Our last full day at Mario Zuchelli Station took sublime to new levels. We had the opportunity to use their coastal vessel, The Skua, for a few hours.
It was great to look over the boat. There are no small boat operations out of Scott Base, nor have there needed to be for there has been a distinct lack of open water for the last decade or more. However this is changing and such operations may one day become part of the science support requirements. The Skua gets used for a range of sampling tasks as ice conditions allow. We took the vessel out to a modest grounded iceberg and sampled the ocean stratification and turbulence right by the berg in order to get a better understanding of how they melt when surrounded by open water rather than ocean capped by sea ice.
Rather than take our large velocity-based turbulence profiler I opted to bring a much more portable device, The SCAMP, that determines the amount of mixing from tiny temperature variations over distances as small as a mm. I’ve struggled to get good results with this device in Antarctica before because there has simply not been enough temperature signal around — basically everything was the same temperature — the freezing point of seawater.
However, being further north and with open seas, there is ample information to work with here. There are two main advantages with this profiler. First, it’s relatively small — shaped sort of like a dandelion seed if it were 60 cm high. The second big advantage is it can start sampling almost as soon as it’s released. The big profiler we were using south of the Drygalski Ice Tongue needs quite a few metres to get up to good sampling speed.
One of the interesting things (to me!) about this sort of sampling is that it is actually doing quite simple things, albeit to incredibly high precision and with high signal to noise ratios. And also that these things were readily achievable to a degree decades ago. Where the challenge comes in is getting to enough places of importance and then getting enough data at the right times to represent the various conditions.
There have been precious few attempts at directly measuring ocean mixing in Antarctic waters. Certainly, much of what goes on is as you would expect anywhere in the world’s oceans. It’s the departures from this, especially relating to unique features like the presence of massive ice shelves and glaciers, that need to be considered.
Being a small berg and seemingly well-grounded there was not much problem with the ice itself. However, even the slow, modest swell was enough to make holding the vessel off from the ice a challenge. The skipper did nicely bringing in the boat right over the spot I wanted and allowing me enough time to get the profiler into the water, the boat would then reverse out and stand off a few tens of meters whilst I paid out the thin line as the profiler slowly sank recording as it went.
The thin kevlar line is only a few mm in diameter but is able to carry several hundred kilograms. Still, it takes some getting used to, throwing expensive equipment over the side of ships with nothing but a thin line for recovery. It could be tougher. There are deep ocean versions of our main profiler that have no line. When they return to the surface the operator has to rely on satellite communications, lights, keen eyes and nervous energy to get the gear and the data back.
One of the curious features of oceanography that a melting berg highlights is a set of processes relating to the combined and separate effects of heat and salt. If you were to throw some ’seawater’ into the ocean it would sink until it found a depth where the ocean density was the same as that of the injected water.
The interesting thing is that the density of seawater mainly comprises the effects of temperature and salt concentration — and these two diffuse at different rates. So our injected patch of seawater might have quite different temperature and salt signatures to the water it found as its neighbour, even though it has matching density.
The result then is the heat and/or salt start to diffuse and equilibrate such that the local density conditions might become unstable. Weird things happens — thin layers of constant density form- thermohaline staircases. If ocean models don’t capture this sort of effect they get the resulting circulation wrong.