The weather continues to be overcast with no breaks to let a little blue sky through during the short daylight period. The winds were in the less than 20 knot range for most of the morning. But now they are up around the 30 kt range and the sea is getting rougher. There are lots of white caps and a developing sea. We are now working at Station # 81, the last one on transect line 12. Our current position at 1543 local time is -69 47.631°S; -76 57.928°W. Winds are out of the northeast (060) at about 30 kts and the air temperature is -.6°C.
The weather on 18 May was moderated significantly from the day before and the seas were much reduced enabling all of the programmed activities to take place. We came close to Alexander Island during the early afternoon, but it was so cloudy and foggy, and there were snow flurries falling, so that there was not much to see. We did get a glimpse of the island through binoculars from the bridge while there was still some light (The sun is now up for about 3.5 hours where we are; more about this below). It is light for some time before and after the sun rise/set, but with the thick layer of clouds, even at high noon, it looks like dusk on a very cloudy day.
Good working weather occurs when the wind speed and associated sea state is 20 knots or less. Between 20 and 30 knots, working conditions become marginal and many pieces of equipment become difficult to deploy off even large oceanographic vessels like the N.B. Palmer. In addition, the movement of the ship causes the bobbing of the instruments up and down on a cable during vertical casts or during oblique net tows, and this can degrade the quality of the data or samples obtained. Above 30 knots, conditions become awful and most over-the-side work must cease. We have done a preliminary analysis of the wind speed (using 5 minute averages) on this cruise from the above perspective starting with the day when the survey was started on 28 April and going to 14 May. The percent of time spent in the three wind speed intervals was: 0 - 20 kts = 39.5%; 20 -30 kts = 38.0 %; >30 kts (up to 50 kts) = 22.4%. Thus, during most of the survey, we had marginal to awful working conditions for about 60% of the time. In spite of this, we will finish the survey tomorrow and will have a remarkably good data set.
Eileen Hofman reports that the calm conditions and light winds made for a pleasant day of CTD and XBT work along survey transects 10 and 11. This was a welcome change from the past few days. CTD casts were completed at survey stations 76, 77, 78 and 79 and XBTS were deployed half way between these stations. The resulting observations allowed us to map the coastal current that flows out of Marguerite Bay onto the west Antarctic Peninsula continental shelf.
The temperature distribution below 200 m indicates that the coastal current either undergoes a large meander or dissipates to the west of survey transect eleven. The additional measurements that we will make as we move offshore along survey transect twelve will allow us to distinguish between these two possibilities.
The vertical temperature and salinity structure at the inner stations
on survey transects ten and eleven indicate considerable mixing and/or
interleaving of different water types. The variability in these profiles
suggest that the inner shelf is a dynamic environment. The upper water
column at these stations shows almost homogeneous properties which indicates
that the Antarctic Surface Water and Winter Water layers have been eroded.
This is a precursor of winter freezing of the surface waters. It is anticipated
that continued cooling over the next few days will reduce the water temperatures
to the freezing point.
Bob Beardsley's Met Notes on May 18: Insolation (or the lack of it!)
The shortwave radiation from the sun that reaches the earth's surface is called "insolation". At a given location, the amount of insolation is a function of the observer's latitude and time of year. As we progress along our CTD survey, the insolation reaching the ship decreases for two primary reasons: a) we move further south from the Antarctic Circle and b) we approach the summer solstice (June 18) when the sun is lowest (highest) in the southern (northern) hemisphere. In addition, the skies have been cloudy much of the time, further reducing the amount of shortwave radiation reaching the ocean surface.
Below is a table showing for each day in May, the mean latitude of the
ship, the length of time in hours that the sun is above the horizon, the
mean measured shortwave radiation averaged over sunup, and the fraction
of the incident shortwave flux that reaches the surface from the top of
|Day||Latitude||Sunup||SW||SW/SW(top of atmosphere)|
Note the general decline in observed insolation SW during May, the decrease in hours of sunlight, and the small values during the last week. The small magnitude of the measured shortwave flux suggests that solar shortwave radiation plays a very small role in the local heat flux during fall, perhaps contributing less than 10 W/m2 heating on average in May. The ratio of shortwave radiation falling at the earth's surface to the shortwave hitting the top of the atmosphere is called the atmospheric transmittance. In the Georges Bank/Gulf of Maine region, the transmittance is typically 0.6 to 0.7. Assuming the atmospheric transmittance over the Antarctic peninsula is 0.6 on a clear-sky day, the measured transmittance listed in the last column is generally much smaller than 0.6, varying from a high of 0.33 to a low of 0.05. In terms of measured daily mean SW divided by the clear-sky values, the amount of insolation reaching the ship varies from a high of 55% to only 8%, with a mean of 30%. These low levels of insolation are due to the persistent cloud cover over the Marguerite Bay and adjacent shelf area. For comparison, during February 1995 on Georges Bank, the minimum transmittance was 11% and the mean transmittance 58% of the clear-sky value. Thus, while cloud cover in both regions can reduce the minimum daily insolation to roughly 10% of the clear-sky value, the Antarctic peninsula is almost always cloudy while clear sunny days do occur over Georges Bank during winter.
These preliminary conclusions assume that the NBP shortwave sensor system is working correctly. A preliminary comparison of the NBP shortwave measurements with those made on the LMGould was made for two daytime periods, when both ships were collocated at Palmer Station and later in Marguerite Bay. The agreement of measured insolation was good, indicating that both systems were working and returning equivalent data. The comparison between the peninsula and Georges Bank transmittances also indicate the NBP system is working fine. Additional intership comparisons will be made when the two ships met over the next few weeks.
Wendy Kozlowski and Michael Thimgan report that over the course of the last twenty days, we have carried out numerous experiments for the determination of primary production. At 71 of the 77 grid stations sampled so far, we've done a total of 104 short term incubations for determination of gross photosynthesis rates. Fourteen of those stations were surface water samples taken by bucket over the side when weather did not permit sampling with the rosette. We've also done fourteen 24 hour, on-deck, incubations for estimations of daily photosynthesis rates at multiple levels throughout the water column. In addition to the water column production work, we were able to collect and process samples from two different types of ice in the southern end of Marguarite Bay. Until its untimely death on May 11th (an as-of-yet undetermined and un-fixable internal circuit failure), we also deployed a Chelsea Instruments Fast Repetition Rate Fluorometer at 40 stations as part of the CTD package, for additional data for use in modeling primary production.
To compliment the production work, we also closely monitor daily surface PAR levels, as well as water column PAR when available from the CTD data set. In addition, we have collected 191 samples for analysis of particulate carbon and nitrogen, preserved 40 slides for microscopic identification and quantification of picoplankton (phytoplankton 2 in size and smaller), preserved 51 samples for quantitative total phytoplankton counts, cast and processed twelve, 5 micron mesh ring net samples, and filtered 54 samples for total particulate spectrophotometric absorption (ap) analysis. With filtration assistance from the CTD group, 476 samples have also been collected for chlorophyll analysis, which we will begin once the grid work has finished. The fluorometer to be used for this analysis was calibrated with specific focus on low level chlorophyll concentrations earlier in the cruise.
Looking at daily production measured at stations through 220.220 (consecutive station 57), values ranged from 136.0 down to only 1.6 mgC/m2/day. Interestingly, the station with the highest measured production was at 340.253 (#23), one of the further off-shore stations, and the lowest production station to that point has been inside the bay at 300.-020 (#33). Once chlorophyll data are complete, we will also look at production numbers in terms of total biomass, and will be able to estimate maximum photosynthetic rates and saturation constants for all stations where PE (photosynthesis vs irradiance) experiments were carried out.
Chris Ribic and Erik Chapman report that on 18 May they surveyed for
3 hours between stations 76 and 77. This transect brought us into cold
coastal water near Alexander Island that has been associated with relatively
large numbers of Snow Petrels in two previous surveys (JD 132 and 136).
Again, they found a large number of Snow Petrels in the coastal water.
Overall, results were similar to those from coastal water just north of
today's survey(JD 136). In both surveys they recorded a relatively large
number of Snow and Antarctic Petrels. Results from all three surveys from
the coastal water mass are given below:
|(JD 132*)||(JD 136+)||(JD 138**)|
|Antarctic Petrel (Thalassoica antarctica)||9||38||61|
|Cape Petrel (Daption capense)||18||6||1|
|Southern Fulmar (Fulmarus glacialoides)||48||18||1|
|Blue Petrel (Halobaena caerulea)||0||40||9|
|Southern Giant Petrel (Macronectes giganteus)||5||6||0|
|Snow Petrel (Pagrodoma nivea)||170||121||96|
* north shore of Alexander Island, 4 hr 19 min
+ west shore of Alexander Island- north, 4 hr 19 min
** west shore of Alexander Island - south, 3 hr
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager and
On the 18th of May, BIOMAPER-II towyo's occurred between stations 75 and 80. The towed body was brought out of the water at Station 76 to make way for a MOCNESS tow and then re-deployed at the end of the station work. While on deck the nose cone was patched to fix the results of a couple of encounters with the stern of the ship during recent recoveries. The topography along this portion of the survey line was shallower and more rugged than on the outer shelf, but not as variable as was observed in the vicinity of stations 68 and 69 a couple of days ago. While patches and layers of high volume backscattering were more numerous than further offshore, no adult krill patches were observed. The VPR images were dominated by copepods (many with large oil sacs indicating an overwintering stage) and medusa at depths around 140 meters. Also observed were pteropods, polychaete worms, larval krill, and siphonophores.
A MOCNESS tow to 340 m (about 10 to 15 meters off the bottom) was taken at Station 76 near the middle of the continental shelf. The eight nets sampled 50 meter intervals from the bottom to 50 meters and 25 meter intervals from 50 m to the surface. The depths between 340 meters to 100 m were dominated by gelatinous animals (salps and jelly fish); copepods, pteropods, and polychaete worms were also noted. The 100 to 50 meter zone was dominated by larval krill; gelatinous animals were largely absent. The 50 to 25 m interval was sparsely populated. Most noteworthy were the presence of a number of fish larvae about 30 to 40 cm in length. Krill larval forms were also noted in the upper 25 m sample, but in much lower abundance. Absent from these samples were adult krill.