Person: Feliks, Yizhak
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Feliks
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Yizhak
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Feliks, Yizhak
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Publication Nonnormal Frontal Dynamics(American Meteorological Society, 2010) Feliks, Yizhak; Tziperman, Eli; Farrell, BrianThe generalized stability of the secondary atmospheric circulation over strong SST fronts is studied using a hydrostatic, Boussinesq, two-dimensional f-plane model. It is shown that even in a parameter regime in which these circulations are stable to small perturbations, significant nonnormal growth of optimal initial perturbations occurs. The maximum growth factor in perturbation total energy is 250 and is dominated by the potential energy, which obtains a growth factor of 219 two to five hours after the beginning of the integration. This domination of potential energy growth is consistent with the observation that the available potential energy (APE) of the secondary circulation is larger by two orders of magnitude than the kinetic energy as well as with the transfer of kinetic to potential perturbation energy at the beginning of the growth of the perturbations. The norm kernel is found to significantly influence the structure of the optimal initial perturbation as well as the energy obtained by the mature perturbations, but the physical mechanism of growth and the structure of the mature perturbations are robust.Publication Asymmetry of Daily Temperature Records(American Meteorological Society, 2008) Ashkenazy, Yosef; Feliks, Yizhak; Gildor, Hezi; Tziperman, EliThe authors study the NCEP–NCAR reanalysis temperature records and find that surface daily mean temperature cools rapidly and warms gradually at the midlatitudes (around 40°N and 40°S). This “asymmetry” is partially related to the midlatitude cyclone activity, in which cold fronts are significantly faster and steeper than warm fronts, and to intrusions of cold air. The gradual warming may be attributed also to the radiative relaxation to average atmospheric conditions after the passage of cold fronts or other intrusions of cold air. At the high latitudes there is an opposite asymmetry with rapid warming and gradual cooling; this asymmetry may be attributed to the radiative relaxation to average cold atmospheric conditions after the passage of warm fronts or intrusions of warm air.Publication Topography and Grounding in a Simple Bottom Layer Model(American Geophysical Union, 1993) Speer, Kevin; Tziperman, Eli; Feliks, YizhakA reduced-gravity model for a dense bottom layer under a motionless deep layer is used to investigate bottom layer flow with topography. The two competing effects in the model are topography, which introduces a topographic β effect, and strong layer thickness variation, which allows for the possibility of zero-layer thickness or grounding. The goal is to arrive at a simple, observationally relevant characterization of the model which can help to distinguish different dynamical mechanisms operating in the ocean. Several regimes are identified, including situations for which the layer depth goes to zero with and without topography. Ways of distinguishing observationally between competing effects are suggested. The study is motivated by the observed shift of Antarctic Bottom Water in the western North Atlantic Ocean from the west to the east; this phenomenon is used to discriminate key elements of the model.Publication Instability of the Thermohaline Circulation with Respect to Mixed Boundary Conditions: Is It Really a Problem for Realistic Models?(American Meteorological Society, 1994) Tziperman, Eli; Toggweiler, J. R.; Bryan, Kirk; Feliks, YizhakA global primitive equations oceanic GCM and a simple four-box model of the meridional circulation are used to examine and analyze the instability of the thermohaline circulation in an ocean model with realistic geometry and forcing conditions under mixed boundary conditions. The purpose is to determine whether this instability should occur in such realistic GCMs. It is found that the realistic GCM solution is near the stability transition point with respect to mixed boundary conditions. This proximity to the transition point allows the model to make a transition between the unstable and stable regimes induced by a relatively minor change in the surface freshwater flux and in the interior solution. Such a change in the surface flux may be induced, for example, by changing the salinity restoring time used to obtain the steady model solution under restoring conditions. Thus, the steady solution of the global GCM under restoring conditions may be either stable or unstable upon transition to mixed boundary conditions, depending on the magnitude of the salinity restoring time used to obtain this steady solution. The mechanism by which the salinity restoring time affects the model stability is further confirmed by carefully analyzing the stability regimes of a simple four-box model. The proximity of the realistic ocean model solution to the stability transition point is used to deduce that the real ocean may also be near the stability transition point with respect to the strength of the freshwater forcing. Finally, it is argued that the use of too short restoring times in realistic models is inconsistent with the level of errors in the data and in the model dynamics, and that this inconsistency is a possible reason for the existence of the thermohaline instability in GCMs of realistic geometry and forcing. A consistency criterion for the magnitude of the restoring times in realistic models is formulated, that should result in steady states that are also stable under mixed boundary conditions. The results presented here may be relevant to climate studies that run an ocean model under restoring conditions in order to initialize a coupled ocean–atmosphere model.