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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

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

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, 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

Abstract

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, 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, Michael Ghil, and Eric Simonnet

Abstract

This study examines the flow induced by an east–west-oriented oceanic thermal front in a highly idealized baroclinic model. Previous work showed that thermal fronts could produce energetic midlatitude jets in an equivalent-barotropic atmosphere and that barotropic instabilities of this jet had dominant periods of 25–30 and 65–75 days.

The present study extends this work to a two-mode baroclinic free atmosphere. The baroclinic jet produced in this case is subject to both barotropic and baroclinic instabilities. A barotropic symmetric instability propagates westward with periods of roughly 30 days and is similar to those found in the equivalent-barotropic model. A baroclinic instability results in standing-dipole anomalies and oscillates with a period of 6–8 months. A mixed barotropic–baroclinic instability results in anomalies that propagate northward, perpendicular to the jet, with a period of 2–3 months. The later anomalies are reminiscent of the 70-day oscillation found over the North Atlantic in observed fields.

The atmospheric flow has two distinct states: the flow in the high-energy state exhibits two large gyres and a strong eastward jet; its antisymmetric component is dominant. The low-energy flow is characterized by small gyres and a weak jet.

The model’s dynamics depends on the layer-depth ratio. When the model is nearly equivalent-barotropic, symmetric oscillatory modes dominate. As the two layers become nearly equal, antisymmetric oscillatory modes become significant and the mean energy of the flow increases.

When the oceanic thermal front’s strength T* is weak (T* ≤ 1.5°C), the flow is steady. For intermediate values of the strength (1.5°C < T* < 3°C), several oscillatory instabilities set in. As the frontal strength increases further (T* ≥ 3°C), the flow becomes more turbulent. These results all depend on the atmospheric model’s horizontal resolution being sufficiently high.

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Yosef Ashkenazy, Yizhak Feliks, Hezi Gildor, and Eli Tziperman

Abstract

The 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.

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