An enlargement of the polynya shows that it is practically open water.
The Weddell Sea polynya is an area of open water that sometimes appears in the Weddell Sea over a relatively shallow region called Maud Rise.
In the latest satellite imagery from the DMSP satellite you can see the lower concentration sea ice as the darker blue colour. If you look at the MODIS imagery for the same date you can clear see black which indicates open water in the pack ice.
Project MIDAS shows us that the iceberg A68 is about one trillion tonnes.
This is the Antarctic Peninsula and the outline of A68 from the satellite image on 14 July 2017 shown in black. The ice front is from the Bedmap2 data set (so a little out of date), and the bathymetry from the IBCSO data set.
One image I haven't seen is how good is knowledge of the bathymetry around A68?
The iceberg is going to drift and likely ground quite quickly. (I wrote about this on the conversation a while ago: When an Antarctic iceberg the size of a country breaks away, what happens next?)
In the map below, the shaded colour is the distance of any point on the sea bed to the closest actual depth measurement.
So the dark blue stripes labelled in the Weddell Sea are actually ship tracks - and the dark colours are good depth data. These measurements will have been made by icebreaker.
Just in front of A68 there is a very large area where no ship has been within ~80 km.
One small note on the size. I digitized the iceberg from a satellite image (a KML File can be downloaded). On twitter today there were satellite images showing fractures already.
But Martin O'Leary of the MIDAS team posted today on twitter that to the untrained eye looks like iceberg, is very likely fast ice (so thick sea ice that is "fast" to A68 - but only a few metres thick.)
At this time of the year we should expect the Antarctic sea ice to be growing rapidly, but after the historic lows of last Antarctic summer, we can see that whilst it is rapidly advancing, the sea ice extent (the area of ocean covered by >15% of sea ice) it is still ~1 ¼ million km2 below the median from 1981-2010.
There is not a consistent trend in Antarctic sea ice extent, and much regional interannual variability. The plot below shows the sea ice extent on 13 May for each of the years 1989-95, and 13 May 2017.
The image above shows the sort of variability we expect in the Antarctic sea ice extent. It is helpful too to see where the sea is currently is and isn't compared with the mean from 1989-93.
The regions in May 2017 with the greatest deficit of sea ice remain the Amundsen and Ross Sea, and the Eastern Weddell Sea and off the coast of Dronning Maud land. As I said in my last Antarctic sea ice post it is likely the freeze up is delayed because of the heat gained by the ocean in the Antarctic summer of 2016/17.
You can also see in the South West Weddell Sea the Ronne Polynya I wrote about in March 2017 is still seen in the sea ice concentration data. In the visible satellite data you can also see this open water.
In the Sentinel 1 SAR data from the 15 May (From PolarView), the growth of the sea ice in the polynya is clear.
This ice growth is important for the ocean as it means the salinity of the waters just beneath the sea ice will be increasing.
I'll keep watching the polynya to see if and when it closes up. And I will also keep looking at the sea ice.
The polynya I saw forming in early February is still clear, and very large in the Southern Weddell Sea. At the moment it is more than than 80,000 km2, although there is clearly a lot of young sea ice covering a large part of the polynya.
In my original post I said this was likely formed by winds from the Ronne Ice Shelf.
Well Dr Stef Lhermitte (Delft) has put together the most amazing movie showing the development of the polynya over January and February. It shows satellite sea ice data with winds from the ECMWF overlain.
You can clearly see the winds pushing the sea ice away from the ice shelf as time progresses.
It is just as @StefLhermitte said in his tweet yesterday:
Trying to understand the geographic nature of the very low Antarctic sea ice extent I made the following animation:
The highlight issues in the graphic are the clear lack of sea ice in the Amundsen/Ross Seas and the Southern Ocean off Dronning Maud land. (If you are not familiar with the names off the seas / locations see the map below).
The Weddell Sea is has a relatively compressed sea ice cover this year - but it's clear there is large inter-annual variability.
Off Wilkes Land the sea ice is heavier this year - and it's easy for this to get lost in the headline story around the very low extent. There are several Antarctic research stations along this coast:
The French Station Dumont d’Urville,
and the Russian Mirny Station.
Here is the NSDIC Antarctic sea ice extent 1 March 2017.
And finally the promised map from the National Snow and Ice Data Center (NSIDC) with regional seas and other features marked.
I noticed yesterday that a polynya had formed in front of the Ronne Ice Shelf over the last 2 weeks.
In that image it is about 27,000 km2 in area.
I mapped the opening of the polynya from MODIS imagery over the last two weeks. There is cloud in the images but the opening of the polynya is fairly clear.
On 31 January 2017 there is no open water, but then over the 16 day period it opens to the ~27,000 km2 in area. If you're eagle eyed you can see that there is thin frazil ice forming in the open water in front of the ice shelf at the end of the sequence.
So what caused it?
Watching the sea ice extent this summer in Antarctica has been a bit surprising. The Antarctic sea ice extent has been tracking at record lows virtually the whole austral summer. We are very close now to the expected sea ice minimum, and this is where we are:
On the left is the sea ice extent from the DMSP satellite 12 February 2017, and on the right the difference between the mean sea ice extent on 12 Feb over the period 1989-93 and 12 Feb 2017. I chose this time period as the cycle has been generally quite stable from year to year.
The current sea ice extent is:
It's clear the sea ice over the summer 2016-17 is showing historic lows. But it's also clear from the sea ice extent above that there is little sea ice left to melt out before the summer turns. Where the sea ice remains - mainly in the Weddell Sea and along the coast of Wilkes Land it is clearly densely packed. If the winds change and the remaining sea ice is decompressed then the extent may fall some more.
This is an animation of the Antarctic sea ice extent from 1 January 2017 to 12 February 2017:
And finally the difference between the mean sea ice extent by day for the 5-year period 1989-1993 minus the concentration from 1 Jan to 12 Feb 2017. Blue shades imply an increased sea ice extent compared with a 5-year mean, and reds imply a decreased sea ice extent.
Not long until the Antarctic sea ice minimum.
The sea ice around Antarctic is currently still at an historic low. As usual I think it is good to look at a geographic perspective on the sea ice distribution. This is the sea ice concentration 22 January 2017 compared with the 1989-1993 mean on the 22 January.
RED shades = less sea ice than the 1989-93 mean on 22 January.
BLUE shades = more sea ice than the 1989-93 mean on 22 January.
The Amundsen Sea has very low sea ice
Probably for me the most striking feature is the extremely low sea ice concentration from Pine Island Bay through to the Ronne Ice Shelf - this is the Amundsen Sea. We are not going to see much more retreat of of the sea ice in this sector as it has already melted. I think it will stay open water until the freeze up begins some around the end of February. It would have been a great year to do ship based oceanography along that coast. I wonder if their could be an impact on ice shelf melt here. It is possible but as you can see from this article - it is water away from the surface and a few hundred metres deep that is in contact with the glacial ice in the Amundsen Sea Embayment, Overall this has to be a result of the recent El Nino, and is a follow on from the polynya events we saw hear in the late winter.
The Bellingshausen Sea has very relatively high sea ice
In contrast you can see it would not be a good year to be working in the Bellingshausen Sea.
Nevertheless, as expected, it was possible to finally relieve the British Antarctic Survey Rothera Base in January.
The Weddell Sea is a game of two halves
The Weddell Sea ice is at this stage in the summer is compressed against the Antarctic Peninsula. This means that there is heavier sea ice to the close to the Peninsula, and much lower sea ice than expected in the rest of the Weddell Sea. If you look at the MODIS Terra Image you can pick out a very sharp sea ice edge.
Shackleton would have been in trouble this summer if he was heading to Elephant Island.
Towards the annual Antarctic sea ice extent minimum.
We expect the sea ice to reach a minimum towards the end of February. Clearly there are places where there is no more sea ice to melt. Where sea ice is present, it is all down to winds over the next month. If they change and move the pack towards open water then - just as we have seen in the Arctic, the concentration could fall much lower. If the winds continue as they have then we could expect the extent fall to slowly as the seasonal melt continues.
Overall it is still to be likely a record breaking year in the Antarctic sea ice extent record.
Project MIDAS publicised on Friday that a huge iceberg is going to calve from the Larsen C Ice Shelf. This was written up a a great story on the BBC news website Huge Antarctic iceberg poised to break away. I understand a little about this stuff so got drawn into the media around it. Here is a BBC News interview on 6 January 2017.
It was great to see Antarctica in the news and it was brilliant to see so many high quality interviews from so many colleagues to different outlets. I may try and collate some of these in the next few days.