Abstract
Abstract. We present a sensitivity analysis and discuss the probabilistic forecast capabilities of the novel sea ice model neXtSIM used in hindcast mode. The study pertains to the response of the model to the uncertainty on winds using probabilistic forecasts of ice trajectories. neXtSIM is a continuous Lagrangian numerical model that uses an elasto-brittle rheology to simulate the ice response to external forces. The sensitivity analysis is based on a Monte Carlo sampling of 12 members. The response of the model to the uncertainties is evaluated in terms of simulated ice drift distances from their initial positions, and from the mean position of the ensemble, over the mid-term forecast horizon of 10 days. The simulated ice drift is decomposed into advective and diffusive parts that are characterised separately both spatially and temporally and compared to what is obtained with a free-drift model, that is, when the ice rheology does not play any role in the modelled physics of the ice. The seasonal variability of the model sensitivity is presented and shows the role of the ice compactness and rheology in the ice drift response at both local and regional scales in the Arctic. Indeed, the ice drift simulated by neXtSIM in summer is close to the one obtained with the free-drift model, while the more compact and solid ice pack shows a significantly different mechanical and drift behaviour in winter. For the winter period analysed in this study, we also show that, in contrast to the free-drift model, neXtSIM reproduces the sea ice Lagrangian diffusion regimes as found from observed trajectories. The forecast capability of neXtSIM is also evaluated using a large set of real buoy's trajectories and compared to the capability of the free-drift model. We found that neXtSIM performs significantly better in simulating sea ice drift, both in terms of forecast error and as a tool to assist search and rescue operations, although the sources of uncertainties assumed for the present experiment are not sufficient for complete coverage of the observed IABP positions.
Highlights
Large changes in the Arctic sea ice have been observed in recent decades in terms of the ice thickness, extent and drift (e.g. Kwok, 2007; Stroeve et al, 2007; Rampal et al, 2011; Stroeve et al, 2012)
We note that the regime we obtain with neXtSIM for the summer 2008 is super-diffusive, with bi2 D ∼ t1.15 for t, and in apparent contradiction with Rampal et al (2009), who found that sea ice follows a same Brownian regime in both winter and summer when averaging over the period 1979–2007. We suggest that this may be the fingerprint of a change in the dynamical behaviour of sea ice in summer that occurred over the most recent years, in which the rheology plays a weaker role than it did in the 1980s and 1990s
The ensemble model sensitivity experiment carried out with neXtSIM and with an FD model reveals the prominent role of the rheology, which marks the key difference between the two models
Summary
Large changes in the Arctic sea ice have been observed in recent decades in terms of the ice thickness, extent and drift (e.g. Kwok, 2007; Stroeve et al, 2007; Rampal et al, 2011; Stroeve et al, 2012). The sea ice models used in these systems are usually derived from the work of Hibler III (1979), and they treat the sea ice as a continuous medium with a viscous–plastic rheology (Hunke and Dukowicz, 1997; Bouillon et al, 2009) In spite of this development, simple free-drift ice (i.e. in the absence of friction and internal forces) forecasts have remained in use by environment agencies (Grumbine, 1998, 2003).
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