Exploration of Tunabreen surge mechanics through a fitted calving SSA model

Surface mass balance (SMB) estimates of glaciers in Svalbard have a relatively high degree of uncertainty despite decades of modeling and observations for the region (van Pelt et al., 2019). This is in part because the surges of marine terminating surging glaciers are complicating estimates of the mass flux towards the ocean (Schuler et al., 2020). Approximately 52% of the glaciers in Svalbard are likely or confirmed surging glaciers (Harcourt et al., 2025), making this complication to estimating the SMB common on Svalbard. The climate in Svalbard changes more rapidly than global averages (Maturilli et al., 2013; Nordli et al., 2014; Isaksen et al., 2016) but it is still unclear to what degree this affects the glaciers in Svalbard as the surge mechanics are still not well understood (Schuler et al., 2020). It does seem that the surges are increasing in frequency over the last years (Farnsworth et al., 2016). Therefore, it is hard to give projections into the future on how glaciers in Svalbard will evolve over the next 50 to a 100 years.

In our study we have looked at Tunabreen, a marine terminating surging glacier that has surge twice in the last 25 years. We use Sentinal-1-based velocity data and digital elevation models (ArticDEMs) to analyze the last surge of Tunabreen. These data show that the surge starts at the terminus and moves up in several distinct phases.

This knowledge is used to fit a calving flowline SSA model to replicate Tunabreen behavior over the last 25 years. The SSA model is based on the model from (Nick et al., 2010) with an adaptable mesh to accommodate a moving terminus as a consequence of calving. Furthermore, the model is adapted from its original form to incorporate the boundary conditions for a grounded marine terminating glacier. The calving law is based on the observed relation between water temperature and terminus retreat (Luckman et al., 2015). To force the model, we used model data of the last 25 years from ERA5-LAND for the air temperature and water temperature.

As such our model is able to replicate the evolution of Tunabreen over the last 25 years to provide insight in the possible surging mechanisms. It shows that a combination from calving, that initiated the surging, as well as subsurface hydrology sustaining the surge, leads to the typical behavior observed at Tunabreen. Using the fitted model, we can estimate how Tunabreen will evolve in the next few decades and what the effect of different climate forecast models or possible pinning points in the bedrock may have on it.