Nathan Paldor

Hydrographic indications of advection/convection effects in the Gulf of Elat

Abstract The convective/advective balance at the northern end of the Gulf of Elat was investigated by comparing observed data to a numerical models predictions. The data, monthly temperature and salinity profiles collected from July 1988 to August 1989, indicate a continuously developing annual cycle, with the water column reaching vertical homogeneity in February and the new thermocline beginning to develop in March. In the summer, an upper 200 m thermally stratified layer (surface temperatures reaching 26°C) overlies a thermally homogeneous layer of 21°C. Salinity is close to 40.5% and varies by less than 0.5% throughout the year, although a salinity minimum develops in the upper layer in late spring and erodes and deepens in the fall and winter as the water column becomes mixed. The strictly one-dimensional convective model successfully reproduces the thermal structure, with a slight lag in summer thermocline development, but is unable to predict the observed salinity minimum. The addition of some advected Red Sea water (40.3%) to the model enables the reproduction of the salinity minimum. This inflow of warmer water also may account for the relatively early thermocline development.

Constraints on effective diffusivity during oxygen isotope exchange at a marble-schist contact, Sifnos (Cyclades), Greece

A smooth sigmoidal δ18O profile is observed at a marble-schist contact within rocks affected by the Miocene greenschist overprint in the Alpine metamorphic complex of the Cyclades on Sifnos. The point of inflection of the profile occurs at the lithological contact, and indicates that advective movement of fluids into the marble was negligible despite previous evidence showing that18O-enriched fluids infiltrated the schist sequence. Least squares model fits of the diffusion equation to the data giveDt products of 0.10 m2 and 0.13 m2 for marbles and schists, respectively (D =effective diffusion coefficient, t =time). To reconcile these figures with quoted free fluid diffusivities one has to propose that either the effective porosities were extremely low and/or the diffusional exchange time was as low as 102–103 years. Low effective porosities in schists are consistent with a model of greenschist evolution in which hydration reactions induced by (18O-enriched) fluids substantially decreased porosity and reduced exchange to fluid diffusional control.

Stability of a potential vorticity front: from quasi-geostrophy to shallow water

The linear stability of a simple two-layer shear flow with an upper-layer potential vorticity front overlying a quiescent lower layer is investigated as a function of Rossby number and layer depths. This flow configuration is a generalization of previously studied flows whose results we reinterpret by considering the possible resonant interaction between waves. We find that instabilities previously referred to as ‘ageostrophic’ are a direct extension of quasi-geostrophic instabilities. Two types of instability are discussed : the classic long-wave quasi-geostrophic baroclinic instability arising from an interaction of two vortical waves, and an ageostrophic short-wave baroclinic instability arising from the interaction of a gravity wave and a vortical wave (vortical waves are defined as those that exist due to the presence of a gradient in potential vorticity, e.g. Rossby waves). Both instabilities are observed in oceanic fronts. The long-wave instability has length scale and growth rate similar to those found in the quasi-geostrophic limit, even when the Rossby number of the flow is O(1). We also demonstrate that in layered shallow-water models, as in continuously stratified quasi-geostrophic models, when a layer intersects the top or bottom boundaries, that layer can sustain vortical waves even though there is no apparent potential vorticity gradient. The potential vorticity gradient needed is provided at the top (or bottom) intersection point, which we interpret as a point that connects a finite layer with a layer of infinitesimal thickness, analogous to a temperature gradient on the boundary in a continuously stratified quasi-geostrophic model.

Stability and stable modes of coastal fronts

Abstract The stability of a single layer, geostrophic front of zero potential vorticity bounded by a vertical coast (wall) is investigated by means of a Rayleigh integral. It is proved that the flow of the density-driven current is stable at all wavenumbers provided the mean velocity of basic flow exceeds fL (where f is the Coriolis parameter and L is the distance between the wall and the free streamline). The frequency of the stable long waves is either zero or super-inertial.

Tropical Plumes over Eastern North Africa as a Source of Rain in the Middle East

Tropical plumes (TPs) reflect tropical–extratropical interaction associated with the transport of moisture from the Tropics to extratropical latitudes. They are observed in satellite images as continuous narrow cloud bands ahead of upper-level subtropical troughs at times when the subtropical jet is highly perturbed. Rainstorms usually develop in the exit regions of TPs, so their presence over northern Africa has an impact on the precipitation regime in the southeastern Mediterranean. Based on satellite images and rainfall measurements from Israel, 10 TPs over eastern North Africa between 1988 and 2005 in which considerable rain was recorded were selected. Using the NCEP–NCAR reanalysis data, the structure and evolution of these TPs were characterized and their regional canonical features were identified. A typical TP that occurred in March 1991 is described in detail. The main canonical characteristics are as follows: the TP development is preceded by an incubation period, expressed either as a stationary upper-level trough, persisting 2–6 days, or as two consecutive TP pulses; the preferred location for TP origin is 5°–15°N, 5°W–15°E; the TP is separated from the underlying dry Saharan PBL; the subtropical trough undergoes a phase locking with the lower tropical trough; the cloudiness in the TP-induced rainstorm is mostly stratified with continuous moderate rain, originating from midlevel moisture; and the TP tends to be followed by a midlatitude cyclogenesis over the eastern Mediterranean. This analysis proposes several explanations for the efficiency of the TPs in transporting moisture over a 2000-km distance.

Inertial Trajectories on a Rotating Earth

Abstract The trajectories of inertial flows on a rotating earth are calculated, in an attempt to reconcile the differing heuristic suggestions in the literature on the subject. It is shown that westward propagating “nearly closed” orbits are possible away from the equator. For orbits crossing the equator, we find a stationary, “figure-eight- like” orbit, together with eastward and westward propagating modes. Near the pole, the convergence of longitudes causes the trajectories to be deflected cyclonically in contrast to the deflection of the Coriolis force, giving rise to a westward propagating mode that meanders about a central latitude.

Instabilities of a two-layer coupled front

We consider the linear instability of a two-layer fluid, whose mean state consists of a motionless lower layer and a surface layer confined between two parallel fronts. An inverted form occurs at many locations in the deep ocean, notably in the Denmark Strait overflow. Because of the vanishing surface layer depth, quasigeostrophy cannot hold, and primitive equations must be used. Two modes of long wave instability are found. The first, valid for intermediate values of the ratio of total fluid depth to surface layer depth, is analogous to a mode found for an isolated front in an otherwise similar geometry. The second mode is the extension to two layers of the mode already discovered for the same geometry but with an infinitely deep lower layer. Numerical extensions of these long wave results to shorter waves show that the former mode would be observed in practice. The theory is applied to laboratory results, and is in excellent agreement with observations.

A Consistent Theory for Linear Waves of the Shallow-Water Equations on a Rotating Plane in Midlatitudes

The present study provides a consistent and unified theory for the three types of linear waves of the shallow-water equations (SWE) in a zonal channel on the plane: Kelvin, inertia–gravity (Poincare ´ ), and planetary (Rossby). The new theory is formulated from the linearized SWE as an eigenvalue problem that is a variant of the classical Schrodinger equation. The results of the new theory show that Kelvin waves exist on the plane with vanishing meridional velocity, as is the case on the f plane, without any change in the dispersion relation, while the meridional structure of their height amplitude is trivially modified from exponential on the f plane to a one-sided Gaussian on the plane. Similarly, inertia–gravity waves are only slightly modified in the new theory in comparison with their characteristics on the f plane. For planetary waves (which exist only on the plane) the new theory yields a similar dispersion relation to the classical theory only for large gravity wave phase speed, such as those encountered in a barotropic ocean or an equivalent barotropic atmosphere. In contrast, for low gravity wave phase speed, for example, those in an equivalent barotropic ocean where the relative density jump at the interface is 10 3 , the phase speed of planetary waves in the new theory is 2 times those of the classical theory. The ratio between the phase speeds in the two theories increases with channel width. This faster phase propagation is consistent with recent observation of the westward propagation of crests and troughs of sea surface height made by the altimeter aboard the Ocean Topography Experiment (TOPEX)/Poseidon satellite. The new theory also admits inertial waves, that is, waves that oscillate at the local inertial frequency, as a genuine solution of the eigenvalue problem.

Shortwave instabilities of coastal currents

Abstract The finite-wavelength instability of a two-layer, inviscid coastal current is investigated numerically over a wide range of parameters, and verified analytically in a simple limiting case. We show that the instability exponents increase linearly with wavenumber and that they increase with the upper layers mean speed. A comparison with Kelvin-Helmholtz instabilities shows both similarities and differences. For large total ocean depth, our theory predicts the existence of very vigorous instabilities whenever the slanting interface between layers extends close to the bottom on the ocean. The energy source of these instabilities is mixed barotropic-baroclinic. The theory presented here ceases to be valid at large wavenumbers, where viscous effects have to be included in the governing equations. An ad hoc extension of the inviscid theory agrees with the spatial and temporal scales of observed, finite-amplitude features in the Algerian Current, for a reasonable value of the eddy-viscosity coefficient....

Finite-Wavelength Instabilities of a Coupled Density Front

Abstract Finite-wavelength instabilities of a coupled density front with zero potential vorticity are found for the single-layer and the two-layer problems. These instabilities result from the resonance between two distinct waves whose real phase speeds coalesce. In the single-layer problem, the range of wavenumbers over which the coalescence takes place decreases with increasing wavenumber; consequently, the instability exponents and the growth rates also decrease. For shallow lower layers, the coalescence range increases with increasing wavenumber; at large wavenumbers, the coalescence range becomes continuous, while the instability exponent is approaching a constant value. The growth rate in the two-layer problem increases, therefore, linearly with wavenumber and the short waves fastest. These short-wave instabilities are qualitatively reminiscent of small-scale features along coastal fronts and in laboratory experiments.