This post started from the ice-drift map (latest attached) and a stray image in my mind. Wind will have very little traction blowing over a flat 100% concentration ice pack (until it hits a pressure ridge). But on a load of ice rubble?
So in an attempt to do something about my total ignorance I googled and found two papers produced in 2014 from a National Science Foundation project..
https://www.arcus.org/witness-the-arctic/2014/3/article/22794Arctic Ocean Sea Ice Loss: Modeling the effect on Wind-to-Ocean Momentum Transfer&
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013JC009425%4010.1002/%28ISSN%292169-9291.FAMOS1Seasonality and long-term trend of Arctic Ocean surface stress in a modelMost of the remarks below are from the first - written so even I could understand (most) of it.
The momentum flux from the atmosphere into the ocean (also known as ocean surface stress) depends on various factors such as wind speed, surface layer stability, surface roughness, and sea ice conditions. Roughness changes in response to changing ocean surface waves and variations in the geometry of ice floes and ridges. Three regimes characterize how sea ice moderates momentum transfer into the Arctic Ocean:
1. At very high ice concentrations, near 100%, the pack ice is so compact that it barely responds to the wind forcing and hence also shields the ocean from the wind.
2. Slightly lower ice concentrations, about 80-90%, allow the ice to drift freely with the wind as pressure within the ice pack is reduced to a minimum, while floe edges and ridges provide high drag (See Figure 1). We refer to this as an "optimal ice concentration", because ocean surface stress is maximal in this case -- as illustrated in the graph of ocean surface stress as a function of sea ice concentration derived from Pan-Arctic Ice Ocean Modeling and Assimilation System1 (PIOMAS) output (See Figure 2).
3. For still lower ice concentrations, stresses decline because open water—even with surface waves—is generally smoother than pack ice.
graph attached
This suggests to me that when looking at the ice drift map the effect of winds will be highest in the high concentration (but less than 100%) areas a shrinking summer sea ice extent means less momentum flux into the ocean in this season. How is that? In the 1980s and 1990s most of the Arctic Ocean featured high ice concentrations, even in summer, with an average close to the 80-90% optimum.
In recent years however, vast areas of open water have reduced the mean ice concentration below this optimum, which results in an overall ocean stress decrease at a small but significant rate in summer.
What does the future hold? The area of high momentum flux (See green in Figure 3a) is shrinking toward Greenland as sea ice continues to retreat. Further, an expanding summer season with increasingly less ice coverage might steepen the negative ocean stress trend and eventually even reverse the positive trends in spring and fall. But this assumes that wind forcing and ocean surface waves do not grow, an assumption that might prove incorrect in a changing climate. This illustrates the fascinating interplay between opposing forces that will determine the magnitude of Arctic Ocean currents in the future.
image attached
The second paper shows show how while in summer ocean stress trend is falling, in s
pring and especially autumn (period of highest winds is in October)
ocean stress is increasing as concentrations in much of the remaining ice have fallen to below 100%.
See last image, and here is their conclusion (edited)Our analysis indicates that sea ice in free-drift amplifies the momentum transfer from the atmosphere into the ocean, which contradicts the general perception that sea ice damps the atmosphere-ocean exchange.
On annual average, most momentum is transferred at an ice concentration of 85%.
On the seasonal scale, sea ice conditions are optimal for maximal momentum flux into the ocean twice a year, in spring and fall. However, wind speeds are much higher in fall
What do I take from this?
- when looking at a sea-ice drift / wind speed map, have the Bremen ice concentration map to hand to see where he biggest impact will be on ice mobility (green and purple not good, yellow and red good?,
- October is the month when winds can have the maximum effect on a weakened ice pack (also Spring?),
- the data goes to 2012. Were there follow-up projects? I hope so.
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I think this is relevant to the end of season prognosis.
I await being shot down with interest.
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