Tag Archives: Weddell Sea

I noticed yesterday that a polynya had formed in front of the Ronne Ice Shelf over the last 2 weeks.

Screengrab from NASA Worldview 17 February 2017
Screengrab from NASA Worldview 17 February 2017

In that image it is about 27,000 kmin 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.

Formation of the Ronne Polynya 30 January to 14 February 2017.
Formation of the Ronne Polynya 30 January to 14 February 2017.

On 31 January 2017 there is no open water, but then over the 16 day period it opens to the ~27,000 kmin 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?

...continue reading

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I've been watching the open water down the eastern side of the Antarctic Peninsula. I said the cause of that was most likely strong westerly winds.

If you look at the sea ice concentration on the western Antarctic Peninsula you can see the effect of these westerly winds.

Towards the end of September 2016 the ice edge is compacted as the sea ice is pushed against the Peninsula.

The Antarctic Peninsula sea ice 24 August to 5 October 2016. Data from DMSP SSMI
The Antarctic Peninsula sea ice 24 August to 5 October 2016. Data from DMSP SSMI

The westerly winds (from bottom left to top right) compress the sea ice against the land (left hand side of the Antarctic Peninsula). This also creates open water on the eastern (right hand side ) of the Peninsula as the sea ice is pushed away from the land.

You can see the very sharp ice edge on the west, and the open open water in the MODIS satellite imagery.

MODIS image of the Antarctic Peninsula 5 October 2016 from the Aqua satellite.
MODIS image of the Antarctic Peninsula 5 October 2016 from the Aqua satellite.

The sea ice concentration anomaly for September 2016 shows that on both sides of the Antarctic Peninsula the westerly winds have reduced the amount of ice we would expect to observe by up to ~40%. On the west side because the sea ice is compressed, on the east side because the sea ice is being pushed away from the land.

Antarctic sea ice concentration anomaly for Sep 2016. Image from NSIDC
Antarctic sea ice concentration anomaly for Sep 2016. Yellow rectangle approx area of images above. Image from NSIDC

This is just late winter weather.

There are a lot of Antarctic research stations on the west of the Antarctic Peninsula, including Rothera, the largest British Base.  If the winds maintain the westerly direction then I can imagine it could be slow to resupply the base this season. There is time for it to change. According to the published schedule the ship is not due to arrive until 27 November 2016.

A slow resupply is not uncommon and I have been on at least one unsuccessful resupply voyage in my career. I took the picture below on 11 December 2004 under similar conditions.

James Clark Ross making very slow progress in compressed sea ice in Marguerite Bay, the Western Antarctic Peninsula.
RRS James Clark Ross making very slow progress in compressed sea ice in Marguerite Bay, the Western Antarctic Peninsula 11 December 2004.

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I noticed in a blog post last week that there was a finger of open water extending down the Western Weddell Sea. I've carried on watching this open water in the MODIS satellite data. Whilst it's been opening and closing, there is a lot of open water. It's clearly a major sea ice generating factory at the moment.

MODIS image of the Western Weddell Sea 30 September 2016. The Open Water is clear.
MODIS image of the Western Weddell Sea 30 September 2016. The Open Water is clear.

The open water is clear in the lower resolution passive microwave sea ice data too.

The sea ice extent along the Antarctic Peninsula 2 October 2016. Data from DMSP SSMI
The sea ice extent along the Antarctic Peninsula 2 October 2016. Data from DMSP SSMI

If you look at some model output there are air temperatures above this open water of between -10° to about -25°C.

Surface temperature at 2m from NCEP output. 3 October 2016.
Surface temperature at 2m from NCEP output. 3 October 2016. From Climate Reanalyzer.org

What is really good is if you look at the temperature anomaly (i.e. the departure from the average with a 1979-2000 baseline), it is very warm over the Weddell Sea.

The temperature departure from average for NCEP output 3 October 2016. Image from climateReanalyzer.org.
The temperature departure from average for NCEP output 3 October 2016. Image from climateReanalyzer.org.

I think the reason it is warmer is because the Weddell Sea pack ice is looser this year. So (as you can see in the picture above) there is lots of open water. The atmosphere is being warmed by the ocean as the sea ice is being generated.

Another pointer to the pack being looser this year is that in August 2016 in the Eastern Weddell Sea there was a rare sighting of the Weddell Polynya.

The Weddell Polynya as observed on 14 August 2016 in passive satellite data.
The Weddell Polynya as observed on 14 August 2016 in passive satellite data. It is a polynya with its own wikipedia page.

I think the Weddell Sea pack ice is more mobile this winter. This is also telling us something about the difference between sea ice extent and sea ice thickness. The sea ice extent is large and easy to measure in the Antarctic - but we don't know how thick it is.

On 26th September 2016 the MODIS sensor on The TERRA satellite captured this beautiful image of South Georgia, with Iceberg A66 drifting past.

Iceberg A66 passing South Georgia captured in a MODIS image on the 26 September 2016.
Iceberg A66 passing South Georgia captured in a MODIS image on the 26 September 2016. The image is available on a KML file on the MODIS Websites.

The iceberg A66 is about 15 km at it's widest point in this image.

We can do a bit simple maths. Estimate the iceberg has a 200 m thickness and it is triangular in shape with a base of ~4 km.

the volume =  0.5 x 15 km x 4 km x 0.2 km = 6 km3.

So the relatively small A66 contains of 6000 gigtons of water. It's a lot. But it's not a lot.

Icebergs get their reference number depending on where they originate from. This one has an identifier "A" which means it came from the sector 0° to 90°W - that's the Bellingshausen and Weddell Sea region. You can track icebergs like this both visually - like in in the image above - or using something called a Scatterometer. A scatterometer can measure the winds over the ocean, and because the winds change over the ice one can track the icebergs. Prof David Long at Brigham Young University provides an excellent database of Antarctic iceberg data based on that idea (this is their research paper on how they do that).

If we look at the location data from the ASCAT sensor you can see that A66 is at the tip of the Antarctic Peninsula - but this data is only from this year. I will have to dig a little deeper. A job for later.

The track of Iceberg A66 from the BYU database
The track of Iceberg A66 from the BYU database

Once the icebergs reach the edge of the Weddell Sea they get to South Georgia very quickly. I did write about that in a paper in the OU database Physical oceanography in the Scotia Sea during the CCAMLR 2000 survey, austral summer 2000.

And some of these icebergs (although not A66) ground at South Georgia and ultimately can affect the ecosystem. Jon Amos wrote about some work I did at a San Francisco conference  in 2010 about that - it's still available on the BBC website: Giant icebergs head to watery end at island graveyard.

Giant icebergs head to watery end at island graveyard
Giant icebergs head to watery end at island graveyard. By Jon Amos

Overall A66 is nothing special, this is not an unusual observation.

It is a beautiful image though.

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The low sea ice extent I noticed in my previous blog post about Antarctic Sound has extended southwards along the east side of the Antarctic Peninsula.

The sea ice extent along the Antarctic Peninsula 24 September 2016
The sea ice extent along the Antarctic Peninsula 24 September 2016. Data from DMSP SSMIS

The open water that shows up as black in the image above extends to at least as far south as the Antarctic Circle (66° 33′S). Open water along this part of the Antarctic Peninsula is unusual at any time of the year let alone the height of winter. The image below is from the National Snow and Ice Data Centre. It shows  sea ice extent >15% with an outline of the typical extent for that day based on a 30-year (1981-2010) median (orange line).

NSIDC Antarctic sea ice extent 24 September 2016 with the median extent (1981-2010) for this day.
NSIDC Antarctic sea ice extent 24 September 2016 with the median extent (1981-2010) for this day.

You can see how unusual this observation is, and I wrote a general post Antarctic Sea Ice Extent a while ago.

The open water is also very clear in the MODIS imagery as the black wedge between the Antarctic Peninsula, and the sea ice of the Weddell Sea.

The MODIS imagery for the Antarctic Peninsula 25 September 2016.
The MODIS imagery for the Antarctic Peninsula 25 September 2016.

In my previous post I pointed at weather systems as likely being responsible. Now to me it looks like a large system is pushing the whole Weddell Sea sea ice to the east and away from Antarctic Peninsula.

There is always some open water in the pack ice at any time of the year, but it's clear that their is a pathway south right now. I imagine it will close soon and wouldn't be keen to be on a ship in that open water heading south.

What is interesting is the heat transfer from the ocean to the atmosphere that far south at this time of the year will be huge. This is what I wrote about that heat loss for the Arctic.