Person: Sayag, Roiy
Loading...
Email Address
AA Acceptance Date
Birth Date
Research Projects
Organizational Units
Job Title
Last Name
Sayag
First Name
Roiy
Name
Sayag, Roiy
6 results
Search Results
Now showing 1 - 6 of 6
Publication Dynamics of the global meridional ice flow of Europa’s icy shell(Springer Science and Business Media LLC, 2017-12-04) Ashkenazy, Yosef; Sayag, Roiy; Tziperman, EliPublication Interaction and Variability of Ice Streams under a Triple-Valued Sliding Law and Non-Newtonian Rheology(Wiley-Blackwell, 2011) Sayag, Roiy; Tziperman, EliIce streams are regions of fast flowing glacier ice that transport a significant portion of the total ice flux from present ice sheets. The flow pattern of ice streams can vary both temporally and spatially. In particular, ice streams can become stagnant and change their path. We study the dynamics of ice streams using an idealized model of an isothermal and power law viscous ice flow that includes horizontal (lateral) shear stresses. The basal sliding law is assumed to be triple-valued. We investigate the spatiotemporal patterns formed because of the flow over a flat bed, fed from an upstream mass source. The ice flows from the mass source region through one or two gaps in a prescribed upstream topographic ridge which restricts the flow, leading to the formation of one or two ice streams. We find a relation between the parameters of the ice rheology and the width of the ice stream shear margins and show how these parameters can affect the minimum width of an ice stream. We also find that complex asymmetric spatiotemporal patterns can result from the interaction of two ice streams sharing a common mass source. The rich spatiotemporal variability is found to mostly be a result of the triple-valued sliding law, but non-Newtonian effects are found to play a significant role in setting a more realistic shear margin width and allowing for relevant time scales of the variability.Publication Spatiotemporal Dynamics of Ice Streams Due to a Triple-Valued Sliding Law(Cambridge University Press, 2009) Sayag, Roiy; Tziperman, EliWe show that a triple-valued sliding law can be heuristically motivated by the transverse spatial structure of an ice-stream velocity field using a simple one-dimensional model. We then demonstrate that such a sliding law can lead to some interesting stream-like patterns and time-oscillatory solutions. We find a generation of rapid stream-like solutions within a slow ice-sheet flow, separated by narrow internal boundary layers (shear margins), and analyse numerical simulations in two horizontal dimensions over a homogeneous bed and including longitudinal shear stresses. Different qualitative behaviours are obtained by changing a single physical parameter, a mass source magnitude, leading to changes from a slow creeping flow to a relaxation oscillation of the stream pattern, and to steady ice-stream-like solution. We show that the adjustment of the ice-flow shear margins to changes in the driving stress in the one-dimensional approximation is governed by a form of the Ginzburg–Landau equation and use stability analysis to understand this adjustment. In the model analysed here, the width scale of the stream is not set spontaneously by the ice flow dynamics, but rather, it is related to the mass source intensity and spatial distribution.Publication Rapid Switch-Like Sea Ice Growth and Land Ice–Sea Ice Hysteresis(American Geophysical Union, 2004) Sayag, Roiy; Tziperman, Eli; Ghil, MichaelRapid and extensive growth of sea ice cover was suggested to play a major role in the sea ice switch mechanism for the glacial cycles as well as on shorter millennial scales [ Gildor and Tziperman, 2000 ]. This mechanism also predicts a hysteresis between sea ice and land ice, such that land ice grows when sea ice cover is small and withdraws when sea ice cover is more extensive. The switch-like sea ice growth and the hysteresis were previously demonstrated using a simple, highly idealized box model. In this work we demonstrate a switch-like sea ice behavior as well as the sea ice–land ice hysteresis using a coupled climate model that is continuous in the latitudinal dimension. It is shown that the switch-like sea ice growth occurs when the initial meridional atmospheric temperature gradient is not too strong. It is also shown that the meridional extent to which sea ice grows in a switch-like manner is not affected by the intensity of the thermohaline circulation, which does, however, influence the climate cooling that is needed to trigger such rapid sea ice growth.Publication A “Triple Sea-Ice State” Mechanism for the Abrupt Warming and Synchronous Ice Sheet Collapses During Heinrich Events(American Geophysical Union, 2004) Kaspi, Yohai; Sayag, Roiy; Tziperman, EliAbrupt, switch-like, changes in sea ice cover are proposed as a mechanism for the large-amplitude abrupt warming that seemed to have occurred after each Heinrich event. Sea ice changes are also used to explain the colder-than-ambient glacial conditions around the time of the glacier discharge. The abrupt warming events occur in this mechanism, owing to rapid sea ice melting which warmed the atmosphere via the strong sea ice albedo and insulating feedbacks. Such abrupt sea ice changes can also account for the warming observed during Dansgaard-Oeschger events. The sea ice changes are caused by a weak (order of 5 Sv) response of the thermohaline circulation (THC) to glacier discharges. The main point of this work is therefore that sea ice may be thought of as a very effective amplifier of a weak THC variability, explaining the abrupt temperature changes over Greenland. Synchronous ice sheet collapses from different ice sheets around the North Atlantic, indicated by some proxy records, are shown to be possible via the weak coupling between the different ice sheets by the atmospheric temperature changes caused by the sea ice changes. This weak coupling can lead to a “nonlinear phase locking” of the different ice sheets which therefore discharge synchronously. It is shown that the phase locking may also lead to “precursor” glacier discharge events from smaller ice sheets before the Laurentide Ice Sheet discharges. The precursor events in this mechanism are the result rather than the cause of the major glacier discharges from the Laurentide Ice Sheet.Publication Spontaneous Generation of Pure Ice Streams Via Flow Instability: Role of Longitudinal Shear Stresses and Subglacial Till(American Geophysical Union, 2008) Sayag, Roiy; Tziperman, EliA significant portion of the ice discharge in ice sheets is drained through ice streams, with subglacial sediment (till) acting as a lubricant. The known importance of horizontal friction in shear margins to ice stream dynamics suggests a critical role of longitudinal stresses. The effects of subglacial till and longitudinal stresses on the stability of an ice sheet flow are studied by linear stability analysis of an idealized ice-till model in two horizontal dimensions. A power law-viscous constitutive relation is used, explicitly including longitudinal shear stresses. The till, which has compressible viscous rheology, affects the ice flow through bottom friction. We examine the possibility that pure ice streams develop via a spontaneous instability of ice flow. We demonstrate that this model can be made intrinsically unstable for a seemingly relevant range of parameters and that the wavelengths and growth rates that correspond to the most unstable modes are in rough agreement with observed pure ice streams. Instabilities occur owing to basal friction and meltwater production at the ice-till interface. The most unstable wavelength arise because of selective dissipation of both short and long perturbation scales. Longitudinal stress gradients stabilize short transverse wavelengths, while Nye diffusion stabilizes long transverse wavelengths. The selection of an intermediate unstable wavelength occurs, however, only for certain parameter and perturbation structure choices. These results do not change qualitatively for a Newtonian ice flow law, indicating no significant role to shear thinning, although this may very well be due to the restrictive assumptions of the model and analysis.