Sutara H. Suanda

A self‐similar scaling for cross‐shelf exchange driven by transient rip currents

Transient rip currents, episodic offshore flows from the surf zone to the inner shelf, present a recreational beach hazard and exchange material across the nearshore ocean. The magnitude and offshore extent of transient rip-current-induced exchange and its relative importance to other inner shelf exchange processes are poorly understood. Here 120 model simulations with random, normally incident, directionally spread waves spanning a range of beach slopes and wave conditions show that the transient rip current exchange velocity is self-similar. The nondimensional exchange velocity, surf zone flushing time, and cross-shore decay length scale are scaled by beach slope and wave properties, depending strongly on wave directional spread. Transient rip-current-driven exchange can be compared to other cross-shelf exchange processes. For example, transient rip-current-driven exchange is stronger than wave-induced Stokes-drift-driven exchange up to six surf zone widths from shore.

Mid- to Inner-Shelf Coupled ROMS–SWAN Model–Data Comparison of Currents and Temperature: Diurnal and Semidiurnal Variability

AbstractAccurately representing diurnal and semidiurnal internal variability is necessary to investigate inner-shelf to midshelf exchange processes. Here, a coupled Regional Ocean Model System (ROMS)–Simulating Waves Nearshore (SWAN) model is compared to observed diurnal and semidiurnal internal tidal variability on the mid and inner shelf (26–8 m water depth) near San Pedro Bay, California. Modeled mean stratification is about one-half of that observed. Modeled and observed baroclinic velocity rotary spectra are similar in the diurnal and semidiurnal band. Modeled and observed temperature spectra have similar diurnal and semidiurnal band structure, although the modeled is weaker. The observed and modeled diurnal and semidiurnal baroclinic velocity- and temperature-dominant vertical structures are similar and consistent with mode-one internal motions. Both observed and modeled diurnal baroclinic kinetic energy are strongly correlated to diurnal wind forcing and enhanced by subtidal vorticity-induced reduc...

Semidiurnal Baroclinic Tides on the Central Oregon Inner Shelf

AbstractSemidiurnal velocity and density oscillations are examined over the mid- and inner continental shelf near Heceta Bank on the Oregon coast. Measurements from two long-term observation networks with sites on and off the submarine bank reveal that both baroclinic velocities and displacements are dominated by the first mode, with larger velocities on the midshelf and northern parts of the bank. Midshelf sites have current ellipses that are near the theoretical value for single, progressive internal tidal waves compared to more linearly polarized currents over the inner shelf. Baroclinic current variability is not correlated to the spring–neap cycle and is uncorrelated between mooring locations. An idealized model of two internal waves propagating from different directions reproduces some of the observed variability in semidiurnal ellipse parameters. At times, the phasing between moorings along a cross-shelf transect are consistent with onshelf wave propagation, a characteristic also present in the out...

Shore-Based Video Observations of Nonlinear Internal Waves across the Inner Shelf *,+

AbstractShore-based video remote sensing is used to observe and continually monitor nonlinear internal waves propagating across the inner shelf. Month-long measurements of velocity from bottom-mounted acoustic Doppler current profilers and temperature from thermistor chains at the 10- and 20-m isobaths are combined with sea surface imagery from a suite of cameras (Argus) to provide a kinematic description of 11 borelike internal waves as they propagate across the central Oregon inner shelf. The surface expression of these waves, commonly seen by eye as alternating rough and smooth bands, are identified by increased pixel intensity in Argus imagery (average width 39 ± 6 m), caused by the convergence of internal wave-driven surface currents. These features are tracked through time and space using 2-min time exposure images and then compared to wave propagation speed and direction from in situ measurements. Internal waves are refracted by bathymetry, and the measured wave speed (~0.15 m s−1) is higher than p...

Wind relaxation and a coastal buoyant plume north of Pt. Conception, CA: Observations, simulations, and scalings

In upwelling regions, wind relaxations lead to poleward propagating warm water plumes that are important to coastal ecosystems. The coastal ocean response to wind relaxation around Pt. Conception, CA is simulated with a Regional Ocean Model (ROMS) forced by realistic surface and lateral boundary conditions including tidal processes. The model reproduces well the statistics of observed subtidal water column temperature and velocity at both outer and inner-shelf mooring locations throughout the study. A poleward-propagating plume of Southern California Bight water that increases shelf water temperatures by ≈ 5°C is also reproduced. Modeled plume propagation speed, spatial scales, and flow structure are consistent with a theoretical scaling for coastal buoyant plumes with both surface-trapped and slope-controlled dynamics. Plume momentum balances are distinct between the offshore (>30 m depth) region where the plume is surface-trapped, and onshore of the 30 m isobath (within 5 km from shore) where the plume water mass extends to the bottom and is slope controlled. In the onshore region, bottom stress is important in the alongshore momentum equation and generates vertical vorticity that is an order of magnitude larger than the vorticity in the plume core. Numerical experiments without tidal forcing show that modeled surface temperatures are biased 0.5°C high, potentially affecting plume propagation distance and persistence.

Statistics of Internal Tide Bores and Internal Solitary Waves Observed on the Inner Continental Shelf off Point Sal, California

AbstractMoored observations of temperature and current were collected on the inner continental shelf off Point Sal, California, between 9 June and 8 August 2015. The measurements consist of 10 moorings in total: 4 moorings each on the 50- and 30-m isobaths covering a 10-km along-shelf distance and an across-shelf section of moorings on the 50-, 40-, 30-, and 20-m isobaths covering a 5-km distance. Energetic, highly variable, and strongly dissipating transient wave events termed internal tide bores and internal solitary waves (ISWs) dominate the records. Simple models of the bore and ISW space–time behavior are implemented as a temperature match filter to detect events and estimate wave packet parameters as a function of time and mooring position. Wave-derived quantities include 1) group speed and direction; 2) time of arrival, time duration, vertical displacement amplitude, and waves per day; and 3) energy density, energy flux, and propagation loss. In total, over 1000 bore events and over 9000 ISW events...

The Effect of Barotropic and Baroclinic Tides on Coastal Stratification and Mixing: Continental shelf tidal effects

The effects of barotropic and baroclinic tides on subtidal stratification and vertical mixing are examined with high-resolution, three-dimensional numerical simulations of the Central Californian coastal upwelling region. A base simulation with realistic atmospheric and regional-scale boundary forcing but no tides (NT) is compared to two simulations with the addition of predominantly barotropic local tides (LT) and with combined barotropic and remotely generated, baroclinic tides (WT) with 100 W m21 onshore baroclinic energy flux. During a 10 day period of coastal upwelling when the domain volume-averaged temperature is similar in all three simulations, LT has little difference in subtidal temperature and stratification compared to NT. In contrast, the addition of remote baroclinic tides (WT) reduces the subtidal continental shelf stratification up to 50% relative to NT. Idealized simulations to isolate barotropic and baroclinic effects demonstrate that within a parameter space of typical U.S. West Coast continental shelf slopes, barotropic tidal currents, incident energy flux, and subtidal stratification, the dissipating baroclinic tide destroys stratification an order of magnitude faster than barotropic tides. In WT, the modeled vertical temperature diffusivity at the top (base) of the bottom (surface) boundary layer is increased up to 20 times relative to NT. Therefore, the width of the inner-shelf (region of surface and bottom boundary layer overlap) is increased approximately 4 times relative to NT. The change in stratification due to dissipating baroclinic tides is comparable to the magnitude of the observed seasonal cycle of stratification.