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Yizhak Feliks


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 and Michael Ghil


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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Eli Tziperman, J. R. Toggweiler, Kirk Bryan, and Yizhak Feliks


A 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.

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J. R. Toggweiler, B. Samuels, Eli Tziperman, Yizhak Feliks, Kirk Bryan, and Stephen M. Griffies


The comment by Rahmstorf suggests that a numerical problem in Tziperman et al. (1994, TTFB) leads to a noisy EP field that invalidates TTFB's conclusions. The authors eliminate the noise, caused by the Fourier filtering used in the model, and show that TTFB's conclusions are still valid. Rahmstorf questions whether a critical value in the freshwater forcing separates TTFB's stable and unstable runs. By TTFB's original definition, the unstable runs in both TTFB and in Rahmstorf's comment have most definitely crossed a stability transition point upon switching to mixed boundary conditions. Rahmstorf finally suggests that the instability mechanism active in TTFB is a fast convective mechanism, not the slow advective mechanism proposed in TTFB. The authors show that the timescale of the instability is, in fact, consistent with the advective mechanism.

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