Modulation of Tidal Currents on Shoaling Internal Solitary Waves

Xiaolin Bai1, Kevin G Lamb2, Jianyu Hu3 and Zhiyu Liu3, (1)University of Waterloo, Waterloo, ON, Canada, (2)Univ Waterloo, Waterloo, ON, Canada, (3)Xiamen University, State Key Laboratory of Marine Environmental Science, Xiamen, China
Internal solitary waves (ISWs) of depression are usually transformed into waves of elevation as they propagate from deep water onto shallow shelves. This is a well-known process called as polarity conversion, which plays an important role in the evolution of ISWs, including dramatic changes in waveform, wave amplitude and vertical structure of wave induced current. After polarity conversion, ISWs of elevation lift nutrient-enriched and high-chlorophyll subsurface water into the euphotic zone where phytoplankton can be exposed to high light levels. Many physical processes, as long as being able to modulate stratification or sheared currents, can affect the polarity conversion of shoaling ISWs, such as mesoscale eddies, internal tides, and seasonal-varying stratification. By contrast, the effects of tidal currents, especially barotropic tidal currents, on shoaling ISWs have attracted much less attention, although they were shown to modulate the propagation speed and wavelength of ISWs. Additionally, our knowledge of spatial-temporal variations of the location of ISW polarity conversion remains limited, especially in the time scale of ISW shoaling given by O(1) day.

In this study, we will focus on the modulation of tidal currents on the spatial-temporal variation of ISW polarity conversion. Our results show that density stratification is significantly modulated by the combined effects of cross-shelf barotropic tidal currents and the flow due to locally generated internal tides, leading to phase-locked tidal variations of the critical point. The barotropic tidal currents push isopycnal up and down as they flow over the slope. As this occurs, internal tides are generated, further modulating the stratification and generating sheared currents. Polarity conversion occurs further onshore (offshore) of the nominal critical point during flood (ebb) tides. These processes are elucidated with idealized numerical simulations of shoaling ISWs in the northeastern South China Sea. The results suggest that the cross-shelf location of the critical point can vary by O(10) km within a tidal cycle. These have important implications for the evolution and dissipation of shoaling ISWs. This study provides an example in the SCS, where the ISWs are among the most energetic in the world ocean. The effects of tidal currents we see in our simulations should have quite generic implications.