dnem asks: is the 15 Sep 2020 slide in #4796 your actual best guess scenario for that date? What would the extent of that scenario be?
No. It is better to focus on weather that has actually happened and document what that has done to the ice. For example, we do not have a grip on what is causing the unprecedented opening between Ellesmere-Greenland and pole so no have clue what comes tomorrow, much less Sept 15th. Even the better ‘operational’ forecasts like hycom haven’t been able find ice physics that reproduces what we can plainly see happening.
Sept 15 will come soon enough, it’s grossly over-weighted in posts. The melt forum is not really concerned with melt season but rather with
loss of the planet’s refrigerator, the darkening solar reflector of high latitude northern hemisphere. Open water having only a tenth the albedo of sea ice, it carves out the lion’s share of growing energy imbalance as it fits increasingly better to seasonal insolation inputs. So it gets a forum.
‘Extent’ isn’t extent but rather a bare number that has forgotten its ice distribution picture. The bizarre opening this summer will go down the rabbit hole of a line graph. Multi-decadal trend lines are a much better use of extent numbers but even there, the x axis intercept has provided a dangerous upper bound to BOE oo far out in the future whereas the end game will likely be vastly faster and different process from the trend.
The 15 Sept scenario in #4796 isn't ‘photoshopping the future’ for the sake of an extent visualization stand-in because there is a
physical basis to it: lowered concentration feeds on itself as enhanced lateral melt takes edges and smaller floes down to open water. So nipping away successively at the lowest concentration remaining in the palette does mimic the dominant process creating open water. However there’s no ability to anticipate less-than-boring weather.
This visualization method also works on hycom thickness though the
physical basis is different (bottom melt). Because the embedded palette is so complex, it is harder to progressively infill the thinnest ice remaining with open water gray but still doable with lots of back and forth on the color picker radius. Hycom goes out six days on the weather so again a 30 day visualization will be thrown off by weather.
Ascat in late-May can supply surprisingly realistic visualizations based on yet another
physical basis, residual brine channel salinity. The grayscale images are whiter on older matured ice and darker on newer FYI and SYI and systematically so. Thus iterated stepping of the contrast slider can incrementally erode higher dielectric ice, mimicking summer melt-off. Ascat does not provide useful images from mid-June to mid-Sept.
Despite the differing physical bases and gimp operations to exploit them, what the three future visualizations have in common is a stepping number. If that’s overdone or underdone, the visualization won’t look much like the actual future, especially if there’s unexpected weather. That departure is of interest in itself. However the images will remain a big step up from bare extent numbers or photoshop artwork.
The first animation below shows major changes are going on every day, just not ones that manifest on the low resolution extent/area forums
The five slides below start with the alarming OsiSaf of 11 Aug 2020, a 48-hour measure of ice pack displacement captured by well-tested AI. The background dark blue is open water; the inner light blue is not quite fully open as it still has vulnerable ice. There’s advantages to simplicity in terms of capturing a good overview; it’s sometimes hard to find a basic take-away from AMSR2 displays with 1% concentration increments that need a hundred colors.
The second slide overlays a reinitialized GFS wind display at 850 hPa in the middle of the OsiSaf date range. This shows winds a couple kms above the surface which again captures the main story without getting into complexities of actual surface air movement. It shows an unremarkable central anti-cyclone surrounded by three even less remarkable small cyclones. A confused jet stream far above has introduced not just surface highs and lows but given angular momentum to them.
The third slide drops the weather pattern onto the ice motion display. It’s a fairly good fit — the ice is moving right along under the anti-cyclone) — considering the 3-hr nullschool dynamic is on a 48-hour OsiSaf summary. The three cyclones are over open water, making waves but not directly moving ice. Persistent winds over long reaches can create swells that greatly damage the ice pack many hundreds of km away if they hit head-on. We have no idea if the next six weeks will bring extreme winds like these.
Air pressure differences per se don’t bring strong cyclonic winds, the gradient between the pattern of highs and lows has to be steep (close isobars). It follows that merely giving the low (970 hPA for the 27 July 2020 event) doesn’t adequately describe the strength, persistence, and ice impacts of the storm.
The fourth slide shifts to where the rubber meets the road: wind power density (WPD) vs frictional resistance of the ice (from pressure ridges, surface roughness and exposed freeboard edges). Wind tunnel data has established that the force exerted on the ice is better fit to the cube of wind speed (rather than speed). That means a mild Arctic zephyr of 5m/s (often seen by the Polarstern) provides 1/8 the push of a brisk 10m/s wind and 1/64 that of a 20m/s gale (rather than the 1/4 expected from assuming a linear relation).
A key feature of WPD in an Arctic cyclone is its strong variation with location and timestamp. Because of the cube, this is exaggerated for WPD relative to plain wind. On this fourth slide, the wind is seen acting very unevenly on the ice, the gold color indicating extreme forces above Ellesmere and Greenland and the blue a far more moderate impulse. Since we have no real idea of the level and distribution of friction across the ice surface, we have no real idea how the ice pack will respond to forces that vary so markedly in magnitude, direction and persistence.
Strong persistent non-cyclonic winds off Siberia brought a most unusual TransPolar Drift in late winter, causing havoc for the Polarstern, putting it months ahead of schedule and prematurely flushing it down the Fram. How does do cyclonic wind gyres differ in ice effect from straight southerlies? A tight radius of curvature means over a short distance through the eye that the wind has reversed direction by 180º. The ice, being pushed in opposite directions, forms a shear zone when cold and thick but more just a jumble of floes when thinner and weakly matrixed. A straight southerly has no curvature but may vary across a transect.
The ice also picks up on the overall cyclonic rotation as seen in this fourth slide. However the torque is not evenly applied. If the ice pack has open leads or polynyas, some of this can be adsorbed by compression or dispersion; if not, by ridging and over-rafting. In summary, though cyclonic events don’t necessarily bring extreme winds (or WPD), the ice pack is stressed in more complex ways. In late summer, on thinning ice, the effects can be dramatic.
The major 27 July 2020 cyclone will have a huge effect on the overall melt season. Even though much of the devastated ice might have melted out anyway just barely before Sept 15th, the early melt wrapping up now has exposed the perimeter of the central older thicker icepack both to enhanced lateral melt and to extreme wind exposure (so far unrealized).