Characterization of episodic rip current pulsations in the inner shelf during RCEX 2007
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Abstract : As the Navy thrusts operations into global brown environments, a more complete understanding of the phenomena ships and swimmers will encounter in nearshore regions is necessary. Rip currents remain infamous and important characteristics of the nearshore environment. These events not only impinge upon swimmers' safety, but may play a key role transferring water, containing nutrients, biologics, and even shore-/ship-borne pollution, between the surf zone and open ocean environments. Vertical and temporal behavior of rip currents outside of the surf zone is poorly understood due to a paucity of comprehensive observations. Observations of two upward-looking Acoustic Doppler Current Profilers (ADCP) deployed in 3 m and 5 m water depth within a rip current (nominally 1.0 and 1.2 surf zone widths from the shoreline respectively) were obtained during April-May 2008 as part of the Rip Current EXperiment (RCEX) at Sand City, Monterey Bay, CA. The ADCPs sampled continuously at 1 Hz. Energetic seaward-directed episodic pulses associated with the rip current obtained velocities up to 0.5 m/s with a frequency of occurrence varying from 1-15 times a day depending upon coincident wave and tidal conditions. Vertical variations of the episodic rip current pulsations ranged depth-uniform to surfacedominated. Cross-rotary analysis and complex correlation, performed in the vertical to describe rotational behavior and temporal lags, show rip currents in the inner shelf exhibit more rotation, up to 20 degrees in both CW and CCW directions, than in the surf zone. High coherence is limited to near-surface levels in the inner shelf, versus more depth-uniform values in the surf zone. Mean vertical profiles show these phenomena exhibit significant shear and structure.Keywords:
Rip current
Surf zone
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Internal tide
Stratification (seeds)
Barotropic fluid
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This thesis investigates the origin of observed current velocities at the Gemini offshore wind farm in 2015. Currents have been measured of 1.2 m/s while, based upon tidal and storm predictions, currents of only 0.7 m/s were expected. The aim of this research is to gain insight into the origin of these higher currents, taking into account the physical oceanography at this location. We find that these high currents measured in August are forced by baroclinic currents. Stratification was present at a measurement ship, 32 km northeast from Gemini. In addition, satellite imagery shows Gemini to be located in a region with fronts, between warm water from the Wadden Sea and colder water from the North Sea. In fact, a tidal mixing front is formed on top of the already established saline stratification [Van Aken, 1986]. The origin of fresh water is found to be from sluices discharging onto the Wadden Sea. Prior to the period with high currents a pulse of fresh water is discharged onto the Wadden Sea. Followed by the lowest winds recorded in 2015, an optimal climate is formed in which fronts can travel offshore. This is supported by the satellite imagery. In addition, density-induced currents are calculated based upon the mathematical model of Heaps [1972]. The analysis if the data suggests that density driven currents of 0.4 m/s are generated in August, which combined with the tide can give 1.2 m/s currents.
Stratification (seeds)
Tidal current
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Mode (computer interface)
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Internal tide
Stratification (seeds)
Current meter
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ABSTRACT Exploration for and production of hydrocarbons from deep water of the northern Gulf's continental slope requires specific information on slope-depth physical oceanographic processes for environmental and operational applications. Few long-term records of slope currents exist in the open literature. In an effort to fill this gap in knowledge, a year-long current meter mooring was deployed off the central Louisiana coast in 600 m (1800 ft) of water. The current meter was situated approximately 10 m (30 ft) above the bottom. Tidal currents modulated a mean offshore flow. Some suggestion of topographic steering is present in the record. The principal axes of the raw data and the low-passed data are not coincident. Bursts of strong current activity are associated with significant variability in the associated thermal signal. The periods of strong variability are slightly longer than one week. These periodic current bursts do not appear to be characteristic of any natural oscillation of the Gulf of Mexico. The current activity is probably associated with meso-scale eddies typically found along the shelf break and upper slope. While important for the long-term transport of suspended materials and the alteration of thermal structure at these depths, the observed currents were, typically, of insufficient strength to resuspend sediment. However, direct Loop Current intrusions and associated eddies have the capacity to resuspend and transport sediment. Such currents were not measured during the deployment of our mooring.
Mooring
Current meter
Eddy
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Current meter
Mooring
Clockwise
Nautical mile
Kelvin wave
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Sound propagation in the waters off Eastern Australia is of keen interest to the Royal Australian Navy (RAN) as it affects sonar operation in one of their prime operating regions. Using temperature and salinity fields from an internal tidal model to determine horizontally and vertically dependent sound velocities, effects of internal tides on sound propagation were investigated along four latitude transects across the continental shelf into the deep basin off Eastern Australia. The sound velocity fields and individual acoustic ray paths varied during both the daily and spring-neap tidal cycles, although their basic characteristics were similar. The most noticeable tidal effects were for sound channel formation. More acoustic rays were present in the mid-water column sound channels over both the continental shelf and in the deep basin during spring tide than neap tide and there was a stronger latitude dependence on this number of rays. However, most of the metrics used to evaluate the acoustic rays did not show appreciable differences for either the spring-neap or the daily tidal cycles. There were strong dependencies of these metrics on latitude and source depth. Here, tidal effects on acoustic propagation appear overpowered by effects from other features such as currents and eddies.
Temperature salinity diagrams
Eddy
Internal tide
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Swash
Internal tide
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Prevailing winds
Fjord
Forcing (mathematics)
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