NBP09-01 Cruise Report - British Oceanographic Data Centre

An automated camera system provided by Steve Ackley was installed on NBP0901 to provide a digitalarchive of the underway sea ice observations. One camera was mounted on an outside railing 1 deck abovethe bridge (outside the ice tower) with an oblique view of the area just aft of starboard. Another camerawas mounted on the 01 starboard railing, pointing down over the bouncing ball. The former view will bedigitally ortho-rectified by Ackley’s UTSA group (led by Blake Weissling) to provide quantitative estimatesof sea ice concentration. The latter ’bouncing ball’ view was intended to archive ice thickness, but given thelack of over-turned floes in summer pack ice, its archive will be of limited use. Also, as first-time installersand users of the camera system, we encountered some difficulties. The 01 ’EisCam’ was not functioning formost of the initial southward transect through ice (likely a wiring issue), and some hourly video feeds werelost due to software glitches. Despite these problems, we managed to archive most of the oblique views, andwe look forward to seeing post-processed results by Ackley/UTSA.Snow and Sea Ice SamplingFloe Selection and Deployment Sampling of sea ice and snow was carried out at a total of 20 icestations. This was facilitated by the “near optimal” ice conditions that were encountered. The lack of ice inPine Island Bay and north of the Getz and Dodson ice shelves permitted timely completion of Autosub andoceanographic work in these areas, and the light conditions in the main pack ice to the north kept transittime between stations relatively low. This allowed more time for sea work than originally anticipated, whilenorth of the polynyas there was sufficient ice along most of the cruise track that a suitable floe could be foundfor sampling along-track. In a few cases the ship was diverted into heavier ice for a few miles - primarily tofind sites suitable for deployment of instrumentation.Because of operational constraints and the thick and heavily deformed ice, sampling was generally confinedto ice in areas of low to moderate concentration (< 70%), or within a few tens of miles of the ice edge.This may introduce a potential bias in the observations to favour areas where melting conditions were moreprevalent than in the inner pack. There is no reason to suspect a bias in ice type or snow conditions, however.The cruise track allowed sampling along both the southern and northern edges of the Amundsen pack, as wellas transits through both the eastern and western sides of the pack and forays deeper into the cental pack,so that the sampling strategy is likely to have captured the overall variability in ice conditions in the region.The possible exception is an area of persistent heavy ice North of the Thwaites iceberg tongue, centered atroughly 72.5 o S and 111 o W.Although the number of ice floes occupied more than doubles the previous total for sampling summer seaice physical properties in the Amundsen, the dataset is fairly small for a robust statistical characterizationof the pack. Focus was therefore placed on resolving processes that control the summer evolution of sea ice,such as flooding and snow ice formation, snow melt and superimposed ice formation, and internal ice meltand the formation of gap layers.Sampling strategy was therefore not to select floes at random as is typical of dedicated sea ice cruises, butrather floes were chosen to best span the range of ice conditions, while focussing on level pans within thicker,older (and moderately deformed) ice floes (Figure 25Top). Despite the surface appearance of level ice atmany of our sampling sites, the underside was often quite a different story (Figure 25Bottom) as revealedby the photographic prowess of Maria Stenzel with logistical prowess from Zim Zimmerman, Julie Jackson,Amy Schaub and Mike Lewis.Heavily deformed sites were avoided to best facilitate interpretation of growth conditions and comparisonwith models. In the outer pack ice region, this was impossible as almost all floes were heavily deformed.At these sites, sampling was carried out away from ridged areas as best as possible. Fast ice sites werepreferentially sampled since these offer the best opportunity for model comparison, as the history of the iceis most readily determined via satellite imagery and deformation is least likely.Most floes selected were a few hundred meters or more across to facilitate station keeping as close-packedice was rare. Once a suitable floe had been selected the ship would either position alongside the floe forNBP09-01 Cruise Report (p. 37 of 83) Revised February 27, 2009