Observations of Diurnal Coastal-Trapped Waves with a Thermocline-Intensified Velocity Field

Abstract

Using 18 days of field observations, we investigate the diurnal (D1) frequency wave dynamics on the Tasmanian eastern continental shelf. At this latitude, the D1 frequency is subinertial and separable from the highly energetic near-inertial motion. We use a linear coastal-trapped wave (CTW) solution with the observed background current, stratification, and shelf bathymetry to determine the modal structure of the first three resonant CTWs. We associate the observed D1 velocity with a superimposed mode-zero and mode-one CTW, with mode one dominating mode zero. Both the observed and mode-one D1 velocity was intensified near the thermocline, with stronger velocities occurring when the thermocline stratification was stronger and/or the thermocline was deeper (up to the shelfbreak depth). The CTW modal structure and amplitude varied with the background stratification and alongshore current, with no spring–neap relationship evident for the observed 18 days. Within the surface and bottom Ekman layers on the shelf, the observed velocity phase changed in the cross-shelf and/or vertical directions, inconsistent with an alongshore propagating CTW. In the near-surface and near-bottom regions, the linear CTW solution also did not match the observed velocity, particularly within the bottom Ekman layer. Boundary layer processes were likely causing this observed inconsistency with linear CTW theory. As linear CTW solutions have an idealized representation of boundary dynamics, they should be cautiously applied on the shelf.

Local winds and encroaching currents drive summertime subsurface blooms over a narrow shelf

“…Strong local winds from passing atmospheric fronts can drive coastal upwelling or downwelling and set off energetic near‐inertial motions (Schlosser et al 2019b). With an average latitude of 42°S, the diurnal tide is sub‐inertial on the 28 km wide shelf, leading to the presence of energetic diurnal coastal‐trapped waves over the outer‐shelf and shelf‐break (Schlosser et al 2019a). There is an energetic internal wave field, even though the incoming semidiurnal internal tide is predominantly reflected offshore by the steep continental slope (Klymak et al 2016; Marques et al 2021).…”

“…Over the majority of this period, clouds were persistent but patchy, Table 1. The position of profilers and moorings deployed in this experiment and their design and sampling (reproduced from Schlosser et al 2019a). Traditional moorings are denoted with "M" and profilers are denoted with "W" and their distance from the shore in kilometers is included in their name.…”

Alongshore Propagation of Subtidal Sea Level Fluctuations Around the Korean Peninsula Over Varying Stratification and Shelf Topography

“…Understanding subtidal (defined as a period from 3 to 20 days) coastal sea level variability is important for effectively adapting to rising global and regional sea levels (Stammer et al, 2013). Subtidal coastal sea level variability is often explained by alongshore propagating coastal-trapped waves (CTWs), which play an important role in coastal ocean circulation and turbulent mixing on continental slopes (Hughes et al, 2019;Schlosser et al, 2019). The CTWs generated by alongshore wind forcing propagate thousands of kilometers away along the coast with changing phase speeds due to local density stratification and shelf bottom topography (Wang and Mooers, 1976;Huthnance, 1978;Brink, 1982;Battisti and Hickey, 1984;Hughes et al, 2019;Schlosser et al, 2019).…”

“…Subtidal coastal sea level variability is often explained by alongshore propagating coastal-trapped waves (CTWs), which play an important role in coastal ocean circulation and turbulent mixing on continental slopes (Hughes et al, 2019;Schlosser et al, 2019). The CTWs generated by alongshore wind forcing propagate thousands of kilometers away along the coast with changing phase speeds due to local density stratification and shelf bottom topography (Wang and Mooers, 1976;Huthnance, 1978;Brink, 1982;Battisti and Hickey, 1984;Hughes et al, 2019;Schlosser et al, 2019). The subtidal sea level fluctuations in many coastal areas related to stratification and shelf topography have been studied using a classical CTW model; for example, phase speeds ranging from 2.72 to 14.10 m s −1 off the South African coast varying with shelf widths of 10-100 km (Schumann and Brink, 1990) and phase speeds ranging from 1.83 to 4.43 m s −1 off the west coast of India varying with shelf widths of 50-80 km (Amol et al, 2012).…”

Internal Gravity Waves Spectrum Generated by a Cylindrical Body Moving in a Stratified Fluid

“…IWs are the factors responsible for the formation of layered structures in marine environments. Schlosser et al (2019) described vertical transport of energy as one of the functions of IWs and mentioned that wind is a major factor responsible for the formation of these waves; according to them IWs can propagate at least hundreds of kilometers from their source. Regarding the usage of IWs' detection, it should be said that, in the design of underwater bodies, they make them invisible to sound waves and to some extent electromagnetic waves, but the changing hydrophysical parameters of the environment caused by moving underwater bodies cannot be reined.…”