Interferometric synthetic aperture radar (InSAR) observations of ice-shelf tidal deformation reveal the wide transition between grounded and floating ice as well as local areas where the ice shelf is only grounded at low tide, a condition that we call ephemeral grounding. Ephemeral grounding creates a subtle, local disturbance on the vertical motion field of the ice-shelf surface in response to changes in occanic tide which is detected with millimetric precision using InSAR. These ice-shelf features are, however, not expected to produce a noticeable disturbance on the ice-shelf velocity field. To illustrate the influence of ephemeral grounding on ice-shelf creep flow, we use a finite-element model in which ephemeral grounding is incorporated through a variable basal friction coefficient. The results show that while ice rises (permanently grounded areas) have a pronounced influence on the ice-shelf velocity field, areas of ephemeral grounding have a vanishingly small influence. What is thus of most interest is the capacity for observations of ephemeral grounding to reveal subtle changes in ice-shelf thickness over time. We discuss an example in the Thwaites Glacier area, West Antarctica, where multi-year data show how ice rises become ephemeral grounding and subsequently disappear. This result is consistent with the grounding-line retreat and ice thinning of Thwaites Glacier.

We use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5 13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5 10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.

Synthetic-aperture radar interferometry data and airborne ice-sounding radar (ISR) data are employed to obtain modern estimates of the inland ice production from Nioghalvfjerdsbrae (NB) and Zachariae Isstrom (ZI), the two largest glaciers draining the northeast sector of the Greenland ice sheet. Ice fluxes are measured at the grounding line (14.2 ± 1 km3 ice a-1 for NB and 10.8 ± 1 km3 ice a-1 for ZI) with an ice thickness deduced from ice-shelf hydrostatic equilibrium, and along an ISR profile collected upstream of the grounding line (14.3 ± 0.7 km3 ice a-1 for NB and 11.6 ± 0.6 km3 ice a-1 for ZI). Balance fluxes calculated from a map of snow accumulation and model predictions of surface melt are 11.9 ± 2 km3 ice a-1 for NB and 10.0 ± 2 km3 ice a-1 for ZI at the grounding line, and 12.2 and 10.3 km3 ice a-1, respectively, at the ISR line. The two glaciers therefore exhibit a negative mass balance equivalent to 14% of their balance flux, with a ±12% uncertainty. Independently, we detect a retreat of the grounding line of NB between 1992 and 1996 which is larger at the glacier center (920 ± 250 m) than on the sides (240 ± 50 m). The corresponding ice-thinning rates (2±1 m a-1 at the glacier center and 0.6±0.3 m a-1 on the sides) are too large to be accommodated by temporal changes in ablation or accumulation, and must be due to dynamic thinning.

Depth-integrated primary productivity (PP) estimates obtained from satellite ocean color-based models (SatPPMs) and those generated from biogeochemical ocean general circulation models (BOGCMs) represent a key resource for biogeochemical and ecological studies at global as well as regional scales. Calibration and validation of these PP models are not straightforward, however, and comparative studies show large differences between model estimates. The goal of this paper is to compare PP estimates obtained from 30 different models (21 SatPPMs and 9 BOGCMs) to a tropical Pacific PP database consisting of ∼ 1000 14C measurements spanning more than a decade (1983-1996). Primary findings include: skill varied significantly between models, but performance was not a function of model complexity or type (i.e. SatPPM vs. BOGCM); nearly all models underestimated the observed variance of PP, specifically yielding too few low PP (< 0.2 g C m- 2 d- 1) values; more than half of the total root-mean-squared model-data differences associated with the satellite-based PP models might be accounted for by uncertainties in the input variables and/or the PP data; and the tropical Pacific database captures a broad scale shift from low biomass-normalized productivity in the 1980s to higher biomass-normalized productivity in the 1990s, which was not successfully captured by any of the models. This latter result suggests that interdecadal and global changes will be a significant challenge for both SatPPMs and BOGCMs. Finally, average root-mean-squared differences between in situ PP data on the equator at 140°W and PP estimates from the satellite-based productivity models were 58% lower than analogous values computed in a previous PP model comparison 6 years ago. The success of these types of comparison exercises is illustrated by the continual modification and improvement of the participating models and the resulting increase in model skill. © 2008 Elsevier B.V.