Large-amplitude mode-2 nonlinear internal waves were observed in 250-m-deep water on the Australian North West shelf. Wave amplitudes were derived from temperature measurements using three through-the-water-column moorings spaced 600 m apart in a triangular configuration. The moorings were deployed for 2 months during the transition period between the tropical monsoon and the dry season. The site had a 25–30-m-amplitude mode-1 internal tide that essentially followed the spring–neap tidal cycle. Regular mode-2 nonlinear wave trains with amplitudes exceeding 25 m, with the largest event exceeding 50 m, were also observed at the site. Overturning was observed during several mode-2 events, and the relatively high wave Froude number and steepness (0.15) suggested kinematic (convective) instability was likely to be the driving mechanism. The presence of the mode-2 waves was not correlated with the tidal forcing but rather occurred when the nonlinear steepening length scale was smaller than the distance from the generation region to the observation site. This steepening length scale is inversely proportional to the nonlinear parameter in the Korteweg–de Vries equation, and it varied by at least one order of magnitude under the evolving background thermal stratification over the observation period. Despite the complexity of the internal waves in the region, the nonlinear steepening length was shown to be a reliable indicator for the formation of large-amplitude mode-2 waves and the rarer occurrence of mode-1 large-amplitude waves. A local mode-2 generation mechanism caused by a beam interacting with a pycnocline is demonstrated using a fully nonlinear numerical solution.

Estuarine time scales including the turnover, particle e‐folding time, the age (calculated with a passive tracer), and residence time (calculated with Lagrangian particles) were computed using a three‐dimensional hydrodynamic model of Galveston Bay, a low‐flow, partially stratified estuary. Time scales were computed during a time period when river flow varied by several orders of magnitude and all time scales therefore exhibited significant temporal variability because of the unsteadiness of the system. The spatial distributions of age and residence time were qualitatively similar and increased from 15 days in a shipping channel to >45 days in the upper estuary. Volume‐averaged age and residence time decreased during high‐flow conditions. Bulk time scales, including the freshwater and salinity turnover times, were far more variable due to the changing river discharge and salt flux through the estuary mouth. A criterion for calculating a suitable averaging time is discussed to satisfy a steady state assumption and to estimate a more representative bulk time scale. When scaled with a freshwater advective time, all time scales were approximately equal to the advective time scale during high‐flow conditions and many times higher during low‐flow conditions. The mean age, Lagrangian residence, and flushing times exhibited a relationship that was weakly dependent on the freshwater advective time scale demonstrating predictability even in an unsteady, realistic estuary.

[1] Field observations and numerical circulation modeling revealed the spatial variability of the tidally driven dynamics in the topographically complex, continuously stratified, macrotidal environment of the Browse Basin on the Australian North West Shelf. Internal wave generation occurred at a number of discrete topographic features, and the resultant interaction of multiple waves led to a spatially variable internal wave climate. The generation of baroclinic energy was most intense in regions where the barotropic tide was aligned with steep topography. Generation of low-mode internal waves occurred at the inner-shelf break, where the ratio of tidal excursion distance to topographic length scale was large. In contrast, generation of high-mode beam-like internal waves occurred at the outer-shelf break, where the ratio of tidal excursion distance to topographic length scale was very small. The most efficient conversion from barotropic to baroclinic energy occurred at the outer-shelf break. The internal waves generated at the inner-and outer-shelf breaks interacted to produce a partly standing internal wave, resulting in a large along-shelf energy flux, and discrete locations with small ratios of horizontal kinetic energy to available potential energy. This phenomenon is likely to occur in other regions with semienclosed topography and multiple generation sites.

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