Variation of Froude number versus depth during the passage of internal solitary waves from the in-situ observation and a numerical model

There are few studies on the variation of Froude number versus depth during the passage of internal solitary waves (ISWs). In this paper, according to the limited observational data of the ISWs near the Dongsha Islands in the northern South China Sea (SCS), it is found that, during the passage of an ISW, the local Froude number near the main thermocline gets maximum and the flow is supercritical, with the Froude number decreasing gradually upward and downward, whilst before and after the passage of the ISW, the local Froude number is less than 1 and the flow is subcritical in the entire water column. Since the observational current data are only available in the upper ocean, a two-dimensional continuously stratified nonlinear model is employed here to study the variation of Froude number versus depth during the passage of an ISW. The modeled ISW induced current field agrees well with that observed during the passage of the observational ISW. It is found that, even during the passage of the ISW, although the Froude number is larger than 1 in the upper layer, it is less than 1 in the lower layer, and the duration when the Froude number is larger than 1 also changes with depth. Thus, it is suggested that, once the Froude number near the main thermocline is larger than 1, the passage of ISWs could be identified. Moreover, it would be better to choose a sampling interval of no more than 1/4 passage period of the ISW during an in-situ observation of ISWs.

► The Froude number near the main thermocline gets maximum during the passage of an ISW.
► During the passage of an ISW, the maximum Froude number changes largely with depth.
► The maximum sampling interval for the ISW observation is suggested.