R/V NATHANIEL B. PALMER DAILY SITUATION REPORT GMT DATE: 17 September 2005 METRICS: SS REMARKS: We spent our last full day at sea transiting with good speed toward the Bahia Posession pilot station and Punta Arenas beyond. We are scheduled to pick up the pilot in a few hours and anticipate an on-time arrival at Prat Pier. This will serve as the last daily sitrep of cruise NBP05-06. GMT TIME: 0000 0600 1200 1800 POSITION: LAT: 53*12'S 53*04'S 52*55'S 52*47'S LONG: 60*36'W 62*15'W 63*48'W 65*36'W WEATHER: TEMPERATURE (C): 6.4 7.0 8.0 9.5 AIR PRESSURE(MB): 1007.5 997.6 992.4 991.1 REL HUMIDITY (%): 79.3 85.9 80.4 72.1 WIND SPEED (KN): 17-20 20-25 20-25 20-30 WIND DIRECTION (T) NW NNW NNW W SWELL HEIGHT (ft): 6-8 6-8 6-8 6-12 SWELL DIRECTION: NW NW NW NW SALINITY(PSU): N/A N/A N/A N/A SEA SURF TEMP(C): N/A N/A N/A N/A VISIBILITY (NM): 3-8 6+ 6 8+ CLOUDS (%): 80% 100% 90% 70% SEA STATE: 2-3 3-4 3-4 4-5 ENGINES/PITCH: 2/75% 2/80% 2/70% 2/80% FUEL ON BOARD (@ 0000): 120,100 Karl Newyear Marine Projects Coordinator R/V NATHANIEL B. PALMER Raytheon Polar Services Company email to: mpc@nbp.usap.gov 4 Sep 05 At the time of writing, we are midway through Phase 3, Drift 11, which will be the final one. We pull the instruments aboard for the last time by 2000 this evening, and will curtail science activities except for a couple of deep CTD stations and several XBT (expendable bathythermograph) launches during egress from the ice, along with continued meteorological observations on the journey back to Punta Arenas. In the past week we continued three Phase 3 drifts (P3D7- 9) in the low stability region on the SW flank of the Rise, and conducted an ad hoc iceberg experiment (see below). After weather forecasts reaching toward the end of the science program indicated no significant changes, we decided to return to the north, first to the transition region where warm core "halo" water interacts with the Maud Rise "Taylor cap" (P3D10) near Phase 1 station 12. Following that we again moved north to the site of Phase 1, station 3, for a drift in the relatively cold, stable water found north of the halo, where we are now. Winds have been quite light – the mean hourly wind speed for the week was around 7 m/s. This is disappointing in that we lacked a really significant wind storm to stir things up after we started Phase 3. On the other hand, we measured several moderately swift events during the ten previous drifts that should provide a wealth of data on what maintains the amazingly delicate balance between the thin ice cover and upper ocean stability. >From a technical standpoint, we demonstrated a number of firsts in how the Palmer can be used successfully as a drifting platform in ice too thin for safe off-ship deployment. If pressed to choose the most significant, I would say that the capability of rapidly clearing the moonpool, along with the van arrangement for housing an automated cycling system, contributed greatly to the success of the experiment. Definitely worth the pre-cruise effort! At times when the other systems required fending off errant ice floes or breaking newly formed ice (with a heavy shackle attached to the sea cable in the case of the forward frame), the moonpool operation has clicked along with very little interruption. No less significant has been the ship-handling technique (described last week) that has evolved rapidly to provide us with long lasting ice free (or at least manageable) "breathing holes." During P3D8 (Mon-Tues), we were in view of several smallish icebergs, drifting as we were, with the ice pack (since their location relative to us did not change). These typically rise about 30 m above the surface, so must extend downward to well below the mixed layer. At J. Morison's suggestion, we decided to spend a few hours investigating, at least qualitatively, what impact a small berg might have on the pycnocline. The strategy was simple: measure the upper ocean with the moonpool profiler on the downwind ("upstream") side of the berg, then move to the opposite side (downstream) to see what if any changes occurred. We also rigged the ship CTD to collect water samples in the wake, to look for O18 depletion indicating water of meteoric origin (glacial ice melting). The results were pretty impressive, with the pycnocline rising from about 150 m depth on the upstream side to around 70 m in the immediate wake. Toward the edge of the wake, the pycnocline was very active. During the study, the berg drifted about 3 km NNE with drift velocity ranging from about 15 cm/s at the start to ~10 cm/s near the end. In addition to its intrinsic interest, the exercise (which as far as we know is unprecedented) again demonstrated the utility of the Palmer as a highly mobile drift station. Tonight we end the intensive data collecting and begin packing for the long haul back to PA. We are thankful for a rich, scientific harvest. Following are brief reports from each PI subgroup: Stanton, Ocean Turbulence (w/Shaw, Stockel): Phase III operation of the deep turbulence frame and temperature, salinity and microstructure profile timeseries systems from the ship have been successful for 11 short drift stations. The deep frame deployments have gone well thanks to the use of the tall forward storage lockers on the Palmer, and we have only had to halt observations twice due to ship or ice movement. After leaving the southwest drift sites, and starting the last two drifts over the outer edge of the halo and north of it, the ice has formed more rapidly and has required frequent clearing of a hole around the frame cable from high up on the forward deck. Processing and analysis of the frame data has shown well resolved shear structures across the pycnocline from the dual ADCP systems on the deep mast. The 40 element thermistor string failed during phase three drift 5 due to a sea-water leak in one of the potted assemblies, but the previous data resolves vertical thermal structure and turbulence within or above the pycnocline depending on the frame position. Preliminary analysis of the 2500 CTD / microstructure profile timeseries from the moon pool-deployed cycling instruments reveal high resolution T/S and turbulence level profiles to 350m throughout the drift operations. These measurements quantify vertical fluxes through the weak pycnocline that are being compared with the concurrent eddy-correlation flux measurements within the lower and upper mixed layer. These observations are being used to understand the processes that maintain a very delicate balance between ice production (and destabilization) from the cold, moderately forced ice cover, and continual vertical transport of heat and salt from the pycocline, due in part to tidal / inertial shear across the pycnocline. The local ship observations of upper mixed layer fluxes are being extended regionally through the 4 ocean flux buoys deployed earlier in the cruise McPhee, Ocean Turbulence (w/Sirevaag): We continued measuring turbulence with the midlevel mast during all the drifts, usually with it situated about 50 m deep in the mixed layer. Preliminary results have shown brief episodes of large upward heat flux, one example reaching as high as 90 watts per square meter (1-h average) during P3D8. Analysis of deep frame turbulence cluster data has also begun. Harcourt, Modeling (w/deSteur, Powell, D. Morison). Float tracking operations concluded without further contact with either float 29 or 33 (that was distinguishable from noise) during subsequent drifts, beyond the initial tracking fixes on float 29 over the two days following its deployment near Station 91. Assisting in Vampire and Yoyo CTD watches (Harcourt). Cable model operational, with critical input during ice drifts with orientations made difficult due to loss of NBP bow thruster. Assisting in Yoyo CTD watches and operations (D.Morison). Ice model is run operationally now and updated daily with each AMPS forecast, and the comparison of motion with both ship drifts and flux buoys has been consistent. The Lagrangian ice model will be updated with fore/nowcasts through the end of September for purposes of ocean modeling and float data interpretation. Assisting in weather observations with Guest. (Powell). Examining Maud Rise flow characteristics in light of new bathymetric data. Assisting in Vampire operations (deSteur). J. Morison, Cycling CTD: The YoYo CTD/LMP continued operating normally this week through the various Phase III drifts. As of Sept. 3 at 1200 we have made a total of 2478 profiles (through about 743 km of water column). On Tuesday, August 30, we took advantage of an opportunity to briefly study mixing in the vicinity of an iceberg we encountered during transit from drift 8 to drift 9. Icebergs are a potentially important source of fresh water and mechanical mixing in the Weddell Sea. Though our study was not exhaustive, as far as we know this was the first time such specialized instrumentation has been used to examine ocean mixing near an iceberg. The "Maudberg" was 200-300 m across and about 30m high. We first set the ship about 400 m upstream of the iceberg and profiled briefly with the YoYo system. We then made longer sequences of YoYo measurements downstream to one side and directly downstream of the iceberg. Additionally we made mid-depth turbulence measurements and used the ship's CTD rossette to obtain a deep profile and water samples for del O18 detection of meltwater at the downstream position. The iceberg geometry was mapped with radar observations and photographs. The pycnocline showed increased variability on the downstream sites and the site directly behind the berg showed a marked shoaling of the mixed layer. We have estimated the reduced gravity or interfacial internal wave speed near the Maudberg to be about 0.27m/s. The speed of the ice relative to the upper ocean as measured by the ship's ADCP was 0.17m/s near the end of the experiment. So the Maudberg was not traveling at the speed of interfacial gravity waves but probably was moving close enough to the critical speed to explain the observed large variations in mixed layer depth in terms of an internal wake. An iceberg moving at near the interfacial wave speed would experience substantial internal wave drag, the "dead water" effect. This may explain the observations that when the wind blows hard sea ice moves around icebergs, but with reduced wind speed, such as during our study, icebergs are towed along by the surrounding sea ice. The sea ice rubble heaps we observed on the "shore" of Maudberg were likely made when the ice drift was 0.5 m/s and Maudberg's drift was retarded by internal wave drag. Guest, Meteorology (w/Powell): We continue rawinsonde and radiation measurements. We performed another low-level series of profile measurements using a kite. Analysis of previous kite flights shows this method can be successfully used to quantify sensible and latent heat fluxes from leads. Goldberg, Notz: ASPECT observations will resume as we depart the ice in the next few days. Padman, Muench: During the last week, Vampire operations from the fantail helo hut during Phase III Drifts 6-10 yielded ~280 microstructure profiles to a maximum depth of ~560 m. Drifts 6-9 provided many examples of the steppy pycnocline that appears to be correlated with low stability of the surface layer relative to the underlying deep water. Our previously expressed hypothesis, that the steps are a response to shear and nonlinear equation-of-state processes (primarily cabbeling), remains viable in light of the current week's data. The just-completed Drift 10 yielded 51 profiles during a period of cold, calm conditions with a slightly more stable stratification due to a fresher surface layer. Active mixing was found throughout the surface layer, with several examples of upward diffusion of heat from the upper pycnocline into the lower mixed layer. We hypothesize that this mixing was associated with convection driven by local ice formation and associated brine rejection in the lead created by the ship. In previous drift stations the drift rate relative to the upper ocean was sufficiently rapid that measured mixed layer conditions were most likely a response more to under-ice processes upstream of the ship drift. During Drift 10 the negligible drift of the ship relative to the upper ocean suggests that local lead processes created the mixing and hydrographic states that we measured. Acoustic backscatter from the hull-mounted ADCPs has helped with interpretation of the high-frequency variability in the pycnocline. It is clear that the signal includes a significant biological component – diel migrations centered around local midday are the dominant signal in backscatter records from all drift stations. Nevertheless, it appears that high frequency variability can be interpreted as pycnocline displacements, resolving time scales faster than either profiling system (yo-yo CTD and Vampire). Miles McPhee, Chief Scientist MaudNESS Science Summary No. 8 11 Sep 05 We finished the final drift station Sunday evening a week ago, pulled the helo hut support bracket back aboard, moved the cycling CTD van to the helo deck, and were underway without much delay. On Monday, we did a deep CTD station near 63*S, 0 E, close to two previous stations. We transferred the forward deep frame winch back to the helo deck by moving it on to the ice with the forward crane, then easing the ship forward enough to pick it with the midship crane. All went smoothly. After it had become apparent from the Terrascan imagery that the SW and W winds of the previous week had moved the ice edge northward thus making our transit in the ice longer, we decided to forego a planned second deep CTD station. We did maintain a schedule of XBT drops at quarter degree (~15 nm) spacing on our NNW track out of the ice pack. We put good use of the time in the ice to pack, label, and secure the scientific gear, which was just as well since when we did exit the ice it was in the face of a 40 kt west wind with enough swell to ensure that the ship was a pretty quiet place on Friday. Yesterday and early today we have been fairly close to the northern ice edge, hence less fetch. We are making up time lost earlier, and will still be able to call at Grytviken, probably in the morning of 13 Sep. Today the wind has shifted to north at 35-40 kt, so we are getting a good roll, as opposed to the pitch that comes with the west wind. A highlight of the ice egress was a gracious offer from Capt. Mike and 2nd mate Rachelle Pagtalunan to let scientists take the helm for short periods during the afternoon watch (under close supervision). So almost all of us now can tell our kids and grandkids that we steered the ship through the treacherous ice pack of the Weddell, with Titanic size icebergs on all sides…(well, I suppose with time it will undergo some exaggeration). Regardless, we all appreciated the unique opportunity. The long transit back to Punta Arenas is providing time for preliminary analysis and collation of the data gathered during MaudNESS. Below is a summary of the measurement activities since the end of Phase 3, Drift 11 a week ago. Meteorology, (Guest w/Powell) The rawinsonde program was completed with a cross-ice edge survey. One more kite flight was performed this week and radiation measurements continue. Goldberg, Notz: ASPECT observations halted on Thurs night as we left the pack ice. Padman/Muench: After Phase III we headed north towards the ice edge, obtaining a transect of temperature with T-5 XBTs (max depth of 1830 m) at 1/4 degree spacing in latitude from 62.75 to 56 degrees S. The transect identified the approximate edge of the "cold regime" (cool T-max water at the northern edge of the Weddell Gyre) near 58.5 degrees S. Occasional anomalies in T_max suggest the presence of eddies or filaments associated with this front. Miles McPhee, Chief Scientist