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.
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.
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!