Generation and propagation of internal tides and solitary waves at the shelf edge of the Bay of Biscay

Abstract

High‐frequency mooring data were collected near the northern shelf edge of the Bay of Biscay to investigate the generation and propagation of internal tides and internal solitary waves (ISWs). During spring tide, strong nonlinear internal tides and large amplitude ISWs are observed every semidiurnal tidal period. While onshore propagation was expected since the mooring is located shoreward of the maximum internal tidal generation location, both onshore and seaward traveling internal tides are identified. Within a tidal period at spring tide, three ISW packets are observed. Like internal tides, different ISW packets have opposite (seaward and shoreward) propagating direction. Based on realistic hydrostatic HYCOM simulations, it is suggested that advection by the barotropic tide affects wave generation and propagation significantly and is essential for the seaward traveling internal tides to appear shoreward of their generation location. A two‐layer idealized nonhydrostatic model derived by Gerkema (1996) further confirms the effect of advection on the generation and propagation of internal tides. Moreover, the two‐layer model reproduces one seaward propagating ISW packet and one shoreward propagating ISW packet, indicating that the offshore and onshore traveling ISWs are excited by nonlinear steepening of the seaward and shoreward traveling internal tides, respectively.

Generation of Internal Solitary Waves by Lateral Circulation in a Stratified Estuary

“…5b). Note that, under wind-forced conditions, Xie et al (2017) also observed one-cell lateral circulation in Chesapeake Bay that was driven by lateral Ekman forcing in the surface layer.…”

“…2c,d), suggesting that these waves may have propagated primarily in the cross-channel direction. For an ISW of elevation (depression) in a twolayer fluid, the wave-induced horizontal velocity in the lower (upper) layer is the same as the wave propagation direction with respect to the background flow (Xie et al 2015). The westward bottom velocity anomaly in the lower layer implies that the ISWs propagated westward.…”

“…We use the velocities measured at the depth of 13.8 m where the wave-induced isopycnal displacements were small but the velocity anomalies were strongest. ADV measurements at 11.5 m depth (close to pycnocline) are not used because wave heaving of the background flow sometimes causes large biases in the estimate of u, as illustrated by Mirshak and Kelley (2009) and Xie et al (2015). Figure 3 shows the inferred wave propagation direction for the ISW observed between 26 September and 2 October, during which the ISW trains appeared roughly every 12 h at the M3 mooring site (see Fig.…”

“…They mainly originate from barotropic tidal currents over abrupt topography such as shelf breaks (Holloway 1987;Gerkema and Zimmerman 1995;Xie et al 2015), banks (Scotti et al 2007), and sills (Farmer and Armi 1999;Cummins et al 2003). In a pioneering study, Maxworthy (1979) showed that a large pycnocline depression is generated downstream of a sill when strong tidal flows pass over the sill.…”

Breaking of internal solitary waves generated by an estuarine gravity current

“…Although the data analysis focused on the wind event on 20 October, the ISWs were observed on all down-estuary wind events during the mooring deployment period. Under weak wind conditions, internal waves occurred regularly Geophysical Research Letters 10.1002/2017GL073824 during certain phases of a tidal cycle and were generated by the interaction between the tidally driven lateral circulation and channel-shore bathymetry [Xie et al, 2017b], much like the ISWs generated by flowtopography interactions over a sill or on the continental shelf [Farmer and Armi, 1999;Klymak and Moum, 2003;Xie et al, 2015;Alford et al, 2015].…”