out on the ice
alongside the lead. This was the first Adélie
Penguin seen in several days.
A summary of the birds and marine
mammals observed on 18 August (YD 230) during 6 hours of survey time as the
ship traveled between stations 65 and 41 is the following:
|
Species (common name) |
Species (scientific name) |
Number observed |
|
Snow Petrel |
Pagedroma nivea
|
66 |
|
Antarctic Petrel |
Thalassoica |
8 |
|
Adélie Penguin |
Pygoscelis adeliae |
1 |
|
Crabeater Seal |
Lobodon carcinophagus |
69 |
Marine Mammal report (
One week has passed since the great
Minke whale day of 11 August. Since then, 15 sightings have been made, counting
30 whales in total in this “Minke Week”. To date, 125.1 hours of observation
have been achieved, 57.7 of which were “effective effort” hours. Eight hours of observation were done on 17
August, but only 45 minutes were effective effort, since most of the day we
were at station 65. In contrast, 7.9 hours of observation were done in 18
August with 6 of them effective hours.
August 18 was characterized by some fog patches and overcast to partly
cloudy skies. There were ice floes of
first year ice, shuga, and grey young ice varying from
6 to 10/10ths coverage, with variable sized leads of open water. In general,
the day presented good viewing and weather conditions for the cetacean survey.
Three whale sightings were made during these two days. A Minke whale was
observed on 17 August during a MOCNESS tow (incidental sighting). This whale
was seen at 1624 (-68º 07.37′S; -74º 39.44′W), 5º to starboard and
0.68 nm from the ship. The first sighting on 18 August was made at 1042 (-67º
31.28′S; -72º 50.65′W) 8º to port and 0.90 nm from the ship. Two
whales, probably Minke (they could not be identified), surfaced in a large lead
of open water. The second sighting of a Minke whale, was done at 1613 (-67º
35.42′S; -72º 01.85′W) 34º to port and 0.78 nm from the ship.
Only ten crabeater seals were
seen on 17 August (seals which were close to the station), whereas 61 seals
were observed on 18 August, most of them (34 seals) seen from 0900 (-67º 37.68′S;
-73º 09.44′W) to 1000 (-67º 35.60′S; -73º 16.90′W). A very peculiar and delightful observation
was made on 17 August while towing the 1-m MOCNESS. The ship was going back and
forth on a transect through a narrow lead of open water, while a crabeater seal
would come along swimming and breaching beside the vessel. What is peculiar about
this behavior is that every time the vessel would make its way back and forth
and would get close to this seal, instead of escaping (as any other “normal”
seal would do) it would get very excited and jump into the water and swim (“play”)
along Palmer's way.
Current Position and
Conditions
The plan to escort the Gould to station 41 was scratched due to bad ice conditions and a second plan to have the Gould make their second process station in the vicinity of grid stations 28 or 27 was ruled out this evening (19 August) for the same reason. We are currently steaming to grid station 43, further offshore where it is hoped that ice conditions will permit the Gould's next process station to be established there. Our current position on 19 August - 2325 hrs is -67º 21.339′S; -71º 25.638′W). Air temperature is -21.9ºC and the barometric pressure is 987.4 mb. Winds are out of 230º (southwest) at 20 to 30 kts. Skies are partly cloudy.
Cheers, Peter