The last major effort of the SO GLOBEC broad-scale cruise to the Western Antarctic Peninsula was completed today (25 August). The work along transect line 3 ended at station #16, which is located about 20 nm west of the middle of Adelaide Island. Unfortunately, the weather changed during the last 12 hours. Around midnight, the stars were shining and a half-moon, low in the sky, was a bright yellow. Winds were out of the southwest. Today, the snow covered peaks of the Island were not visible, because a weak low pressure system moved in and brought with it low clouds, brisk northeast winds, and snow for a good portion of the day.
There are still some tasks that remain to be finished as we make our way north out of the pack ice and into open Antarctic waters. Among these is a transit tomorrow along the ice edge zone surveying for birds and marine mammals, which may be much more abundant in this zone than in the survey area. We are currently steaming towards station 4 where a repeat 1-m2 MOCNESS tow is planned on the way to the ice edge. This was the station where this net system had it most serious collision with the pack ice and the samples were lost. Our position at 2044 is -66° 51.742S; -69° 28.411W. Winds are out of the northeast (050) at ~18 kts, down from the 25 to 30 kt winds around mid-day. The air temperature is -10.1°C and the barometer, at 989.3 mlb, is coming up after dipping down just a bit as the weak low pressure system passes us by.
Work on 24 August was completed at stations #24, 21.5 (halfway between stations 21 and 22), 13, and 14. XBTs were used to obtain temperature profiles at stations #24, 21.5 and at halfway points between them. CTD casts to the bottom were done at station #13 and 14 as part of the study to look at the influence of offshore water on the shelf along transect line three and XBTs were dropped at the halfway points. A partially successful 10-m2 MOCNESS tow was done off the shelf at station #13. The tow was taken into the wind i.e., to the southwest, but the Antarctic Circumpolar Current was coming at the ship moving towards the northeast. As a result, the wire tended to the port and got frequently hung up on the ice. About 2/3 through the tow, it was aborted, but it still took a long time to get it up and on board. Pulling the nets on board was a real struggle. The nets froze almost immediately after they came out of the water and pulling them up out of the water and over the stern was like pulling up leaden sheets. BIOMAPER-II was deployed at the end of the work at station #13 and towyoed on the transits to stations #14 and 15. The Plummet Net was used at station #14 to try and sample one of the small patches of strong acoustic scatterers that were observed between 60 and 90 m as the station was approached.
John Klinck reports that on 24 August, the CTD group has gotten busy lately launching 10 XBT probes and doing two ctd casts. The XBT probes were dropped along the shelf break between stations #46 and 13 (including 46, but not 13) at about 20 km intervals. The CTD stations were on the outer end of the 420 line, which is being repeated. Detailed comparison of theses repeat stations will be reported in the next day or so.
The XBT series covers 6 stations along the shelf break. Ten probes were used; there were 5 failures or incomplete profiles. The probes were good to 500, 270, 466, 760, 760 and 760 m, respectively, with maximum temperatures of 1.82, 1.78, 1.70, 1.89, 1.86 and 1.71, respectively. The warmest water indicates the ACC along the shelf break, while the 1.71 at the northern end of the section indicates that the ACC is offshore of the shelf break.
Station #13 (840 m) is on the shelf break with a mixed layer to 90 m, although there is a significant salinity gradient across this layer. A relatively smooth pycnocline does not show much layering activity. A broad oxygen minimum spans 300 to 500 m depth while a temperature maximum (1.45° C) occurs at 420 m. Conditions at this station have not changed substantially between occupations.
Station #14 (530 m) has a mixed layer to about 80 m with no structure. A bumpy pycnocline extends to 250 m or so. A broad oxygen minimum occupies the depth range 250 to 400 m. A 50 m thick temperature maximum (1.5° C) layer is centered on 350 m. A layer of uniform temperature (50 m thick) exists at the bottom.
Ari Friedlaender reported that on 24 August, marine mammal observations began at 1215 en route to station 14 (-66° 21.632S,- 71° 21.124W). Skies were cloudy, becoming less so as the day wore on. Ice conditions were 9/10 small and medium cakes with brash between them and some small newly frozen pools. As well, there were several bands of larger (300 m across) leads that extended for several kilometers. Visibility was good, at 2 nm. We arrived on station at 1515 having seen 2 crabeater seals and no cetaceans.
Chris Ribic and Erik Chapman report that on 24 August (JD236), they
surveyed for 2 and a half hours between stations #13 and 14. Ice conditions
varied between 9 and 10/10ths concentration and the dominant ice type was
cake floe. The survey took us over the shelf break where just 12 Snow Petrels
and no other birds were seen.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, and S. Gallager):
The BIOMAPER-II section along transect 3 was quite successful. The system worked as well as it can at this stage in the cruise with all subsystems (Acoustics, VPR, ESS) functioning at a performance level that was OK. The towyoing through the pack ice was reasonably good in the early afternoon when the Palmer steamed from station #13 to 14, and early on the way from #14 to 15. Ice thickened up and required 4 engines about midway to station #15 and there were a number of wire snags on large chunks of ice when the wire was paid out maximally i.e., 250 mwo. But because they were caught quickly, they only resulted in BIOMAPER-II dipping deeper down into the water column. Only once was the backing of the ship and "blowing" the ice away with the ships propellers required for an extended period (~10 minutes) to get the wire loose from the ice. After station #15, the towyos got easier in thinner ice and more leads, and with the approach to station #16 large leads became available to get to the last station on the transect line. As we came onto station #14, a series of "krill" patches showed up on the echograms between about 60 and 80 meters. A Plummet net cast was done to see what was in the layer and a some euphausiid furcilia were caught. Much deeper in the water column, a moderately strong backscattering layer extended from the sea floor to more than 100 m above it.
There were no 1-m2 MOCNESS tows taken on 24 August.