Mar 03 - Ice work
Noon Position : lat 74 18.0 S, long 36 06.0 W
Course made good: Variable
Air temperature @ noon today : -1.3 degrees C
Sea temperature @ noon today : -1.1 degrees C
Wind: Direction E, Force 4
Sea Ice Science
This week we have been concentrating on sea-ice science. Jeremy Wilkinson explains a little more;
Measurements of new ice formation
Once the surface of the ocean is cooled to freezing any additional heat loss produces a slight supercooling of the water. Once the upper surface of the ocean is below freezing ice formation can occur. The first evidence of ice formation are small crystals known as frazil. If both the wave and wind effects are reduced the agitation of the frazil ceases and the surface layer of frazil can begin to consolidate. This in turn enables the frazil fuse together to form a continuous sheet known as nilas. Nilas is observed to be extremely plastic and can easily bend with the waves. Further thickening of nilas will continue as a unidirectional process by which seawater freezes directly to the underside as heat is conducted through the ice. This is known as congelation growth. Nilas can also become thicker through the rafting or ridging events.
However if wave action continues pancake ice forms rather than nilas. The diameter of primary pancakes depends on the high-frequency part of the wave spectrum. Around the circumference of pancake ice is a raised ridge initially 1 or 2 cm higher than the surrounding plane of the pancake. These ridges, which are generally uniform in height, are the first part of the pancake to protrude above the water surface and are therefore the first indication that pancake ice is forming. They are formed by the piling of frazil ice around the edges of the pancakes by friction between them. This can be from general jostling and collisions between pancakes and the cyclic pressure pulses of the wave field i.e. as pancakes are compressed together at the trough of a wave frazil ice is pushed up on the sides of the pancake. The edges appear grey at first, but as the sea water drains become white in appearance.
Inside the raised rim the surface is usually quite smooth and moist. The watery nature of the centre of the pancake is due to the greater salinity of mixed separated brine and seawater. Snow falling on central area of the pancake will melt due to this salinity, however over time it will be slowly diluted enabling a full snow cover to be developed.
Immediately upon formation pancakes, like nilas, are very porous and have little strength as the frazil bonds between the newly formed pancakes are very weak. When touched newly formed pancakes will break into horizontally floating discoids and they will not sustain their own weight if removed from the ocean. However as time passes, and sub-freezing air temperatures continue, the interstitial water between frazil crystals freezes, individual frazil crystals grow and brine drainage occurs, all of which strengthen the pancake.
Over the years we have studied brine rejection in both frazil and pancake ice in the Arctic. This cruise give us the opportunity to extend this study to the Antarctic.
During the cruise two satellite-transmitting buoys were released on the sea ice. These buoys send back information about the movement of the sea ice, the air temperature, air pressure and wave field. All data is processed by the buoys and them sent to our office back at SAMS. Once deployed the buoys are not expected to be recovered.
The area chosen for the buoy deployments were large, thick multiyear floe deep inside the pack. These sites were chosen as it should enable the buoys to survive any compaction of the pack due to storm activity.
Hand in hand with the buoy deployments was the characterisation of the floes the buoys were deployed on. Measurements included ice thickness through drilling, salinity and ice density measurements from coring and snow density measurements from snow pit measurements.
Yet more penguin photos.......
With all of the excitement on the ice floe, a couple of Emperor Penguins came over to investigate. They weren't quite sure what to make of the science team, their equipment or the holes that were dug in the snow. Here is a photo of the intrepid party leader, Jeremy Wilkinson, with the two visitors. Is Jeremy really that short?! (With thanks to Martin Stott and Povl Abrahamsen for the picture and the photoshop skills!)
Okay, so they're not penguins. This week we had a wonderful opportunity to watch these Minke
whales as they were playing, just a few metres from the side of the ship. We think they may have been flirting with the CTD! Thanks to Martin Stott, who was quick off the draw with
The James Clark Ross and the Ernest Shackleton
This week, we headed further south towards the Fichner ice shelf. If you remember, this area has a lot of sea ice at the moment and we weren't sure that it would be possible to get to the ice shelf at all. As we headed south it became apparent that it would have been very difficult to work the ice shelf programme in the present ice conditions. However, it didn't stop us getting on with the sea ice projects outlined above. As we travelled south we passed the Brunt ice shelf. Unfortunately the closest we got to Halley was seeing the drum lines, (these are guidance lines from the creek to the base), in the distance. We did get to stop to pick up some of the outgoing Halley summerers. These people had been housed on the Ernest Shackleton overnight and then we picked them up at creek N9 on the way past. As they transferred across, there was much frantic waving and shouted messages between the two ships, as well as a lot of activity from the photographers in the small boat that went from one to the other. Photo by Simon Wright
These two photos show just how lovely being in the Weddell Sea can be. We have had our fair share of cloud and snow over the last ten days, but probably not as much snow as there has been in the UK by the sounds of it! We are just coming to the end of this science programme and will soon be on out way north heading for the city lights of Stanley. It will take about a weeks cruising to get there and we will be moored alongside the Ernest Shackleton again. Until then, goodbye.