William D. Hibler

Bio: William D. Hibler is an academic researcher from University of Alaska Fairbanks. The author has contributed to research in topics: Sea ice & Arctic ice pack. The author has an hindex of 34, co-authored 73 publications receiving 5436 citations. Previous affiliations of William D. Hibler include Dartmouth College & University of Texas at Austin.

A Dynamic Thermodynamic Sea Ice Model

Abstract: A numerical model for the simulation of sea ice circulation and thickness over a seasonal cycle is presented. This model is used to investigate the effects of ice dynamics on Arctic ice thickness and air-sea heat flux characteristics by carrying out several numerical simulations over the entire Arctic Ocean region. The essential idea in the model is to couple the dynamics to the ice thickness characteristics by allowing the ice interaction to become stronger as the ice becomes thicker and/or contains a lower areas percentage of thin ice. The dynamics in turn causes high oceanic heat losses in regions of ice divergence and reduced heat losses in regions of convergence. TO model these effects consistently the ice is considered to interact in a plastic manner with the plastic strength chosen to depend on the ice thickness and concentration. The thickness and concentration, in turn, evolve according to continuity equations which include changes in ice mass and percent of open water due to advection, ...

Modeling a variable thickness sea ice cover

Abstract: A numerical model simulating a variable thickness sea ice cover over a seasonal cycle is presented. The model includes a fixed depth mixed-layer formulation with open water heat absorption and lateral melting terms, and a mechanical distribution function consistent with the physics of the ridging process. The equibrium simulation results in realistic geographical ice thickness variations of April ice along the Canadian Archipelago which exceed 7 m, and thicknesses of about 2 m along the Alaskan North Slope. Ice velocity fields were realistic in shape but 25% larger than the net ice station drift over a year; sensitivity simulations indicated a reduced average annual ice export rate of 0.04 Sv as compared to 0.09 Sv for the equilibrium simulation.

Modeling Pack Ice as a Cavitating Fluid

Abstract: Polar ocean circulation is influenced by fluxes of salt and freshwater at the surface as ice freeze in one location, is transported by the winds and currents, and melts again elsewhere. The motion of sea ice, moreover, is strongly affected by internal stresses that arise from the mechanical strength of the ice cover. A simple sea-ice dynamics model, allowing these effects to be included in large-scale climate studies, is presented. In this model a cavitating fluid behaviour is assumed whereby the ice pack does not resist divergence or shear, but does resist convergence. While less realistic than other rheologies that include shear strength, this assumption has certain advantages for long-term climate studies. First, it allows a simple and efficient numerical scheme, in both rectangular and spherical coordinates, which as developed here along with a generation to include shear strength via the Mohr-Coulomb failure criteria. Second, realistic ice transport is maintained, even when the model is driv...

On an efficient numerical method for modeling sea ice dynamics

Abstract: A computationally efficient numerical method for the solution of nonlinear sea ice dynamics models employing viscous-plastic rheologies is presented. The method is based on a semi-implicit decoupling of the x and y ice momentum equations into a form having better convergence properties than the coupled equations. While this decoupled form also speeds up solutions employing point relaxation methods, a line successive overrelaxation technique combined with a tridiagonal matrix solver procedure was found to converge particularly rapidly. The procedure is also applicable to the ice dynamics equations in orthogonal curvilinear coordinates which are given in explicit form for the special case of spherical coordinates.

A Diagnostic Ice–Ocean Model

Abstract: A coupled ice–ocean model suitable for simulating ice–ocean circulation over a seasonal cycle is developed by coupling the dynamic thermodynamic sea ice model of Hibler with a multilevel baroclinic ocean model (Bryan). This model is used to investigate the effect of ocean circulation on seasonal sea ice simulations by carrying out a simulation of the Arctic, Greenland and Norwegian seas. The ocean model contains a linear term that damps the ocean's temperature and salinity towards climatology. The damping term was chosen to have a three-year relaxation time, equivalent to the adjustment time of the pack ice. No damping, however, was applied to the uppermost layer of the ocean model, which is in direct contact with the moving pack ice. This damping procedure allows seasonal and shorter time-scale variability to be simulated in the ocean, but does not allow the model to drift away from ocean climatology on longer time scales. For the standard experiment, an initial integration of five years was per...

Anomalous collapses of Nares Strait ice arches leads to enhanced export of Arctic sea ice

Abstract: The ice arches that usually develop at the northern and southern ends of Nares Strait play an important role in modulating the export of Arctic Ocean multi-year sea ice. The Arctic Ocean is evolving towards an ice pack that is younger, thinner, and more mobile and the fate of its multi-year ice is becoming of increasing interest. Here, we use sea ice motion retrievals from Sentinel-1 imagery to report on the recent behavior of these ice arches and the associated ice fluxes. We show that the duration of arch formation has decreased over the past 20 years, while the ice area and volume fluxes along Nares Strait have both increased. These results suggest that a transition is underway towards a state where the formation of these arches will become atypical with a concomitant increase in the export of multi-year ice accelerating the transition towards a younger and thinner Arctic ice pack.

The simulation of SST, sea ice extents and ocean heat transports in a version of the Hadley Centre coupled model without flux adjustments

Abstract: Results are presented from a new version of the Hadley Centre coupled model (HadCM3) that does not require flux adjustments to prevent large climate drifts in the simulation The model has both an improved atmosphere and ocean component In particular, the ocean has a 125° × 125° degree horizontal resolution and leads to a considerably improved simulation of ocean heat transports compared to earlier versions with a coarser resolution ocean component The model does not have any spin up procedure prior to coupling and the simulation has been run for over 400 years starting from observed initial conditions The sea surface temperature (SST) and sea ice simulation are shown to be stable and realistic The trend in global mean SST is less than 0009 °C per century In part, the improved simulation is a consequence of a greater compatibility of the atmosphere and ocean model heat budgets The atmospheric model surface heat and momentum budget are evaluated by comparing with climatological ship-based estimates Similarly the ocean model simulation of poleward heat transports is compared with direct ship-based observations for a number of sections across the globe Despite the limitations of the observed datasets, it is shown that the coupled model is able to reproduce many aspects of the observed heat budget

Tipping elements in the Earth's climate system

Abstract: The term "tipping point" commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term "tipping element" to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.

GFDL's CM2 global coupled climate models. Part I: Formulation and simulation characteristics

Abstract: The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved. Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, wi...