Abstract
AbstractRemotely sensed images document the occurrence of multiple packets of internal solitary waves (ISWs) in the Rhine River plume at the same time. We use a combination of field observations, and non‐hydrostatic and hydrostatic modeling to understand the processes that lead to the generation and retention of multiple ISW packets within the Rhine plume. Previous numerical modeling shows that the tidal plume front is trapped in the mid‐field plume for more than one tidal cycle due to tidal straining and recirculation within the plume, resulting in the presence of multiple fronts in the near‐and mid‐field plume regions. In this work, we show how variations in the strength of these fronts can lead to the release of ISW packets. We conclude that the retention of the fronts in the mid‐field region of the plume and modulation in the strength of the fronts can explain the presence of multiple ISW packets. A frontal Froude number analysis shows that fronts generated during the previous ebb tide can release ISWs in addition to the newly released tidal plume front.
Highlights
Internal solitary waves (ISWs) are nonlinear-dispersive waves that propagate in stratified waters (Ostrovsky & Stepanyants, 1989), which support vertical and horizontal transport of nutrients, phytoplankton, and fine sediment (Bogucki et al, 1997; Da Silva et al, 2002; Klymak & Moum, 2003; Sandstrom & Elliott, 1984)
Multiple internal solitary waves (ISWs) packets are likely to occur in other frictional river plume systems with semidiurnal tides, a large Stokes number, and where tidal advection dominates over the intrinsic frontal speed
Various data sources allowed us to document the presence of multiple ISW packets in the Rhine region of freshwater influence (ROFI), generated by different tidal plume fronts
Summary
Internal solitary waves (ISWs) are nonlinear-dispersive waves that propagate in stratified waters (Ostrovsky & Stepanyants, 1989), which support vertical and horizontal transport of nutrients, phytoplankton, and fine sediment (Bogucki et al, 1997; Da Silva et al, 2002; Klymak & Moum, 2003; Sandstrom & Elliott, 1984). Several different ISW generation mechanisms have been observed; the most common being the nonlinear-dispersive evolution of internal waves generated by the tidal flow of stratified fluid over topography (Farmer & Armi, 1999; Maxworthy, 1979; Pietrzak et al, 1990). Some novel ISW generation mechanisms have been observed, including generation by river plumes (Li & Pawlowicz, 2018; Nash & Moum, 2005; Osadchiev, 2018), intrusion fronts (Bourgault et al, 2016), and bottom gravity currents (Xie et al, 2017). Tidal plume fronts are important in the near-field plume because they interact strongly with the ambient water, influencing vertical mixing and changing local water properties (Garvine & Monk, 1974; Jay et al, 2009; Kilcher & Nash, 2010; Luketina & Imberger, 1987). Hetland (2005) showed the importance of the processes in the near-field plume for the downstream
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