Cyclone warning!
by neven, August 5, 2012
I have postponed this post until I was sure that what follows is going to happen.
Remember the term 'flash melting'? That's when from one day to the next large swathes of ice disappear on the University of Bremen sea ice concentration maps. We witnessed one such instance last year when a relatively large and intense low-pressure area moved in from Alaska over the ice in the Beaufort and Chukchi Sea regions (see blog post). It lasted about a day or two and then quickly faded, but the effects were spectacular.
Well, it looks like we have something bigger coming up this year. This is the ECMWFweather forecast for the coming four days (click for a bigger version):
Not only does this low-pressure area, or cyclone, look bigger, more intense and longer-lasting than the one from last year, the ice also seems to be in a weaker state than ever, as evidenced by the fact that 2012 trend lines on both sea ice area and sea ice extent graphs track lower than previous record years, despite weather that until recently would completely stall the decline.
There are many things that cyclones do to cause flash melting:
- Diverge the ice pack through wind force (gales), creating open water between ice floes and pushing ice floes towards warmer waters, for instance in this coming event towards the Beaufort and Laptev Seas, where waters are anomalously warm.
- Churn the ice, fragmenting it into smaller pieces which are easier to melt out, turning floes upside down even, with their darker bottoms (due to algae, etc.) showing up, soaking up more sunlight.
- Increase wave action, especially when there's no thick ice to dampen the waves, flooding floes with saltier water that melts the ice, but can also temporarily fool satellite sensors into thinking there's open water. Therefore some of the flash melting 'unflashes' the next day, and then flashes again, and unflashes, until it's really gone.
- Increase vertical mixing of the waters below the ice. Remember, the top layer is much colder and fresher due to the melting of sea ice than the layers below. Wave action and something called Ekman pumping (which I'll explain in the near future, but here is a good discussion) cause upwelling of these warmer layers. As Artful Dodger commented earlier today: "Predicted wave heights in the Chukchi sea are 10-12 feet during the gale. That means about 30-36 feet of the sea surface layer will be churned by wave action. This will bring warmer, saltier water up from the depths and inundate the sea ice. All surfaces of the ice flows will be exposed to warmer water, and the fresh melt layer will be quickly and continuously washed away, preventing refreezing. Look for massive loss of SIA, which may not be detectable until after the storm clears, due to the clouds masking the passive microwave satellite sensors."
So that's what we can expect: big losses in sea ice area, and perhaps in sea ice extent as well (with a lag). I'm going to try to cover this as real-time as I can, so make sure to check in for updates. This could become very big and should make for some spectacular images. Too early for conclusions wrt to minimum records. We'll know more Thursday/Friday.
Posts
No comments:
Post a Comment