From the Rignot paper, some historic(2000) seasonality variation:
"We examined the seasonal variability in flow
speed of major glaciers in fall 2000. We found
no velocity change from September to January at
the 1% level over the 24-day averaging period of
Radarsat-1. On the Petermann Glacier (1 in Fig.
1), a continuous set of observations in 2004
reveals an 8% increase in the summer months
compared to winter (Fig. 2A). A similar sea-
sonality is detected on Nioghalvfjerdsbrae and
all southeast Greenland glaciers and has been
observed on Jakobshavn Isbrae (
and Colum-
bia Glacier, Alaska (9). Winter velocities are
therefore only 2% lower than the annual means,
and flow changes must exceed 8% to be sig-
nificant."
I atttach the figure referred to, y axis is meter/year