lifeblack read this I'd be interested in your opinion it's mostly about tidal effects on the glacier it's movement and calving, bear in mind that due to it's geographical position the tides in Jacobshvn fjord are about 3-4hrs behind the 'natural' tide time so in some sense out of synch with any effects on the glacier. I found nothing in it that confirmed or challenged my view, apart from the step change of melange height above the cill [fig2] which, to me, suggests a deep logjam of melange, and if so then voids enough for tidal seawater/meltwater exchange.
So, here's my take on the paper: It's a derivation of the location of the grounding line of the glacier, and whether the glacier front coincides with the grounding line. The technique used is flow rate variation along the line of sight from a vantage point to various locations over a period of several days (1). The flow rate variations are (via
math) transformed to a spectral distribution and plotted, the three major tide components are also marked in these plots.
A phase offset is seen at the glacier front, and some vertical motion is found during early summer (2). A step change is seen in the flow rate of the melange of the ice floating in the fjord, along with a decrease in height above surface that coincides with this point.
Paper concludes with logical argument consistent with presented evidence that a short tongue of ice exists in early summer (3) and that the glacier calves it off by the end of summer and the front is grounded. At no point in the paper does it suggest that seawater is penetrating past the grounding line - in fact, given the height increase at the grounding line (and according to Sidd's explanation) it appears that seawater will NOT be moving upstream past the sill until the glacier is thin enough to float at that location (or pretty darn close to it) (4)
1. I'm going to trust that this is done correctly. I got a little bit confused with the description and how topography affects things.
2. I'm not entirely clear on how they extract vertical movements from their LOS measurements. See 1.
3. There's also vertical movement associated with what they think is the tongue, and the glacier height is low enough to support being floated at that location.
4. Seawater at the base of the glacier will definitely help melt it, but I don't think it's capable of generating a cavity extending upstream until the height of the glacier at that point is low enough to be floated (and not pressing down hard on the bed).