Genesis of the coupling of internal wave modes in the Strait of Messina

Abstract

The internal waves in the south of the Strait of Messina (Italy) are studied using observational data and numerical simulations. The observational data consisted of SAR images, XBT probes, CTD yoyos and thermistor string profiles from the Coastal Ocean Acoustic Changes at High Frequencies (COACH06) cruise. The numerical Lamb model was used to solve the fully nonlinear, nonhydrostatic Boussinesq equations on an f-plane. The model is 2.5 dimensional with spatial variation in a vertical plane extending along depth and in along-stream direction, and neglects derivatives in cross-stream direction. Eleven out of fifteen SAR images contained internal wave events for the month of October 2006. From these images, we estimated some of the internal wave characteristics: a least square fit of the front positions of the southwards propagating internal wave trains gives a propagation speed of 1.00 m s − 1 and a time of release of the bore of about 2 h after maximum northwards tidal flow at Punta Pezzo. Nonhydrostatic dispersion leads to a wavelength increase between the first two internal solitary waves of the trains of 40 m km − 1 . Our in-situ data were used to initialize and evaluate the Lamb model. An EOF analysis was applied to the data and the model outputs. The first three empirical functions contain over 99% of the variability. The data and the model results are in very good qualitative and quantitative agreement, giving a propagation depth of the internal solitary wave train around 90 m with a pycnocline oscillating from 80 to 130 m. Using the first two EOFs, an original method for detecting internal waves was developed by analyzing the scatter diagram of the first EOF projection coefficient versus the second EOF projection coefficient. This distribution has a specific “crescent shape” showing that the first two projection coefficients are not independent in the presence of internal wave events. The “crescent shape” signature of the internal waves can be used as an internal wave detector.