Since we arrived within the marginal-ice zone, and especially now in the sea-ice zone, the veracity of the Southern Ocean has been damped by ice floes at the water surface. The sea ice converts kinematic energy (oceanic surface waves) into mechanical energy (break up of ice floes) and potential energy (ice-ridge building). In doing so, the sea ice not only modulates the polar ocean-atmosphere system, but it also makes our journey more pleasant — we went from a confused swell of 2.5 to 3m in the open water just north of the ice edge, to about 0.5m of unidirectional swell in the marginal-ice zone, to none within the pack. This meant that most of us on the vessel could get some sleep (instead of holding on tight so as to not fall out of bed), and allowed our group to unpack some fragile gear, to test it in the cold environment.
Rob unpacked his Conductivity-Temperature-Density (CTD) instrument and gave it a dry run. I unpacked our Alaska-built Magna Probe, which is an autonomous snow-thickness gauge with integrated GPS position receiver. This probe has specifically been developed to efficiently obtain the snow-thickness distribution over ice (or frozen ground). The fast-ice work at Davis will be our first opportunity to use it to collect snow-thickness data. Once the integrated GPS was within good satellite range, the probe was ready for calibration and testing on a skinny layer of snow on the helicopter deck. Our measurements compared well to ruler measurements and the probe has been packed, ready to be flown off to Davis station for our fast-ice work.
Observing the sea ice along our track (see Blog 3), we have noted a large variety of ice types at a range of ice (and snow) thicknesses. So far the vessel’s Master and mates have been able to find sufficient open water within the ice pack to allow the vessel to travel at up to 11 knots. Their ice navigation is supported by a suite of satellite images, which are delivered daily to the vessel by the ACE CRC remote-sensing specialists.
At the vessel the large-scale ice situation has been tracked using relatively coarse (12km) passive microwave data, which depicts changes in ice concentration and also reveals the ice movement. In addition, 250m (so-called high-resolution) visual imagery are used to derive information on ice types and thickness. Unfortunately the visual imagery is often affected by clouds, meaning that little or no sea ice information can be derived.
Luckily, active microwave (SAR — synthetic aperture radar) data can be ordered on demand, providing very high spatial resolution information (in the order of tens of metres), independent of any cloud cover. However, so far the pack ice along our path has been relatively low in concentration, and SAR have not been required.
At this stage we look forward to an approximately 60 nautical-mile-wide belt of highly concentrated and thick ice, shown along our route by the passive microwave data. Once this has been navigated we anticipate light ice condition all the way to the fast-ice edge off Davis Station.
Petra Heil — sea ice physicist