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Yizhak Feliks

Abstract

Analytical solutions to the nonlinear equations of motion are used to describe the sea breeze front.

It is found that the front can develop when the atmosphere stratification in the heated layer is neutral or unstable. The temperature drop due to front passage is proportional to the square of the difference between the front speed and the synoptic wind. This square of the difference, on the other hand, is proportional to [2θ()/θm]ga where θ() is the mean potential temperature drop across the front and a is the front radius. Fronts which have the same speed of propagation and size, have a larger temperature drop when the synoptic wind blows in the opposite direction to the direction of propagation.

Onshore winds associated with strong convergence are obtained in the lower part of the front, while the return current associated with strong divergence is observed in the upper part. The vertical velocity reaches values of some meters per second in the front region.

The front propagation is studied in terms of the vorticity equation. The buoyancy term always tends to propagate the front inland. The nonlinear advective term in most of the cases tends to slow this propagation. In some of the cases when buoyancy is very low the advective term tends to propagate the front inland.

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Yizhak Feliks

Abstract

The most prominent winter storms in the eastern part of the Eastern Mediterranean are known as Cyprus cyclones. The surface wind speed is between 15–30 m s−1, and about five such cyclones occur in a typical winter. The cyclone radius is between 500 and 1500 km. The evolution of the sea structure under such atmospheric forcing is examined with a two-dimensional numerical model in the vertical cross section perpendicular to the shore line. Two distinct regions result in the sea. A downwelling zone near the coast, about 100 km wide, and a horizontally homogeneous zone in the open sea, where vertical mixing is the important dynamical process. In the open sea the final profiles turn out to be similar to those observed in the Levantine Intermediate Water (LIW) in their formation region. We suggest that the LIW forms in the region under the influence of these Cyprus cyclones.

In the downwelling zone the 14°–17°C isotherms decline by more than 250 m. This water has the same T-S properties as the water in the anticyclonic eddies found along the Asia Minor coast and other parts of the Eastern Mediterranean. A very deep mixed layer is obtained, deeper than 300 m. The water in the downwelling zone near the sea surface is warmer by 0.50°–1°C than in the open sea. This last result is observed in winter IR satellite images.

The downwelling rate increase with increasing wind stress and decreasing horizontal eddy coefficient. This rate is not influenced by the evolution in the mixing layer.

Along the coast in the downwelling front a prominent jet developed. The scale of the jet is proportional to the square root of the horizontal eddy coefficient.

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Yizhak Feliks

Abstract

In this work the evolution of the sea and land breeze is studied using a nonlinear model under calm synoptic conditions and diurnal periodic forcing of ground temperature. The breeze is examined as a function of the strength of the heating amplitude of ground temperature θ 0. For θ 0 ≤ 6°C, the solution is quasi-periodic with two incommensurate oscillations of 24 and 22.6 h; the last is the inertial oscillation at latitude 32°N. A very low frequency oscillation (VLFO) of 16 days, which is the linear combination of the two incommensurate oscillations, is also obtained. For θ 0 = 7°C, the solution becomes nonperiodic. For θ 0 ≥ 10°C, chaotic solutions are obtained. In the chaotic regime the prominent oscillations can be divided into two classes. One class includes short-time-scale oscillations, such as the 24-h oscillation, the 22.4-h slightly modified inertial oscillation, and their harmonics. The second class incorporates time scales that are larger than a week, such as 15 days, which is a linear combination of the 24- and 22.4-h oscillations. The flow in the second class is in geostrophic balance. The kinetic energy, which manifests spells of very large energy fluctuations, is examined. During these spells the amplitude of the VLFO is large, and the amplitude of the 24-h oscillation is small compared to the spells where the fluctuations in the kinetic energy are small.

Analyses of the wind observations in the central coast of Israel in the summer months show great similarity to the model simulation in the chaotic regime. A VLFO of 10 days, which is prominent in its parallel to the shore component, is interpreted to be the result of the nonlinear interaction between the inertial oscillation at the central latitude of the eastern Mediterranean, 33.5°N, and the 24-h oscillation as obtained in the present model.

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Yizhak Feliks

Abstract

The diurnal oscillation of the height of the inversion due to the sea breeze is studied analytically by use of a linear model. The base of the inversion over the sea moved downward during daytime and upward during nighttime. Over the land the diurnal movement of the inversion base is opposite to that over the sea. The change in the height of the inversion base reached about 250 m. An increase in the mean height of the inversion's base and the stability of the inversion results in a smaller diurnal oscillation. The amplitude of this oscillation is almost independent of the latitude and of the diffusion above the boundary layer. Weak gradient wind blowing in the inland direction increases the amplitude of the oscillation, while stronger gradient wind decreases the amplitude of the oscillation due to a decrease in the intensity of the sea breeze.

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Yizhak Feliks and Michael Ghil

Abstract

The instability of the downwelling front along the southern coast of Asia Minor is studied with a multimode quasigeostrophic model. Linear analysis shows that the most unstable wave has a length of about 100 km, The wavelength depends only very weakly on the transversal scale of the front. The wave period is larger by an order of magnitude than the e-folding time; that is, rapid local growth occurs with little propagation. The growth rate is proportional to the maximum of the speed of the downwelling westward jet.

The evolution of the frontal waves can be divided into three stages. At first, the evolution is mainly due to linear instability; the second stage is characterized by closed eddy formation; and finally, isolated eddies separate from the front and penetrate into the open sea. The largest amount of available potential energy is transferred to kinetic energy and into the barotropic mode during the second, eddy-forming stage, when several dipoles develop in this mode. The formation of anticyclonic eddies is due to advection of the ridges of the unstable wave's first baroclinic mode by the barotropic dipole. The baroclinic eddies ride on the barotropic dipoles. The propagation of such dipole-rider systems is determined mainly by the evolution of the corresponding barotropic dipole.

These results suggest that the warm- and salty-core eddies observed in the Eastern Mediterranean are due, at least in part, to the instability of the downwelling front along the basin's northeastern coastline. There is both qualitative and quantitative similarity between the observed and calculated eddies in their radius (35–50 km), thermal structure, and distribution along the coast.

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Yizhak Feliks, Ehud Gavze, and Reuven Givati

Abstract

In this paper a method of optimal interpolation of the vector field is suggested. It is shown that the vector estimator and its associated error are independent of the choice of the coordinate system in which to represent the wind. The use of the method as a tool for determining the optimal number and sites of observation stations to represent the wind in a given area is demonstrated. A vector correlation coefficient is defined from the regression matrix in a manner analogous to the scalar case. It is independent of the coordinate system and is symmetric in its arguments. The method was applied to a dataset of surface wind measured in a complex terrain in Israel during the summer 1993. Comparison of the wind field interpolated by this method to that interpolated by the commonly used 1/r 2 method shows the suggested method is a significant improvement both in magnitude and direction.

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Yizhak Feliks, Andrew W. Robertson, and Michael Ghil

Abstract

This paper addresses the effect of interannual variability in jet stream orientation on weather systems over the North Atlantic basin (NAB). The observational analysis relies on 65 yr of NCEP–NCAR reanalysis (1948–2012). The total daily kinetic energy of the geostrophic wind (GTKE) is taken as a measure of storm activity over the North Atlantic. The NAB is partitioned into four rectangular regions, and the winter average of GTKE is calculated for each quadrant. The spatial GTKE average over all four quadrants shows striking year-to-year variability and is strongly correlated with the North Atlantic Oscillation (NAO).

The GTKE strength in the northeast quadrant is closely related to the diffluence angle of the jet stream in the northwest quadrant. To gain insight into the relationship between the diffluence angle and its downstream impact, a quasigeostrophic baroclinic model is used. The results show that an initially zonal jet persists at its initial latitude over 30 days or longer, while a tilted jet propagates meridionally according to the Rossby wave group velocity, unless kept stationary by external forcing.

A Gulf Stream–like narrow sea surface temperature (SST) front provides the requisite forcing for an analytical steady-state solution to this problem. This SST front influences the atmospheric jet in the northwest quadrant: it both strengthens the jet and tilts it northward at higher levels, while its effect is opposite at lower levels. Reanalysis data confirm these effects, which are consistent with thermal wind balance. The results suggest that the interannual variability found in the GTKE may be caused by intrinsic variability of the thermal Gulf Stream front.

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Yizhak Feliks, Eli Tziperman, and Brian Farrell

Abstract

The 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.

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Yizhak Feliks, Michael Ghil, and Eric Simonnet

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This study examines the flow induced in a highly idealized atmospheric model by an east–west-oriented oceanic thermal front. The model has a linear marine boundary layer coupled to a quasigeostrophic, equivalent- barotropic free atmosphere. The vertical velocity at the top of the boundary layer drives the flow in the free atmosphere and produces an eastward jet, parallel to the oceanic front's isotherms. A large gyre develops on either side of this jet, cyclonic to the north and anticyclonic to the south of it. As the jet intensifies during spinup from rest, it becomes unstable. The most unstable wave has a length of about 500 km, it evolves into a meander, and eddies detach from the eastern edge of each gyre.

The dependence of the atmospheric dynamics on the strength T∗ of the oceanic front is studied. The Gulf Stream and Kuroshio fronts correspond roughly, in the scaling used here, to T∗ ≅ 7°C. For weak fronts, T∗ ≤ 4°C, the circulation is steady and exhibits two large, antisymmetric gyres separated by a westerly zonal jet. As the front strengthens, 4 < T∗ < 5, the solution undergoes Hopf bifurcation to become periodic in time, with a period of 30 days, and spatially asymmetric. The bifurcation is due to the westerly jet's barotropic instability, which has a symmetric spatial pattern. The addition of this pattern to the antisymmetric mean results in the overall asymmetry of the full solution. The spatial scale and amplitude of the symmetric, internally generated, and antisymmetric, forced mode increase with the strength T∗ of the oceanic front. For T∗ ≥ 5°C, the solution becomes chaotic, but a dominant period still stands out above the broadband noise. This dominant period increases with T∗ overall, but the increase is not monotonic.

The oceanic front's intensity dictates the mean speed of the atmospheric jet. Two energy regimes are obtained. 1) In the low-energy regime, the SST front, and hence the atmospheric jet, are weak; in this regime, small meanders develop along the jet axis, and the dominant period is about 25 days. 2) In the high-energy regime, the SST front and the jet are strong; in it, large meanders and eddies develop along the jet, and the dominant oscillation has a period of about 70 days. The physical nature of the two types of oscillations is discussed, as are possible transitions between them when T∗ changes on very long time scales. The results are placed in the context of previous theories of ocean front effects on atmospheric flows, in which baroclinic phenomena are dominant.

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Yizhak Feliks, Michael Ghil, and Andrew W. Robertson

Abstract

Oscillatory climatic modes over the North Atlantic, Ethiopian Plateau, and eastern Mediterranean were examined in instrumental and proxy records from these regions. Aside from the well-known North Atlantic Oscillation (NAO) index and the Nile River water-level records, the authors study for the first time an instrumental rainfall record from Jerusalem and a tree-ring record from the Golan Heights.

The teleconnections between the regions were studied in terms of synchronization of chaotic oscillators. Standard methods for studying synchronization among such oscillators are modified by combining them with advanced spectral methods, including singular spectrum analysis. The resulting cross-spectral analysis quantifies the strength of the coupling together with the degree of synchronization.

A prominent oscillatory mode with a 7–8-yr period is present in all the climatic indices studied here and is completely synchronized with the North Atlantic Oscillation. An energy analysis of the synchronization raises the possibility that this mode originates in the North Atlantic. Evidence is discussed for this mode being induced by the 7–8-yr oscillation in the position of the Gulf Stream front. A mechanism for the teleconnections between the North Atlantic, Ethiopian Plateau, and eastern Mediterranean is proposed, and implications for interannual-to-decadal climate prediction are discussed.

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