Dr Beatriz Pena-Molino (ACE CRC) - reposted from Sciblogs
Location: 65.395795°S, 142.682921°E
Sea State: Calm
If you have been following this voyage for the last fortnight then you may well be wondering “what is a polynya?”
Polynya is a Russian word used to describe areas of open water surrounded by ice. They occur both in the Arctic and Antarctic oceans and they are often found around the coast. These were once believed to be rare, but are now known to exist all around the Antarctic continent.
How do we know this? The answer is up there, in the sky. With the help of satellite images, our ability to say when and where polynyas occur has increased enormously. So, unlike early Antarctic explorers, we now have the assistance of daily satellite images like the in this blog post to help guide the RV Tangaroa. The only problem is that clouds can sometimes mask the underlying water.
But why do we care so much about polynyas?
During the winter it is not uncommon for temperatures to go down to -40 °C around Antarctica. When this cold air is blown over the frozen surface of the ocean, the sea ice insulates the water below from these extreme temperatures. However, in certain parts of the coast, some very strong “katabatic” winds blow from the land towards the ocean and push the ice away from the coast, creating the open patches of water we call polynyas.
Once the polynya has been created, the water is no longer protected by the ice and is exposed to the extreme Antarctic air temperatures. The surface waters freeze and form more sea ice, but the katabatic winds blow the ice away from the coast, allowing yet more sea ice to form. In this way the polynya becomes a very effective sea ice factory.
Sea ice formation not only shapes the upper part of the ocean around the Antarctic continent, but also affects the characteristics of the water below.
As sea ice forms, the salt crystals dissolved in the water are left behind, increasing the salinity of the surrounding water. As a result the water in the polynya is now much denser than the surrounding water and called High Salinity Shelf Water. This dense water sinks to the bottom, eventually forming Antarctic Bottom Water (see future blog posts).
Even though polynyas are a very persistent feature throughout the Antarctic coastline, they are rather delicate. Changes in the wind and distribution of ice can effectively shut down these ice factories. An example of this was the calving of the Mertz Glacier in 2010. Since the breaking of the Mertz Glacier Tongue, the former Mertz Polynya area has been covered by some very thick ice – making it very difficult for us to get in on the ship in the last couple of years.
While satellite measurements show us the location of the polynyas around Antarctica, sea-going oceanographers still need to get in and physically measure them to understand why they occur. We need to measure temperature, salinity and current speed and direction using a CTD and ADCP (see blog post 5: The Oceanography Team). This will provide some very valuable information about the current state of the polynyas around the Mertz glacier.
But we are here in summer and most of the “action” happens during the winter time, when the temperatures are the coldest and the Antarctic coastline is virtually inaccessible. This means that for us to measure this part of the ocean even when we can’t actually get there, moored instruments are the solution. These instruments are anchored to the bottom of the ocean and left for a year or two to record what is happening in the polynya. We still hope to retrieve a series of moorings that have been deployed in the region for the last couple of years, to download this important data.
See other posts in this series on the Mertz Polynya blog homepage.