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A classical non-linear quasi-geostrophic model of wind-driven ocean circulation is integrated by using new additional boundary conditions conceived to: a) preserve, as far as possible, the typical structure of the Sverdrup flow in the southern and eastern regions of an idealised sub-tropical gyre and b) eliminate any additional boundary conditions in the northern and western regions of the fluid domain. Numerical simulations produce current and transport values close to those found in the literature whereas instability occurs at relatively low values of non-linearity, consistently with the well-known Sverdrup-like behaviour of large-scale transport in the world-wide ocean. A strict correlation emerges between the stability and the persistence of the Sverdrup-like behaviour of the so obtained solutions. [ABSTRACT FROM AUTHOR]

In the framework of stable free modes, a criterion is derived for the localisation in latitude of the intensified sector of the mode, i.e. northwards, southwards, or equally distributed in the north and south. The criterion is based on the sign taken by the inverse of the total vorticity at the middle latitude of the reference basin, and resorts to the necessarily westward orientation of an unforced current according to one of Greenspan's theorems. Some solutions, also of non-linear modes, elucidate the result. [ABSTRACT FROM AUTHOR]

Ierley's regional model of a single-gyre wind-driven circulation is an interesting compromise between the need to account for flow nonlinearity and its Sverdrupian behaviour far from the western area of the basin. The model assumes a priori that current intensification is westwards, but it can be easily reformulated, as the authors do, by setting the intensification region at the east. However, in this case the energetics of the flow exhibits an inconsistency that shows that westward intensification has no alternative. This is in accordance with all the previous conclusions on the subject but, for the first time, with the novelty of having taken into account also the nonlinear character of the flow outside the Sverdrupian area. [ABSTRACT FROM AUTHOR]

A new reading of westward intensification is put forward by decomposing the transport stream function in the sum of a couple of symmetric and anti-symmetric terms, relatively to the mirror reflection with respect to the central longitude of a reference basin, rectangular in shape and included into a certain beta plane. The consequence of the decomposition is twofold: first, the anti-symmetric term turns out to be a strict consequence of the latitudinal dependence of Coriolis acceleration, in the sense that it disappears in a hypothetical f-plane; second, whatever the wind forcing may be, the symmetric and anti-symmetric components of the meridional transport are always parallel in the western region of the basin and anti parallel in the eastern one. As a consequence of their superposition, the meridional transport is more energetic into the western region and it gives rise to the phenomenon of westward intensification. On the other hand, the components of the meridional transport tend to compensate each other in the eastern region in accordance with the Sverdrup balance. [ABSTRACT FROM AUTHOR]

In this work, we use a set of recent multiyear simulations to develop a simplified sea surface height index (SSH). The index characterizes the dynamics of Ionian upper layer circulation and its links with sea surface height and salinity in the Southern Adriatic and Aegean Seas during the period 1987-2008. The analysis highlights a covariant behavior between Ionian Sea and Aegean Sea associated with a mutual zonal exchange of water masses with different salinity characteristics. Our analysis confirms that the variability observed in the period 1987-2008 in the upper layer circulation of the Ionian was driven by the salinity variability in the Southern Adriatic and Aegean Sea. This study supports and reinforces the hypothesis that two observed BiOS-like reversals reflect the existence of multiple equilibrium states in the Mediterranean Thermohaline circulation in the Eastern Mediterranean and that a complete characterization of observed variability needs to take into account a fully coupled Adriatic-Ionian-Aegean System. [ABSTRACT FROM AUTHOR]

The observed pseudoperiodic reversal of the upper-layer circulation of the Ionian Sea has been assumed to be related to some internal feedback processes (density driven) by the so-called BiOS (Adriatic-Ionian Bimodal Oscillating System) hypothesis. The idea behind this paper is that the Ionian circulation is driven by what happens in the boundary of the region, and so a very simple dynamical model has been derived by integrating in the bounded flow domain (the Ionian Basin) on the f-plane of the twolayer quasi-geostrophic evolution equations. Furthermore, a nonlinear term which mimics the mutual interaction between the moving layers has been introduced. By considering a stochastic Gaussian noise acting as a forcing and a decadal periodic input term having an amplitude much smaller than the noise variance, a stochastic resonance appears in the solution for a certain range of the values of the parameters. The decadal forcing at the boundary of the Ionian basin can be related to the lowfrequency atmospheric variability in the surrounding basins. The resonance has a period consistent with the observed decadal time scale of the reversal between cyclonic and anti-cyclonic phases. [ABSTRACT FROM AUTHOR]

Subaqueous, asymmetric sand waves are typically observed in marine channel/canyon systems, tidal environments, and continental slopes exposed to strong currents, where they are formed by current shear resulting from a dominant unidirectional flow. However, sand-wave fields may be readily observed in marine environments where no such current exists; the physical processes driving their formation are enigmatic or not well understood. We propose that internal solitary waves (ISWs) induced by tides can produce an effective, unidirectional boundary 'current' that forms asymmetric sand waves. We test this idea by examining a sand-wave field off the Messina Strait, where we hypothesize that ISWs formed at the interface between intermediate and surface waters are refracted by topography. Hence, we argue that the deflected pattern (i.e., the depth-dependent orientation) of the sand-wave field is due to refraction of such ISWs. Combining field observations and numerical modelling, we show that ISWs can account for three key features: ISWs produce fluid velocities capable of mobilizing bottom sediments; the predicted refraction pattern resulting from the interaction of ISWs with bottom topography matches the observed deflection of the sand waves; and predicted migration rates of sand waves match empirical estimates. This work shows how ISWs may contribute to sculpting the structure of continental margins and it represents a promising link between the geological and oceanographic communities. [ABSTRACT FROM AUTHOR]

Two years of sea level data obtained at Trieste, Italy, and Paranaguá, Brazil, were used to compare the performances of two tidal analysis methodologies, one in the time domain (HMB) and other in the frequency domain (HMF). For each station, the first year was analyzed to estimate the tidal constituents while the second year was used to compare observations against forecasted sea levels. With respect to tidal dynamics, results from both methods showed that the semi-diurnal atmospheric tide S2p is disturbing the sea constituent S2 as well as other frequencies, particularly in the diurnal species in the subtropical case of Paranaguá. These results are in agreement with theoretical development and numerical simulations available in the literature. On the methodological side, both methods showed equivalent performances although HMF is more user-friendly and offers better and more comprehensive results. The main reason seems to be linked to the deepen exploitation of astronomical and physical aspects of the tidal constituents generation and discrimination, considering that HMF can estimate more than 170 while HMB estimates around 110. Also, HMF showed better results for shallow water and long term components. However, the residuals showed that a significant amount of oscillating energy is left behind by both methods, suggesting that other deterministic signals not present in the astronomic tidal frequencies of sea water have to be considered. It is concluded that a better stochastic model for tidal analysis and forecast needs to be formulated in order to better represent the physics of sea level: while tidal forecast with the usual methods seems to work well in many practical cases, the high dependence of numerical models on initial and boundary conditions suggests that sea level harmonic constituents estimation has to be improved. [ABSTRACT FROM AUTHOR]