Beaufort Gyre guest blog

Not an update of current conditions in the Beaufort Sea, but some science for your reading pleasure.

This is a guest blog by Alek Petty, a postdoc at NASA’s Goddard Space Flight Center and the University of Maryland, specializing in Arctic and Antarctic sea ice variability. Alek has just published a paper in the Journal of Geophysical Research called Sea ice circulation around the Beaufort Gyre: The changing role of wind forcing and the sea ice state, and below he explains in detail what the paper is about (also be sure to check out his website).

Sea ice circulation around the Beaufort Gyre

The Beaufort Sea ice pack has experienced a significant breakup event in recent weeks. This has coincided with some classic Beaufort Gyre ice drift circulations. In this blog, I want to talk a bit more about the sea ice circulation around the Beaufort Gyre, why it’s important, and why it might have changed over the last few decades.

The Arctic is replenished with around ten thousand gigatons of fresh water every year from river run-off into the Arctic basin, Pacific Water inflow (through the Bering Strait), precipitation and – increasingly – sea ice melt.

Figure 1: Mean dynamic topography (MDT) of the Arctic Ocean [from Farrell et al., 2012, GRL]. Grey lines are bathymetry contours [Jakobsson et al., 2012] and the black boxes indicate the Beaufort Gyre study region.

The predominantly clockwise winds over the northern Beaufort Sea draw in a significant fraction of this freshwater – through a process called Ekman pumping – resulting in a characteristic doming of the ocean surface (a ‘spin-up’ of the ocean as it is often called).

The combined forces of gravity and Coriolis drive a clockwise ocean current that circulates this relatively fresh surface water – along with its overlying sea ice cover – around the center of the dome (also clockwise). The end result is one of the most prominent features of the Arctic Ocean: the Beaufort Gyre (see Figure 1).

The sea ice sandwiched between the atmosphere and ocean responds to the combined effects of wind and ocean drag and thus modulates how effectively the winds can ‘spin up’ the ocean. The interaction between winds, ocean currents and sea ice are intrinsically linked, making it an extremely challenging system to study. We’ll get back to this point later.

For those concerned with understanding the fate of Arctic sea ice, the circulation of ice in this region is crucial – as thicker, older ice enters the Beaufort Gyre north of the Canadian Archipelago, but often melts out as it enters the warmer waters north of Alaska. Several decades ago it wasn’t uncommon for sea ice to survive a full Beaufort Gyre circuit, although this appears to be happening less and less in recent years, as the younger, thinner ice melts out in the Beaufort Sea in summer (see recent studies here and here). The decline of sea ice within the Beaufort Gyre region over the last few decades has been one of the strongest declines observed across the Arctic, so these processes are worth understanding in more detail.

In our recent study, we used satellite tracking of sea ice floes to investigate the changing circulation of sea ice around the Beaufort Gyre. Our results show that the sea ice circulation around the Beaufort Gyre increased rapidly in the 2000s, despite no real trend in the strength of the wind circulation over the same time period.

Figure 2: Ice drift curl trends from 1980-2013 using the NSIDC Polar Pathfinder ice drift data [Fowler et al, 2013]. JFM: January-March etc.

The changes were very seasonal, however, with the biggest increase in sea ice circulation occurring in autumn – coinciding with significant decreases in sea ice concentration (Figure 3), thickness (Figure 4), and a latter freeze-up of sea ice in the region.

Figure 3: Seasonal sea ice concentration in the Beaufort Gyre region, using the NASA Team (solid lines) and Bootstrap (dashed lines) processing of passive microwave data. Black lines indicate the annual mean ice concentration.

Figure 4: Seasonal ice thickness in the Beaufort Gyre region from the PIOMAS ice-ocean model (black lines), and upward looking sonar moorings in the Beaufort Sea (a-d, shown in Figure 1) from Krishfield et al., (2014, JGR). The gray stars/lines in spring (AMJ) indicate the thickness in the Beaufort Gyre region from IceBridge remote sensing data.

Let’s look at the possible causes of this enhanced ice circulation in a bit more detail (a summary of this discussion is given in the schematic below). When sea ice is old, thick, and strong – internal ‘ice-ice’ stresses make it harder for the winds to drag the sea ice and ocean around. Old sea ice, however, can also be very rough (compared to the ocean), providing more obstacles for the winds to push against – increasing the effective drag. Alternatively, young sea ice – which is increasingly dominating the Arctic sea ice cover – is often much smoother than old ice, and may in-fact reduce the effective wind drag. Young ice is obviously thinner and weaker too. Melt ponds and broken up sea ice floes provide more complexity to this ice state-effective wind drag relationship, however, especially in summer.

Figure 5: Candidate mechanisms that could explain the enhanced Beaufort Gyre ice circulation.

Reductions in concentration (Figure 2) can significantly reduce the internal ‘ice-ice’ stresses, increasing the effective ice circulation. When these ‘ice-ice’ stresses are negligible, we often say the ice is in ‘free drift’ as the sea ice provides negligible resistance to the wind and/or ocean drag. The strong concentration declines observed in summer, however, are thought to be somewhat irrelevant to this discussion, as the ice concentration was already low (less than 80% in the 1980s/1990s) and the ice was probably already freely drifting. In autumn, however, the concentration declines are more significant as the ice wasn’t in free drift (in the 1980s/1990s), but likely entered a state of free drift in the 2000s. The ice-ocean model PIOMAS suggests declines in thickness across all seasons in the Beaufort Gyre – including autumn declines of ~50% since the 1980s.

The increased heat flux from the ocean to the atmosphere, driven by recent increases in Arctic open water (or decreased ice concentration), has also reduced the stability of the atmospheric boundary layer. A less stable boundary layer can increase the effective wind drag through enhanced turbulent mixing.

If the winds do spin-up and strengthen the underlying ocean currents, this can also increase the sea ice circulation as the sea ice and ocean circulate more in tandem, reducing the ocean-ice drag. We currently lack reliable, seasonal estimates of the Beaufort Gyre ocean currents, so we still don’t know how much of an influence the ocean is having on these changes. We do know that there was a significant ‘spin-up’ of the Beaufort Gyre in the 2000s (i.e. it domed and accumulated more freshwater than usual), so it’s likely a reduced ocean drag may have contributed to an increased ice circulation also. The role of sea ice declines in potentially enhancing this ‘spin-up’ of the Beaufort Gyre in the 2000s (and the role it may play in future) is also open to speculation.

There are other processes that may also be important (e.g. a freshening of the surface waters), so the quest to understand this complex, coupled system goes on. What we really need now are more detailed observations, and a well calibrated, sophisticated, fully coupled atmosphere-sea ice-ocean climate model to test out some of these ideas in more detail. Do get in touch if you have one!

Alek

Comments

[Magma]

Excellent guest post, Alek. Logical and clearly written explanation of a phenomenon that I (and likely some other readers) have been watching without necessarily grasping many of the interesting details, including the changes over the past decades. Thanks.

[wayne]

I particularly like the sketched about old and new sea ice with respect to winds. The missing one may be new sea ice, mostly flat, only having "sails" by much lower smaller pressure ridges. This fits with respect to another missing component, the newish counterclockwise gyre driven by persistent lower pressure cyclones during summer and also very lately winter, in effect, creating no gyre at all. There is no circulation, just melting in place ( summer 2013-2014). It is a natural defense against total eradication by compaction. Sea ice onto itself is equally complex, either thin or thick, boundary layers exist, and sometimes boundary layers increase surface interface wind speeds, a particular not so rare occurrence especially during the long night.

[D]

FishOutofWater aka George here.

With the warming of Alaskan waters the Bering strait is ice free longer and more warm moderately salty water is entering the Arctic from the Pacific, especially in the fall. Good measures of the mass balances of salt and water are needed to model the stability of the water in the Beaufort sea. Unfortunately, many of the buoys have gone out of service and sampling of the depth profiles and currents is very thin.

This year the high pressure is much more persistent so the wind push on the ice is probably more efficient than when the winds are briefly strong but of variable direction. Summer 2014 was good for building ice in the Arctic because there was little net motion despite all the highs and lows.

Good post. Thank you for telling us about the excellent research.

[Aaron Lewis]

Thank you very much. I had thought about this, and it is very nice to see that others get the same answers.

Um, does the rapidly moving keels of thin ice stir the fresh water lens more than the slowly moving keels of thick ice? That is, with increased continental melt has the fresh water lens gotten thicker and more stable?, Or, less stable?

I tend to think that thinner ice allows more light in, warming the top of the lens, and heat from the mid-waters warms the bottom of the lens making it over-all warmer, lighter, and more stable, but less protected from cyclones.

I still think that the sea ice will continue to diminish as long as the fresh water lens is intact. Then, when a cyclone drives deep mixing, the Arctic will go abruptly blue.

[fryingpan136]

When the Central Arctic basin eventually becomes mostly free ice, hopefully in the distant future, will it result in a large clockwise gyre about the North Pole? Will the Beaufort Gyre still be strong, or will it get sucked into the polar gyre? Will the increasing polar warmth tend to decrease the gyre strength?

[Chris Reynolds]

Thanks for the post Alek,

PIOMAS Gice reflects the presence of exported MYI in the East Siberian Sea, for example in 2010 in the following graph. And it seems that since the early 2000s (approx 2002), the previous regime of regular presence of volume from the thickest ice ends. This has ushered in a new regime of 'patchy and sporadic' appearance of large slugs of MYI volume.

https://c2.staticflickr.com/8/7212/27189627866_40e07bd140_o.png

This seems to have happened as both Chukchi and the ESS have begun to exhibit predominant September open water, implying a collapse of summer ice survival rates in these regions.

ESS
https://c2.staticflickr.com/8/7099/27223724265_9ea3f6eebe_o.png

Chukchi
https://c2.staticflickr.com/8/7095/27127120702_0a557c07c3_o.png

Of course a reduction in thick ice volume reduces summer extent in turn.

This means that the mass circulation of the Arctic Ocean ice pack has been interrupted.
https://nsidc.org/cryosphere/seaice/processes/circulation.html
Resulting in the collapse of the Beaufort Gyre Flywheel. The BG flywheel is described by the BG Exploration project.
http://www.whoi.edu/page.do?pid=66596

[Susan Anderson]

Beautifully done, thank you.

[Alekpetty]

Thanks very much for the comments so far everyone! It's very inspiring to observe this active sea ice blogging community, and many of the ideas discussed here (and in other blog entries) have definitely given me food for thought.

Just a response to a few comments:

wayne - yes, I should have made it clearer that this mean anticyclonic (clockwise) circulation is the average of very variable ice circulation patterns, including cyclonic (anti-clockwise) drifts on occasion. I've not looked to see if those cyclonic drift patterns (holding the sea ice in the Beaufort) have increased in recent years - as you suggest - but I find the idea of storms contributing to this an interesting one for sure.

Young flat sea ice with more pronounced sails was represented by mechanism 3 in Figure 5. I still expect this to contribute to a reduction in drag compared to the rougher multi-year ice, despite the sails being less weathered. I'm actively looking into this at the moment, however!

D - good point about the importance of repeated wind patterns. And you're right we need more direct observations to understand the freshwater/salt budgets. I was involved with a cruise around the Beaufort Gyre (the same one mentioned by Chris Reynolds) which is trying to do just that. The cruise is limited to late summer, but many of the buoys and the moorings, give us year-round data. More data would be nice though!

Aaron - good point about the stirring. People are actively trying to work out what a younger, thinner sea ice cover might do to momentum transfer into the Arctic Ocean (stirring of the ocean if you like). A new paper came out in the same journal last month by some colleagues of mine (http://onlinelibrary.wiley.com/doi/10.1002/2015JC011186/abstract)which I briefly mentioned in the blog. They concluded that the ice-ocean stress may be reducing (on average across the Arctic) due to the decreased roughness of the younger ice. This was a model study, however, so we still need to figure out how realistic those results were.

There definitely is more heat being absorbed by the upper ocean (thinner ice and more open water) which would make that freshwater lens more stable as it's lighter and thus more buoyant, as you said. The cold layer of Atlantic water below this means the sea ice is somewhat protected from the warmer Pacific Water that resides deeper in the water column. Clearly an idea worth exploring more though.

fryingpan136 - I would say this is a huge unknown! We don't know how much the removal of that insulating 'sea ice lid' will change the Arctic atmosphere/circulation patterns. Clearly there is a lot more heat/moisture being exchanged with the atmosphere now - but I'm not sure exactly what that might do to the location/strength of the BG? Maybe it could become more variable in strength and location?

Chris - I agree with the points about the MYI export variable and melt out increasing in recent years (perhaps's doesn't even matter if it's MYI or FYI, it looks like a lot of that doesn't survive the summer in the Chukchi and ESS as you say). Not sure this will lead to a collapse of the BG flywheel, as that still depends on the wind forcing and ability of the winds to maintain a spun-up ocean, however?

[wayne]

Thanks Alek,

It is good to have focus on a subject, apparently easy, but deeply complex. Good tidings for your further research!

[Chris Reynolds]

Alek,

To be clear: I'm not saying the Beaufort High will necessarily collapse or shift, I've not read anything hard and fast about the future prospects in modelling studies. So the circulation will, I expect, continue.

What I am saying is that if the sea ice 'mass' is stripped off the flywheel every summer it no longer provides the 'smoothing' function that keeps summer extent up in Beaufort/Chukchi/East Siberian Sea. Meaning that open water formation efficiency remains high in those regions.

[Colorado Bob]

Current atmospheric models underestimate the dirtiness of Arctic air

Black carbon aerosols--particles of carbon that rise into the atmosphere when biomass, agricultural waste, and fossil fuels are burned in an incomplete way--are important for understanding climate change, as they absorb sunlight, leading to higher atmospheric temperatures, and can also coat Arctic snow with a darker layer, reducing its reflectivity and leading to increased melting. Unfortunately, current simulation models, which combine global climate models with aerosol transport models, consistently underestimate the amount of these aerosols in the Arctic compared to actual measurements during the spring and winter seasons, making it difficult to accurately assess the impact of these substances on the climate.

Link

[Colorado Bob]

Sorry Neven , but everyone needs to this -

Sculpture by Isaac Cordal, entitled “Politicians discussing global warming.”

http://d2jv9003bew7ag.cloudfront.net/uploads/Isaac-Cordal-Politicians-Discussing-Global-Warming-from-the-series-Follow-the-leaders-2011-Berlin-Germany-installation-view-photo-credits-Isaac-Cordal.jpg