Report 9: More sunshine and sunsets

Aurora Australis helicopter deck
Chess amongst the bergs (Photo: Steve Nichol)
SunsetCTD (Sonde) oceanography instrument

Steve Nicol, Voyage Leader

Between Transects 3 and 4 we had another stunning day sailing eastwards along the continent through pack ice and past massive icebergs in sparkling sunshine. The wildlife put on a special display with sightings of Ross and Leopard seals, orcas and minkes and flocks of small grey birds that kept the ‘birdos’ in such a state of over-excitement that the doctor was nearly called to help sedate them. Unfortunately, Cath had just seen her first ever Ross seal and was in no state to administer calmatives to anyone but herself. At one point the wall of a nearby iceberg collapsed precisely at the only moment during the entire voyage when no cameras were present on the bridge leading to a matching avalanche of colourful language. The sunsets were, however, a highlight and as the sun slowly lowered itself to the horizon between the bergs all aboard slowly fried their retinas waiting for a green flash that never came, but with the wonders of digital photography doubtless there will several images from that night purporting to represent this sought after phenomenon.

Most people when they think of the Southern Ocean think cold and wet but especially, cold. To oceanographers it is these things and more. Some definitions of the Southern Ocean have it starting relatively far north at a boundary called the subtropical front but for many, the true Southern Ocean starts south of the Sub Antarctic Front. This boundary, also referred to as the Antarctic Convergence marks the point where the cold polar waters flowing northwards dive beneath the warmer northern waters and there is often a distinct and noticeable drop in temperature associated with crossing this system from north to south. There is also, as with many boundary systems, a localised increase in biological activity in this area. Once within the ring of the subantarctic front one enters the Antarctic Circumpolar Current (ACC) which is the world's largest current system and the only truly global ocean current as it flows eastwards around the Antarctic continent in an unbroken stream. In some sectors of the Antarctic, such as south of the Pacific the current is very wide, but in others it is squeezed between the Antarctic continent and other land masses. The best example of such a pinch point is at the Antarctic Peninsula where the entire ocean flow is compressed into a very narrow opening south of Patagonia with an associated increase in speed. In the region where we are conducting our survey the ACC is mostly fairly wide but it does get constricted as it passes over the Kerguelen Plateau. The ACC is also affected by a number of large-scale gyres and the one that affects the BROKE-West survey area is the Weddell Gyre which touches on our western-most legs with its southward flowing flank.

Perhaps the most dramatic feature of the survey area, though, is not the wide oceanic river of the ACC but is the narrow (<50 Km wide) current jet that flows towards the west just offshore of the continental shelf break. Instead of the sluggish eastwards offshore flows of around few cm per second we see here, speeds of between 10-20 cm sec-1. This westward current flow has a marked effect on both the physics and the biology of the region. The flow rates of these currents are measured in a number of ways. Data from the CTD allow calculations to be made of sea-surface height (or pressure) in the ocean, and because water flows along lines of constant pressure, just like in weather maps, the strength of flow can be estimated, and this is particularly useful for basin-scale estimates of water transport. More directly, satellite tracked buoys can periodically send data on their position and this can be used to detect ocean movements and these are showing up the complexity of the eddy structures in the upper ocean. Recently, acoustic current profilers have been used and these provide direct measurements of velocity from the particulate material in the water – generally the organisms living there which gives oceanographers an indirect interest in biology – and these measurements can provide us with horizontal and vertical profiles of current speeds. The detail of the information from these acoustic instruments (and the CTD data) gives us new insights into the structure of the ocean and how the animals and plants are distributed relative to the oceanographic landscape. Putting all these data together over the entire survey area will result in a three dimensional picture of the structure and movements of the water masses of a whole ocean basin in the sort of detail that has not been attempted before.

Part of the scientific process is making predictions and it is easy to see how this gets perverted into gambling. The tonnage of krill that will be detected on the voyage is already subject to a sweepstakes that has spread far beyond the confines of the ship but the contents of the net are always fair game for a discrete wager. Toby, rather prematurely, bet his life on the identity of a particular acoustic target and was proved disastrously wrong. Esmee and Ruth are now the proud owner of a slightly used acoustician and are wondering what other function this specialised creature could be put to. Luckily there are 82 others aboard only too willing to offer helpful suggestions and these discussions will help to sustain us as we complete our next loop up to the grey and bumpy north and then back into the sunshine and calm waters near the continent.