Topographic Rossby modes in the Strait of Sicily

Abstract

In this paper, numerical results of a barotropic circulation model are presented, indicating that topographic Rossby modes can be excited in the area of the Strait of Sicily. The results also suggest that such motions are robust features, since they can be resonantly generated by different forcings. A numerical barotropic shallow‐water model is implemented in the central Mediterranean Sea, and, in order to perform a spectroscopic analysis, the initial value problem is solved, both in the presence of a white wind with spatially constant curl and of a white boundary flow representing an equivalent remote pressure forcing. A spectral analysis in selected points in the region of the strait reveals five peaks with periods ranging from 2 to 5 days. The associated circulation patterns are then studied by forcing the system with monochromatic winds or boundary flows at the periods of the energy peaks. Traveling patterns are recognized, whose spatial/temporal structures and periods are found to compare well with those of analytical Rossby modes, thus obtaining a clear evidence of their dynamical nature. In order to investigate the actual excitation of Rossby modes in realistic conditions, the system is then forced by the 1980 National Meteorological Center momentum flux data. The main mode at nearly 3 days obtained from the preceding spectral analysis is indeed found to be excited in the narrowest section of the strait during winter and autumn months. Rossby modes in the numerical time series are identified by means of the analysis of the energy spectra and of the coherence and phase of current signals between two points along the theoretical propagation direction. Finally, the existence of topographic Rossby modes in the Strait of Sicily is discussed in connection with the local properties of horizontal mixing in that area.