« CT SIA anomaly drops below 2 million km2 | Main | Unprecedented Greenland Ice Sheet Surface Melt »

Comments

Feed You can follow this conversation by subscribing to the comment feed for this post.

Wipneus

Nice post Neven!.

Normally I am not watching the weather very closely, but this got me interested. So allow me a silly question:
Why does low pressure "make the ice pack diverge" ?

Werther

Thanks Neven,
In preparation, I’ve been doing some CAD work on todays MODIS tiles. Thus, I have a somewhat more numerical approach to the holes.
First, the central Arctic Basin, about 2,2 MK2, is still unified (the maze within in leads).
Second, the rest surrounding that, 3,8 MK2, consists of floes without a structural pattern. The biggest one I can find is about 1600 km2, within the Basin, north of the ESAS/Laptev boundary.
The holes, while the word supposes weaknesses that collapse, may be better resembled through the word ‘patches’. They appear in a sheer endless way, ever changing, between the torn and floating floes. They are everywhere in that 3.6 MK2 area. The biggest, in your Laptev Bite, measures almost 5000 km2 open water (though with a lot of free floating debris inside). It gives an incredible 2740 km1 stretch of ‘floe-coastline’.
The pattern reminds of the lake structure in FI the Rainy Lake region, Minnesota/Canadian border.
Overall, the quality is just a percentage away from what was there 09 september last year. I’d say it’s even worse than that now north of the St.Anna Trough (the connection between the Barentsz Sea and the Eurasian Basin.
I wonder whether the ‘Bite’ (no French, please) indicates a sort of whirl within the general flow of the subsurface ocean currents. If so, could it be that warmth out of that layer has an easier way of radiating up to the surface layer? A sort of Sargasso Sea process?

To top off the alarmist tone of this contribution, almost all the bigger floes within this 3,6 MK2 area are covered with melt ponds (showing up as grey tones instead of ‘black and blue’). The melt pond-induces break-up of floes has already splintered all the large floes in the Beaufort sector.

Fifty days to go. Pray for cloud cover…

Kevin McKinney

"Why does low pressure "make the ice pack diverge" ?"

Actually, that's a good question. Naively, you might think the opposite, since divergent flows are associated with high-pressure systems (anti-cyclones.) Quick searching didn't turn up an answer... enlighten us, O wise ones!

Werther

My grumpier than wise take on Neven's 'diverge'is: the lows don't do that in themselves. Kevin's naivety is more common sense; lows suck in air on surface level. Well, that's what I've learnt watching 'Twister'.
But their on-and-off and criss-cross activity prohibits compaction. That's why extent doesn't take a nosedive.

Neven

Good question, wipneus! I don't know the answer, actually. Here are a few guesses:

I have a feeling that the wind in lows blow harder. Maybe that's not true, but you often hear of cyclones (ie low-pressure systems) doing damage, but not about anti-cyclones. Usually the isobars of lows on weather maps are closer together than isobars of highs. If it's true that the winds blow harder, the ice moves faster, making it more difficult to keep it together. Like a car that goes fast into the curve will tend to swerve out.

Perhaps the Coriolis effect has something to do with it? From NSIDC: "In the Northern Hemisphere, the Coriolis force causes objects to deflect to the right, and in the Southern Hemisphere, objects deflect to the left." If objects move in an anti-clockwise motion and deflect to the right, they will go away from the center and thus diverge (leaving holes). If they move in a clockwise motion and deflect to the right, they will converge (compacting the ice). Also see Ekman Spiral.

Finally, it could also have to do with the geographical configuration of the Arctic.

Neven

BTW, the Laptev Hole can clearly be seen on today's satellite image.

Kevin McKinney

I'm sure you're right about the windspeed, Neven--I know I've read a number of things about cyclones versus anti-cyclone corroborating that.

About the Coriolis speculation, I'm a bit more doubtful--but I trust my gut feeling not much on this issue! The Ekman Spiral phenomenon is certainly real enough.

I had a third thought: while cyclonic systems create convergent flows toward their centers, that doesn't mean that their only effects are convergent. Consider the 'edge' (defined in some arbitrary fashion) of such a system: 'outside' the system, there will be zero inward flow, but at the edge there will be inward flow. So the system will be trying to 'stretch' any ice spanning the boundary.

The same could be true, though perhaps to a lesser extent, within the low, if the wind field is not uniform. If windspeeds drop toward the center (as we know they do in the extreme case of a hurricane's 'eye'), then the difference in windspeeds will once again create a longitudinal stress--a 'stretch'--in the ice.

Highs should be able to act the same way, but in reverse--which brings us back to the point you made, Neven, about windspeeds characterizing the two sorts of systems.

Thinking about it this way also makes it intuitively obvious why thinner or broken ice is more vulnerable than multiyear slabs.

Musings...

Kevin McKinney

"If windspeeds drop toward the center (as we know they do in the extreme case of a hurricane's 'eye'), then the difference in windspeeds will once again create a longitudinal stress--a 'stretch'--in the ice."

Oops. Logic fail. In this case, the force would compact, not stretch, the ice. Got to think about which way the vector runs...

But correctly characterizing the wind field within a low would let you know where (if anywhere) it was tending to spread or concentrate the ice.

Apocalypse4Real

Neven,

Great post with comparative sources. While we are interested in peeking through the clouds, another source for what melt conditions are like is the buoys in the Arctic Basin.

One I have been watching is #48520. It is reportng above freezing air temps for most of the last 10 days.
http://www.sailwx.info/shiptrack/shipposition.phtml?call=48520

Ishouldbegolfin

Re: convergence around atmospheric high and divergence around atmospheric low pressure systems:

Sea Ice Motion in Response to Geostrophic Winds:

http://www.agu.org/pubs/crossref/1982/JC087iC08p05845.shtml

Apocalypse4Real

Another example of ice melt indicators even in the MYI north of Greenland is buoy
#47537, which currently is reporting a water temp of 2.7 C at N 83°41' W 051°53'

http://www.sailwx.info/shiptrack/shipposition.phtml?call=47537

SteveMDFP

Apocalypse-4Real--

Those buoy readings are eye-popping to me. Temp readings on the buoys I've been checking seem rarely to go above 2 dC (O-buoy and army bouys shown on Neven's daily graphs page).

You've got one showing 6 and 7 degrees in air well north of the Nares straight, and this one shows water temps at 2.7.

I gather that the buoys I've looked at have their thermometers quite close to the ice, thus rarely straying far from O degrees. And they won't then stray far from it until they're floating in open water.

With this much warmth above and below the ice, and plenty more warmth in the air and waters all around the arctic, I'd say we're in store for a lot more melting.

I think we saw a report here about an icebreaker reporting the polar ice to be just 60 cm thick. I can't see how much of it will survive the rest of the summer.

Rlkittiwake

Looking at the CT maps for the last few days... something wicked this way comes.

ThE SnYpEr AzZ

How much rain typically falls under Arctic Ocean lows this time of year ? I would think significant rainfall on thin, patchy ice could cause as much destruction as wind.

Apocalypse4Real

SteveMDFP,

These are not isolated cases:

Buoy 48600, 23 July 03:10, N 79°47' W 153°48', Air temp 4.6 C, Water 1.0 C

http://www.sailwx.info/shiptrack/shipposition.phtml?call=48600

Buoy 47506, 23 July, 0000, N 83°48' W 032°25' (Just 16 miles north of Greenland), Water temp 3.2 C

http://www.sailwx.info/shiptrack/shipposition.phtml?call=47506

What this demonstrates is warm currents are under the thickest MYI.

Finally, Buoy 48698 fascinates me, because if its reports are accurate, there are significant fast, dramatic shifts in water temps in the East Siberian Sea over the last 10 days, ranging from -0.7 to 22.9 C.

The buoy is currently at N 71°00' E 172°16' or approx. 100 miles NNE of Pevek, Russia.

http://www.sailwx.info/shiptrack/shipposition.phtml?call=48698

These resources seem to substantiate SST warmth that create conditions favorable for ongoing melt in the AB and East Siberian Sea.

AmbiValent

Comparing the navy ARC thickness maps to one and two years ago, this year's ice looks thinner except for a stretch just north of Greenland and the Archipelago. Are they still using the same algorithm? If so, it looks scary...

Espen Olsen

It looks like the first snow at the Web cams since summer solstice:

http://psc.apl.washington.edu/northpole/NPEO2012/9.jpg

Rob Dekker

ThE SnYpEr AzZ, for many climatic/weather variables, including percipitation rates, NCEP/NCAR re-analysis is a helpful tool.
For example, here is the plot from 2011
which shows less than 1 mm/day precipitation over the Arctic, or this one from 2010 which shows a couple of mm/day.

Either way, the rain over the Arctic is typically just a few degrees above freezing, so does not add much 'punch' to melting sea ice. I leave it up to you to calculate how much melt a 2 mm/day rain of (at most) 5 C contributes to melt. It ain't much.

Lows moving in from lower latitudes however may be completely different cookies. For example, the very significant low south of Iceland that Rlkittiwake refers to is currently moving over anomalously warm waters south of Iceland, at 13-15 C. It's soaking up a lot of warm moisture, and thus has a lot of latent heat. I'm not sure which way this puppy moves, but one way or the other it will hit sea ice or land ice pretty soon. There, it will release it's latent heat, which may cause some significant melt around West Greenland. Are there any not-so-stable glaciers left over there that may want to shed some mass ?

Neven

Ishouldbegolfin, thanks for the link to that paper. Unfortunately it's behind a paywall (after 31 years).

I can't seem to find a free version, and haven't been able to find anything else either, but probably didn't search hard enough.

I still think it's a combination of things like wind speed, Coriolis effect, and what Werther said: lows are too fickle to stay in one place and make the ice converge.

Rob Dekker

That said about the minimal effect of rain over the Arctic, what rain does is destroy whatever crystaline structure was left over in the snow cover, which will seriously reduce albedo. Thus, when the sun does come out after the rain, the sun's still strong insolation will have no mercy with the ice (at least in July).

Rob Dekker

Neven, I think all three of your arguments regarding ice dispersion during low pressure over the Arctic will hold. Also, a fourth argument may be that the thickest (and thus slow moving) ice in the South-East (North of Greenland) while this thin ice and open water is in the South-West (Siberian coast), thus your argument that the geographical configuration of the Arctic may play a role is I think very well founded too.

Still, I think that the Coriolis effect may be the strongest of them all.
Remember that the Coriolis effect exerts an acceleration southward on anything moving eastward, to the extent of :

Asouth = Veast * f

where f = 2 * omega * sin(phi)
where omega is 2*pi/24hrs and phi is the latitude.
Physics tells that V = A*T, so after 24 hrs of ice moving in eastern direction, it's southward speed has become :

Vsouth = Veast * 2 * 2*pi/24hrs * 24hrs = Veast * 4*pi.

In other words, after 24 hours, this eastward moving ice is now moving mostly southward, causing significant dispersion of the ice.

Werther

Hi neven,
I saw one of these 'three sigma events' nearly came to your doorstep. Steiermark was hit by heavy rain, triggering mud-slides.
How are things over there?

Rob Dekker

Sorry guys, my non-native English is again revealed : 'dispersion' is not the right term for ice being pulled apart. Ishouldbegolfin's 'divergence' sounds much better.
Also, please remove the '-' sign from my calculations.

Neven

Also, please remove the '-' sign from my calculations.

Done.

I've made this illustration to show how I think the ice moves due to the Coriolis effect:

Remember the definition of the Coriolis effect: "In the Northern Hemisphere, the Coriolis force causes objects to deflect to the right".

Depending on the direction of the winds the ice will either converge or diverge. In high-pressure areas (left) the winds blow in a clockwise fashion, the ice follows and goes to the right, or inwards. In low-pressure areas (right) the winds blow in a anti-clockwise fashion, the ice follows and goes to the right, or outwards.

Thanks again for a great question, wipneus. And thanks for all the answers.

Wipneus

Neven:

I don't think those diagrams are correct.
Compare with the sketches on the top right hand side of this wiki page:

http://nl.wikipedia.org/wiki/Wet_van_Buys_Ballot


The Buys Ballot law is directly caused by the Coriolis forces.

Neven

Indeed, wipneus, the Coriolis effect has an effect on the wind as well, which is why the wind doesn't go straight from high to low, but (due to the planet's rotation) the wind bends and goes around an area of low or high pressure. This is the cause of the clockwise or anti-clockwise blowing of winds.

But the Coriolis effect influences the movement of objects as well, such as ice floes. They also are deflected to the right (on the Northern Hemisphere), and that is, I believe, one of the main reasons that that ice floes under a high-pressure system will converge. And diverge under a low-pressure system.

Neven

I have finally found something that I can copypaste:

The force exerted by the wind will start moving the ice into the direction of $\vec{U}_ a$. As soon as it has gathered sufficient speed, the Coriolis force will kick in, diverting its direction off to the right of the wind direction. The water drag will slow the floe, but as long as it is still accelerating, the Coriolis force will keep turning it. Eventually, as shown in the diagram, the direction of motion is far enough to the right of the wind for the Coriolis force, wind drag and water drag to balance in a perfect triangle of forces. The floe moves steadily at an angle of about 45 degrees to the right of the wind and at about 3% of the wind speed.

This phenomenon was discovered by the famous polar explorer Fridtjof Nansen during the drift of his ship Fram across the Arctic in 1893-1896, and has become known as the Nansen rule.

From Wikipedia:

Nansen had proved the polar drift theory; furthermore, he had noted the presence of a Coriolis force driving the ice to the right of the wind direction, due to the effect of the Earth's rotation. This discovery would be developed by Nansen's pupil, Vagn Walfrid Ekman, who later became the leading oceanographer of his time.
Wipneus

The floe moves steadily at an angle of about 45 degrees to the right of the wind and at about 3% of the wind speed.

That is the solution of he paradox! 45 degrees should be more than enough to make ice diverge when the wind are converging.

Thanks for your patience.

Neven

I saw one of these 'three sigma events' nearly came to your doorstep. Steiermark was hit by heavy rain, triggering mud-slides.
How are things over there?

No problems in my part of Styria. I'm living a bit further away from the Alps where most of the trouble is. The river Mur that runs through Graz has never been as high as last Saturday. They even had to shut down a bridge. Average nationwide precipitation for July has already fallen, most of it in the south.

No mention of climate change in the local media, just that there has been an increase in extreme precipitation events since 2005.

We're going to have more rain tomorrow, and hopefully the northern parts of Styria will be spared, as the soil is still completely saturated.

Artful Dodger

Neven, you're right about low pressure leading to sea ice diversion... at least Dr. Peter Wadhams agrees with you!

"The wind stress which drives the sea ice through frictional drag is integrated over a large area - it has been estimated that in concentrated pack ice a piece of sea ice responds to wind fields integrated over a distance of 400 km upwind. Therefore a large-scale divergent wind field, created by an appropriate pressure pattern, can also create a divergent stress over a large area of icefield. Since ice has little strength under tension, this divergence can open up cracks which widen to form leads."

from "How Does Arctic Sea Ice Form and Decay?"
Peter Wadhams, Professor of Ocean Physics, Scott Polar Research Institute, University of Cambridge, UK
1 Jan 2003

Werther

Dispersion...divergence...whatever, minus 120K again on CIA. Let's try some latin...veni, vici, evanui???

Wouterlefebvre

All,

what you're discussing is the Ekman spiral and the Ekman transport.

http://en.wikipedia.org/wiki/Ekman_transport

Ekman is a known Swedish Oceanographer who was the first to discuss this point.

Wouter

Andreas Muenchow

Geostrophic dynamics (Coriolis balancing pressure gradients) only work away from boundaries or when friction is negligible. At boundaries (such as the ice or the ocean or the bottom of the ocean or atmosphere) friction enters. If friction is balanced by the Coriolis force, then you get an Ekman spiral. So, lets put this together (Neven is right, largely):

Low pressure system atmosphere, counter-clockwise (geostrophic) flow aloft, friction between air and ice/ocean results in convergence of air (bottom boundary layer atmosphere) and upward motion (like in hurricanes), counter-clockwise winds force ice and/or surface ocean at an angle to the right of the winds (northern hemisphere) which gives a divergence in the ocean boundary layer, the divergence lowers sealevel (ocean) causing pressure gradients to which then the ocean currents below the boundary adjusts geostrophically, that is, low pressure atmosphere, low pressure ocean, counter-clockwise circulation in both. Boundary layer flux, however, is in opposite directions in atmosphere (bottom boundary) and the ocean (surface boundary).

Search Ekman pumping ... this is how most of the oceans are forced by the winds, indirectly via pressure gradients due to convergences or divergences in boundary layers ... all physics is beautiful ... geophysical fluid dynamics even more so ;-)

Rob Dekker

Thanks Wouter, Andreas, for identifying the divergence of surface water/ice as Ekman transport (caused by Coriolis forces).
I understand that the Ekman spirals have been actually observed and confirmed under Arctic sea ice (since the late 50's) and appear affect the top 20 meters of ocean water. The water that replaces the diverging surface water in the low pressure zone will thus well up from a depth greater than 20 meters.

Now it just happens to be that in much of the West Arctic, there is a stratification layer between 20 and 75 meters, which contains water which can be up to 1 C above the freezing point of sea ice. Here an example from ITP41 in the Northern Beaufort :
http://www.whoi.edu/itp/images/itp41dat3.jpg
If this water is pulled to the surface by the Ekman pump, it will cause significant bottom melt of the ice in the depression zone.

I used a back-of-the-envelope calculation to quantify this effect of a moderate depression of a couple of 100 km radius, with ice speeds in the range of 0.3 - 0.5 m/s on the edges, which show that easily 100 W/m^2 bottom heat flux rate could be expected over the entire area of the depression, and maybe much more in the center (if it's a pretty strong depression).

So it seems to me that a low pressure zone over (at least the West Arctic) does not just rip the ice apart and form holes, but also attacks the ice from below with heat from the 20-75 meter stratification layer..

Can this be true ?

Werther

Rob,
Since last years' Kara Bulge musings I've been imagining how a changing, dense pattern of local eddying might bring the Atlantic layer warmth up to the ice pack bottom.
I find it exciting to follow how we as a group are getting closer to defining one of the actual physical processes that's missed in the models.

Werther

Peter,
Relevant...hot for reading this evening. When was this published?

Peter Ellis

2008, apparently
http://www.whoi.edu/page.do?pid=97670&tid=3622&cid=44107

I don't think we should get above ourselves and claim we're spotting stuff missing from the models. If it's been known since Nansen, I think it's unlikely we'll have added anything new in the last 24 hours, particularly when doing "research" via, um, Google.

Werther

Thanks for the warning, Peter.
But we live in dark times... getting a general idea, being an amateur, is only possible through linking all the specific scientific information and imagining.

Neven

getting a general idea, being an amateur, is only possible through linking all the specific scientific information and imagining.

Indeed. In our own small way we are contributing to collective knowledge and consciousness. In that sense this blog is everything I hoped it would be. I would never have learned as much if it weren't for others chiming in. And most of the recent blogs in Greenland, Petermann and this one on holes in the ice pack have been inspired by comments from others.

So, thanks to Rob, Wouter and Andreas. I'm still getting to grips with the third dimension of ocean as well as atmosphere. It adds to the complexity, to put it mildly.

Ekman pumping...

Wouterlefebvre

@Rob: this seems correct to me. As you get divergence of the water and ice in the low pressure area, water will be pumped up in the centre of the low (Ekman pumping).

Artful Dodger

... death spiral.

This characteristic of a large, deep SLP system explains why clouds don't seem to be a net negative forcing to arctic sea ice during the melt season.

When winds swirl around a 400 km diameter low, they bring 100 w/m^2 of bottom melt (thanks Rob D) to an area of 125,000 km^2. As well, it fractures and spreads the ice.

On the other hand, the clouds associated with that low do increase the optical depth of the atmosphere, decreasing incoming shortwave radiation (insolation). By how much? Barrow breakup data plots tell us it can be about 50% or so, which is really close to that 100 watts.

CAPIE 66 % x 300 W insolation ~= 100 W absorbed.

So large Arctic lows are death spirals indeed. Mixing the Arctic Ocean Cold Halocline Layer over a sufficiently large area would be a tipping point.

Peter, thanks for that dig! Do you have information that any models employ vertical Ekman transport as a forcing? This is direct communication to the warm, salty Atlantic Water Layer, so potentially fundamental.

Neven, herein lies your answer to the Kara bite. Let's reexamine the weather/wind patterns leading up to the event. ie: how big/deep, and what duration where SLP systems prior to the appearance of the bite?

Cheers,
Lodger

Werther

Hi Lodger,
we have holes, bites, bulges, gyres to name some, now be careful not to toss them over any Sea on the consequence of having us all arctic sea(ice)sick!

Espen Olsen

When the clouds disappear north of Norway, we will again see that spectacular algae show, as we did last year.

Twemoran

Neven
Any chance you'll be doing a piece on Ekman pumping?

Questions:

Does ice have to be highly mobile for Ekman to have an effect? It seems as though a solid, immobile cap of ice would be unaffected, while open water or very loose ice would experience the maximum.

While reduced shortwave insolation and Ekman pumping may more or less balance each other, will increased long wave radiation tip the scales?

Would a short duration storm (or a mobile one) over open water or fractured ice initiate long lasting upwelling in return for a short term decrease in insolation?

Terry


Apocalypse4Real

I have been "peeking around" (pun intended) at various alternate data/imagery sources for Arctic observation. Here are two new ones (for me):

1) Arctic SST: Multi-Sensor Improved Sea Surface Temperature for GODAE (MISST).

What it produces is global SST coverage through a Google Earth kml at 9km resolution from combined USN, NOAA, and NASA IR satellite data updated every 3 hours. The SST legend is in the upper left corner of the Google Earth display and is a little hard to read, but the data is very useful. The link is:

http://www.misst.org/gearth/google_earth.html

2) NCOF-Godiva Sea Ice Thickness:

The data comes from the National Centre of Ocean Forecasting (UK) and uses the GHRSST data to calculate thickness at a 5km resolution as a Google kml display file.

How to get to the kml: The link is: http://behemoth.nerc-essc.ac.uk/ncWMS/godiva2.html

1) Open the NCOF folder
2) Open the Global-Global Ocean folder
3) Click "sea ice thickness"

The map will be displayed as a global map, but when opened in Google Earth you can display the Arctic image.

Also, you can set the ice thickness variable for display - I set mine for 0-6 meters.

The data is useful because you can compare days back through 2010 by changing the calendar dates.

Twemoran

Apocalypse
I particularly like the new(to me)MISST interface.

Lodger
If I'm understanding Ekman pumping correctly it seems as though a high CAPIE - indicating more mobile ice - would enhance the pumping causing more melting which would raise the CAPIE index. A vicious vortex indeed.

While the cloud cover is blocking incoming insolation, it should also be blocking outgoing long wave radiation, allowing the up welling heat to concentrate on melting ice as opposed to being radiated away.

If the ice cover were solid (immobile) no pumping would occur, but every polynia that a low pressure area passes over could be affected.

The more I learn, the more I find I need to learn. - another vicious vortex

Terry

Wouterlefebvre

@Neven: you're not in trouble with the third dimension, as you cover three dimensions very well (two horizontal + time dimension). You just have to cope with one more. ;-)

@Artful Dodger: Coupled sea-ice ocean models do take into account Ekman transport and Ekman pumping; as these are major players in the sea-ice world. I have been working myself with this type of models, hence the knowledge.
Some links to my previous work (all on the other side of the world):
http://www.agu.org/pubs/crossref/2004/2004JC002403.shtml

http://www.ocean-sci.net/1/145/2005/os-1-145-2005.html

http://www.agu.org/pubs/crossref/2008/2006JC004032.shtml

http://www.springerlink.com/content/m262u05132584004/

If anyone interested in one of these papers, do not hesitate to contact me: wouterlefebvre at hotmail (you know what follows)

Neven

Any chance you'll be doing a piece on Ekman pumping?

I just might, Terry, if the fourth dimension (the real one) will allow me.

Wouter, since you seem to be an expert, I wanted to ask about the other influence on ice pack divergence under low-pressure areas I invented:

"I have a feeling that the wind in lows blow harder. Maybe that's not true, but you often hear of cyclones (ie low-pressure systems) doing damage, but not about anti-cyclones. Usually the isobars of lows on weather maps are closer together than isobars of highs. If it's true that the winds blow harder, the ice moves faster, making it more difficult to keep it together. Like a car that goes fast into the curve will tend to swerve out."

Is this influence negligible, or do you think it also plays a role? Because if I write a blog post about this, I have to try as much as possible not to look like a fool. ;-)

SteveMDFP

To all here--

What a fabulous discussion!! The education we are stimulating for each other surely compares favorably with a grad-level seminar course.

One small conclusion. I think with the forces we're discussing (low pressure systems tending toward divergence, Ekman pumping, etc.) we have an explanation for the macro-level observation that dramatic arctic ice loss is paradoxically accompanied by a preserved sea ice extent.

That is, while the ice sheet is largely solid, an atmospheric low is unable to scatter arctic ice. Under thinner ice conditions, we don't have immobile sheets of ice, we simply have floes packed next to each other. These move in response to the wind, and are spread centrifugally through complex geophysical forces whenever a low pressure system passes.

Global warming deniers are fond of pointing out how relatively normal sea ice extent remains, even as ice mass and even ice area decline. The preservation of sea ice extent is NOT a reassuring indicator, it's reflection of the arctic ice sheet becoming like an ocean of slush.

Quite obviously, as sea ice become thinner and more dramatically spread out, it becomes more vulnerable to catastrophic melt, not more resilient.

Wouterlefebvre

@Neven: in my opinion you're both correct and not correct. You're correct that wind speeds in low pressure areas is higher than in high pressure areas; indeed, the isobars tend to be closer in low pressure areas than in high pressure areas. However, the effect seems negligable to me. The centrifugal acceleration is equal to v²/r ( see for instance http://en.wikipedia.org/wiki/Centrifugal_force ). Let's try some numbers: v = 20 m/s (not bad wind speed already) and r = 500 km = 500000m, we get an a of 0.0008 m/s². This seems quite small to me; indeed, I always learned that on these scales the coriolis force is dominant (wiki: "In low-pressure systems, centrifugal force is negligible and balance is between Coriolis and pressure forces." http://en.wikipedia.org/wiki/Coriolis_force ).

Hmmm..., on a side note, I have the impression that my knowledge on this is getting a bit rusty, too long that I'm not in climate sciences anymore (since 2008).

Apocalypse4Real

I thought I'd revisit a few buoys from yesterday while peaking under the clouds - or high pressure areas.

Buoy 48600 is at N 79°43' W 153°32' or approximately 590 miles north of Barrow, AK.

On 2012-Jul-25 00:20
Air temp: 10.0 C
Water temp: 0.6 C

http://www.sailwx.info/shiptrack/shipposition.phtml?call=48600

Buoy 47537 is at N 83°41' W 051°54' or approx 80 miles north of North Greenland.

On 2012-Jul-25 00:00
Water temp: 2.0 C

http://www.sailwx.info/shiptrack/shipposition.phtml?call=47537

Buoy 25619 is at N 87°33' W 011°51' or approximately 175 miles south of the North Pole.

On 2012-Jul-25 01:10
Air temp: 0.6 C
Water temp: 0.3 C

Water temps at this buoy have been above freezing for at least the last 10 days, and the ice thickness is approximately 1.2-1.4 meters.

http://www.sailwx.info/shiptrack/shipposition.phtml?call=25619

Finally, Buoy 48698 is at N 70°58' E 172°22'
or approximately 100 miles NE of Pevek, Russia.

On 2012-Jul-25 00:00
Water temp 3.5 C

http://www.sailwx.info/shiptrack/shipposition.phtml?call=48698

While other buoys may reflect colder temps, these demonstrate from a variety of locations that favorable melt conditions continue, even in areas of 1 m+ ice.

The ice thickness estimates came from the Godiva 2 data.

Rob Dekker

Neven, Wouter,
Coriolis acceleration Acoriolis = V * 2 * 2*pi/24hrs, and centrifugal acceleration is Acentr = V^2/R. Divide the two Acoriolis/Acentr = R/V * 4*pi/24hrs. Fill in R=200km and V at 0.4 m/sec, shows that Coriolis forces are 72 times stronger than centrifugal forces.

So for sea ice under a low pressure zone, Coriolis forces are far stronger than centrifugal forces.

Rob Dekker

Wouter,
Question for you : Since the occurrence and intensity of low pressure zones are essentially stochastic processes, how do GCMs (such as the ... model you worked with) model them ? Are they modeling long-term probability of effects such at AO and the NAO, or short-term effects of whatever the GCM stochastically generates based on weather events elsewhere ?

Peter Ellis,
Thanks for the interesting note (by Jiayan Yang) on the correlation between increased upwelling (and decreased CHL layer) in some areas of the Arctic duing the 9-year period (1988-1997), and the AO index over the same period. I noticed though that Yang did not present any data after 2002, only the note that the CHL "partially recovered" in the later 90's.

Yang mentiones that he did a data analysis until 2006. Do you know where he published that, and if that alleged correlation between AO and upwelling still holds after 2002 ?

Rob Dekker

Wouter, I'm sorry, I forgot to fill in something : The "..." should be read as "ORCA2-LIM", which I understand is the global sea ice-ocean model you worked with.

Wouterlefebvre

@Rob: the model was only used in retrospective mode, so it was forced by the ERA40 or the NCEP reanalysis. As such it took into account NAO/AO and its southern ocean equivalents (which were the most important for me).

However, the model can also be coupled to an atmospheric model, which I did not do, but others did.

Wouter

Rob Dekker

Thanks Wouter, that makes sense.

Many of us here at Neven's are trying to figure out why GCM consistently underestimate sea ice loss, so any info on how much (or how little) GCMs differ between forecasts and (NCEP/NCAP re-analysis forced) hindcasts is valuable.

Thanks a lot for your insights and your willingness to share with us your experience and thus I hope you stick around here at Neven's !

michael sweet

A4R,
I having been watching the NOAA sea surface temperature anomaly maps and they have shown above freezing temperatures under the ice in a large area of the Arctic. Yesterday the temperature and area covered by anomaly increased substantially. I thought they were too high in error, but they match the bouy data you provided. How can the temperature in contact with the ice be substantially above freezing? What is the melt rate? Do you know the depth that the bouys sample the temperature?

The areas of high temperature north of Alaska show large holes in the ice. Will the ice melt out before winter sets in?

Wouterlefebvre

@Rob: I'm already a reader of neven's site since the last melt season, although I'm not commenting often. I'll continue reading, and if I think that I can add to the discussion, I'll comment.

dabize

First time poster but long time lurker here.

I thought I might start by putting up this weeks composite image (I hope this works)

http://www.americanwx.com/bb/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=68916

I further "declouded" it in Photoshop by montaging it with last weeks image and defaulting "cloud" pixels to that image. I then removed the red channel, which removed most of the remaining cloud. The left hand panel is the raw image for comparison.

Neven

Welcome, dabize. I can't see the image because I don't have permission (because I'm not registered).

dabize

Ah....blast. How can I fix that?

I'm well known in my circle to be a null mutant for computer/web skills!

Neven

You can send the image to me and I'll upload it to Picasa. My e-mail address is in the menu at the top, just above 'climate disclaimer'.

dabize

Will do.

Neven

Dabize sent me the cleaned up version of the composite image. I've put it in the post. Thanks, dabize.

Apocalypse4Real

@ Michael Sweet

My highly intelligent answer to most of your questions is "I don't know."

That said, as far as I am aware, the buoys are reporting air temps within a few meters of the ice surface. One contributor to high temps over the ice have been strong south winds drawing heat into the Arctic.

Buoy water temps are likely readings near the surface. Where there is open water, these readings are likely higher. I have observed high variation in water temps in a matter of hours/days on some buoys. It hints at high mix rates or current changes along with perhaps the closing of leads in the ice.

Melt rates will change based upon a host of variables as ably demonstrated by other contributors. Whether we have full melt in some areas will be based upon ongoing heating, winds, SLP's, leads in ice, ice thickness, near surface water temps, etc. We will only really know in September.

I keep watching the SST anamoly maps as well, and they are impressive. They are of significant concern in the East Siberian/Laptev Sea areas prone to methane release.

Espen Olsen

I was interviewed by the national radio this morning about ice and icebergs and the quote of the day was : The best action film ever, but in super slow motion!

Janne Tuukkanen

In this paper

http://www.agu.org/pubs/crossref/2000/2000GL011495.shtml

it is observed that ice drift direction is practically parallel to the isobars. (or geostrophic winds) At least nothing like 45 degrees to the right.

SteveMDFP

Janne Tuukkanen--

That's an interesting point. But the isobars are (roughtly) circular. Judging by clouds at least, winds around low pressure areas seem to be spiral. Thus, there may not be a contradiction between ice moving at 45 degrees to the right of the wind, and the ice moving parallel to isobars.

Now, it may get more complicated for wind and ice when movement is in the middle of a dipole anomaly, I'm not sure....

Artful Dodger

Individual floes move right at 45 to the wind. Summed over the entire SLP system, the net motion is outward from the center, or 90 degrees to the wind.

Janne Tuukkanen

Steve, you are right. I happened to find this wonderfully titled and a bit old fashioned document "Handbook for Sea Ice Analysis and Forecasting"

http://www.nrlmry.navy.mil/forecaster_handbooks/SeaIce/Handbook%20for%20Sea%20Ice%20Analysis%20and%20Forecasting.02.pdf


3.2.1 Geostrophic Winds and Direction of Floe Motion

Conclusion: Surface winds should be corrected ca 25 degrees to the left, and coriolis effect for Arctic sea ice is something like 30-40 degrees to the right. This should make ice drift 5-15 degrees to the right from the isobars.

Janne Tuukkanen

And about the Kimura et al. 2000 paper. There it is stated, that:

Concerning the turning angle 0, almost all areas have ranges within 4-10 degree. This result implies that sea ice moves nearly parallel to the geostrophic wind.

Nearly... practically parallel... But around a low, a little bit outward :)

Artful Dodger

Janne, what does it say about net motion?

Janne Tuukkanen

Kimura? Nice ice drift maps with vectors paralleling isobars.

This different paper (Kawaguchi and Mitsudera 2008, not behind a paywall) about ice divergence under low pressure systems looks promising. Looks also long and exhaustingly technical:

http://journals.sfu.ca/coaction/index.php/tellusa/article/download/15308/17138


We have found a characteristic length scale, r∗1 , based on the influence of the Coriolis term.

Near the centre where r∗ < r∗1 , the outward radial velocity, which causes the divergence and
SIC reduction, quadratically increases with distance.

In contrast, where r∗ > r∗1 , the increase of ur∗ gradually becomes gentle, and consequently, ice-drift divergence assymptotically approaches a constant value.

As a result, SIC reduction is largest at about
r∗ = r∗1 .

Eh, right.

FrankD

After three days under the clouds, the NWP is peeking through again.

And just like that, it's ahead of 2011...

Espen Olsen

Jøkelbugt, North East Greenland:

The only sizeable fast ice left in all of Greenland, will very soon be broken up, the ice is thinning and holes are appearing inside the remaining consolidated ice, I believe within a week it will be gone, more or less.Interesting to watch whether the "lagoons" in front of 79 and Zachariae will be open too?
http://rapidfire.sci.gsfc.nasa.gov/cgi-bin/imagery/single.cgi?image=crefl2_143.A2012209051000-2012209051500.250m.jpg

Verify your Comment

Previewing your Comment

This is only a preview. Your comment has not yet been posted.

Working...
Your comment could not be posted. Error type:
Your comment has been posted. Post another comment

The letters and numbers you entered did not match the image. Please try again.

As a final step before posting your comment, enter the letters and numbers you see in the image below. This prevents automated programs from posting comments.

Having trouble reading this image? View an alternate.

Working...

Post a comment