Abstract. On the basis of observations of the time-dependent, tidally forced flow over a long sill we find that aspiration and channel curvature set the flow structure and condition the flow to allow intense vertical mixing. Aspiration reduces the potential energy of the water column by thinning it while maintaining its density contrast. Channel curvature induces a cross-channel circulation that can rapidly overturn a stratified flow. Eighteen Mong-channel sections of density, velocity, and dissipation rate of turbulent kinetic energy e were collected in and around the Tacoma Narrows of Puget Sound, a site suspected of driving a strong vertical circulation in adjoining Main Basin. Rapid inflow to the Narrows on flood from one channel of a triple junction reduces dynamic pressure, allowing dense water from below sill depth to be uplifted, or aspirated, into the Narrows. Basin, enhancing the impact of mixing at the Narrows by discharging the mixed product far from the source. Scaling of the cross-channel momentum equation suggests that, below a threshold value of along-channel velocity, stratification should suppress secondary circulation for a given vertical shear, radius of curvature and channel width. Above the threshold velocity the magnitude of the cross-channel velocity is roughly consistent with predictions for unstratified flow. We estimate the maximum effective eddy diffusivity that aspiration and mixing in the Narrows can produce in Main Basin to be 10 -3 m 2 S --1.

Pelagic larval dispersal is thought to be the main mechanism connecting many marine populations and is an important determinant both of an individual's success and a population's distribution and spatial structure. Thus, quantitative estimates of the retention or dispersion of larvae from spawning grounds is important for the determination of recruitment success in fisheries. Models can be used to study connectivity through a dispersal curve or dispersal kernel: the probability that a larva will settle at a given distance from its release location. We applied a 3-dimensional circulation model and a Lagrangian particle tracking model to the southeast US continental shelf to describe dispersal kernels in 2 dimensions. We used a fully orthogonal design to assess the importance of factors that influence the dispersal kernel, including spawning time, spawning location, larval behavior (vertical position in the water column), larval duration, and turbulent dispersal. Our results indicate that adult behavior, in the form of spawning time and location, may be more important than larval behavior in determining larval dispersal on the inner-and mid-shelves in this region.KEY WORDS: Dispersal kernels · Larval fish · Lagrangian particle tracking · Southeast US continental shelf Resale or republication not permitted without written consent of the publisher

There has been wide interest in Marine Heatwaves and their ecological consequences in recent years. Most analyses have focused on remotely sensed sea surface temperature data due to the temporal and spatial coverage it provides in order to establish the presence and duration of Heatwaves. Using hydrographic data from a variety of sources, we show that an advective Marine Heatwave was initiated by an event in late December of 2016 south of New England, with temperature anomalies measuring up to 6 • C and salinity anomalies exceeding 1 PSU. Similar features were observed off of New Jersey in February 2017, and are associated with the Shelfbreak Front migrating from its normal position to mid-shelf or further onshore. Shelf water of 34 PSU was observed just north of Cape Hatteras at the 30 m isobath and across the continental shelf in late April 2017. These observations reveal that the 2017 Marine Heatwave was associated with a strong positive salinity anomaly, that its total duration was approximately 4 months, and its advective path extended roughly 850 km along the length of the continental shelf in the Middle Atlantic Bight. The southward advective velocity implied by the arrival north of Cape Hatteras is consistent with previous estimates of alongshelf velocity for the region. The origin of this Marine Heatwave is likely related to cross-shelf advection driven by the presence of a Warm Core Ring adjacent to the shelfbreak south of New England.

Abstract. Simultaneous profiles of microstructure, horizontal velocity, and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date. Shear increased rapidly after passing through a lateral constriction which formed a hydraulic control. A kilometer-long set of 20-m-tall billows grew on a middepth density interface where the Richardson number fell below 0.25. The velocity interface thickened steadily after the billows formed, consistent with rapid momentum -mixing across a shear layer with a Reynolds number of 3 x 106. The billows generated large W_ "density overturns and dissipation rates greater than I0-' W kg-1 , even within the first large Woverturn, indicating that these structures were fully turbulent early in their development.1 ,M As the billows grew, a well-mixed layer developed at the interface and survived as an W actively turbulent layer for up to 6 buoyancy periods, 3 times longer than in laboratory 1 5 50 studies at low Reynolds number. Variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where ____ the billows first formed. With data from the large overturns within the shear layer, we find e/IN 2 --3 x 10', an average root-mean-square overturn scale (L--' of 2.6 m, and a I%* buoyancy scale (Lb) of 2.7 m. Despite having sampled the billows at varying stages of their evolution, we find no indication that the ratio Lrm/Lb is ever significantly different than 1 for this shear instability.1. Introduction from changes in conditions where the billows first form. The mean density of individual billows varies within While steaming with the current along a deep tidal the set; we propose that this results from changes in channel, we had the good fortune to sample a kilometer-vertical offset of the velocity and density interfaces. The long set of large billows (Plate 1). As the billows faded longitudinal density structure we observe has important on the echo sounder, we turned the ship around to run implications for the final state of the patch of mixed upstream back through them. Because of the increasing fluid produced by the instability. The dissipation rate tidal currents and problems retrieving our microstruc-within the billows varies with billow density, suggesting ture profiler, we fell back and then repositioned several that the interface offset significantly affects the intensity times. But the current swept the billows downstream by of turbulence. us, allowing us to collect a second set of measurements.Section 2 describes our instrumentation and then the This paper is a detailed presentation of our fortuitous setting and spatial organization of the sampling. We encounter. Our simultaneous measurements of micro-give an overview of the data in section 3, including a structure, acoustic backscatter, and horizontal currents simple model of the event that is consistent with the provide a unique realization of energetic, naturally oc-observations. Section 4 a...

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