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Experiments on the collision of mode-1 and mode-2 internal-solitary-like waves

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HY2W05 - Physical applications

Magda Carr (Newcastle University), Marek Stastna (University of Waterloo) & Peter. A. Davies (University of Dundee) Internal solitary-like waves (ISWs) propagate along density interfaces and are a common feature in stratified flows. In the ocean, the majority of ISWs observed are of the first baroclinic mode (mode-1). ISWs of mode-1 displace isopycnals in one direction only. Mode-2 ISWs, on the other hand, displace isopycnals in opposite directions and most typically are convex (upper isopcynals are displaced upward and lower isopycnals are displaced downward). Recent observations (Magalhaes & da Silva 2018) suggest that mode-2 ISWs may be more prevalent than previously thought but still the number of observations of them are far fewer than their mode-1 counterparts. Stastna et al., (2015) numerically investigated head-on and overtaking collisions between mode-1 and mode-2 ISWs. They showed that there is strong modal coupling between mode-1 and mode-2 ISWs during collision. The mode-2 ISWs were significantly deformed by the mode-1 wave-induced currents and in most cases lost coherence altogether. Stastna et al., (2015) attributed the destruction of the mode-2 wave to the mode-1 wave-induced shear across the pycnocline centre and hypothesised that this could be the reason why fewer mode-2 waves are observed in the ocean than mode-1. In this study, a laboratory investigation is made of the head-on collision between mode-1 and mode-2 ISWs. Excellent agreement is found with the findings of Stastna et al., (2015) when the mode-2 wave amplitude is relatively small compared to that of the mode-1 wave. However, it is shown that if the mode-2 wave is of large enough amplitude it can retain a mode-2 structure after collision albeit somewhat altered. It is shown that the ratio between the two wave amplitudes is crucial in determining the fate of the mode-2 wave after collision. References Magalhaes, J. M. & da Silva, J. C. B. 2018 Internal solitary waves in the Andaman Sea: New insights from SAR imagery. Remote Sensing. 10, 861; doi:10.3390/rs10060861 Stastna, M., Olsthoorn, J., Bagalaenko, A., & Coutino, A. 2015 Strong mode-mode interactions in internal solitary-like waves. Phys. Fluids. 27, 046604

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