Matthew S. Spydell

Observing Surf-Zone Dispersion with Drifters

Abstract Surf-zone dispersion is studied using drifter observations collected within about 200 m of the shoreline (at depths of less than about 5 m) on a beach with approximately alongshore uniform bathymetry and waves. There were about 70 individual drifter releases, each 10–20 min in duration, on two consecutive days. On the first day, the sea-swell significant wave height Hs was equal to 0.5 m and mean alongshore currents |υ| were moderate (<0.1 m s−1). On the second day, the obliquely incident waves were larger, with Hs equal to 1.4 m, and at some surf-zone locations |υ| was greater than 0.5 m s−1. The one-particle diffusivity was larger, with larger waves and stronger currents. On both days, the one-particle diffusivity tensor is nonisotropic and time-dependent. The major axis is initially parallel to the cross-shore direction, but after a few wave periods it is aligned with the alongshore direction. In both the along- and cross-shore directions, the asymptotic diffusivity is reached sooner within, r...

Lagrangian Drifter Dispersion in the Surf Zone: Directionally Spread, Normally Incident Waves

Abstract Lagrangian drifter statistics in a surf zone wave and circulation model are examined and compared to single- and two-particle dispersion statistics observed on an alongshore uniform natural beach with small, normally incident, directionally spread waves. Drifter trajectories are modeled with a time-dependent Boussinesq wave model that resolves individual waves and parameterizes wave breaking. The model reproduces the cross-shore variation in wave statistics observed at three cross-shore locations. In addition, observed and modeled Eulerian binned (means and standard deviations) drifter velocities agree. Modeled surf zone Lagrangian statistics are similar to those observed. The single-particle (absolute) dispersion statistics are well predicted, including nondimensionalized displacement probability density functions (PDFs) and the growth of displacement variance with time. The modeled relative dispersion and scale-dependent diffusivity is consistent with the observed and indicates the presence of ...

Observations of drifter dispersion in the surfzone: The effect of sheared alongshore currents

reaching a local maxima of approximately 1.5 m 2 s � 1 between 160 and 310 s, before decreasing to about 1 m 2 s � 1 at 1000 s. The alongshore diffusivity kyy increased monotonically to 1000 s and was variable between the 5 days. For times greater than 30 s, the alongshore diffusivity is greater than the cross-shore diffusivity, consistent with previous observations. The observed diffusivities are fit to analytic functional forms, from which asymptotic diffusivities and Lagrangian timescales are determined. The asymptotic alongshore diffusivity ^yy varies between 4 and 19 m 2 s � 1 , and this variation is ½�

Observed and modeled drifters at a tidal inlet

Material transport and dispersion near the mouth of a tidal inlet (New River Inlet, NC) are investigated using GPS-tracked drifters and numerical models. For ebb tide releases, velocities are largest (>1 m s−1) in two approximately 30 m wide channels that bisect the 1–3 m deep ebb shoal. In the channels, drifter and subsurface current meter velocities are similar, consistent with strong vertical mixing and 2-D hydrodynamics. Drifters were preferentially entrained in the channelized jets where drifter cluster lateral spreading rates μin were small ( μin≈0.5 m2 s−1). At the seaward edge of the ebb shoal, jet velocities decrease linearly with distance (to ≤0.2 m s−1, about 1 km from shore), and cluster spreading rates are larger with μout≈3 m2 s−1. Although the models COAWST and NearCom generally reproduce the observed trajectory directions, certain observed drifter properties are poorly modeled. For example, modeled mean drifter velocities are smaller than observed, and upon exiting the inlet, observed drifters turn north more than modeled drifters. The model simulations do reproduce qualitatively the spreading rates observed in the inner inlet, the flow deceleration, and the increase in μout observed in the outer inlet. However, model spreading rates increase only to μout<1 m2 s−1. Smaller modeled than observed μout may result from using unstratified models. Noncoincident (in space) observations show evidence of a buoyant plume ( Δρ=1 kg m−3) in the outer inlet, likely affecting drifter lateral spreading. Generally, drifter-based model performance is good within the inlet channels where tidal currents are strongest, whereas model-data differences are significant farther offshore.

Baroclinic Modes in a Two-Layer Basin

Abstract The objective of this study is to investigate the time-dependent circulation in a closed basin where the steady circulation is included and long Rossby wave speeds are consistent with observations. Specifically, the large-scale baroclinic eigenmodes of a two-layer rectangular basin forced by surface wind stress in the limit of small dissipation are examined. Low-frequency modes with small decay rates independent of friction result when the constraint of mass conservation is enforced. The magnitude of the wind stress is found to be critical to the eigenspectrum. For all forcing magnitudes, including forcings with closed geostrophic contours, oscillatory modes with decay rates independent of friction emerge. For forcings with closed geostrophic contours, two important classes of eigenmodes with comparable decay rates emerge: purely decaying modes confined to the region of closed contours, and basin-scale oscillatory modes. The purely decaying modes also exist without the constraint of total mass co...

Relating Lagrangian and Eulerian horizontal eddy statistics in the surfzone

Concurrent Lagrangian and Eulerian observations of rotational, low-frequency (10−4 to 10−2 Hz) surfzone eddies are compared. Surface drifters were tracked for a few hours on each of 11 days at two alongshore uniform beaches. A cross-shore array of near-bottom current meters extended from near the shoreline to seaward of the surfzone (typically 100 m wide in these moderate wave conditions). Lagrangian and Eulerian mean alongshore velocities V are similar, with a midsurfzone maximum. Cross-shore dependent Lagrangian (σL) and Eulerian (σE) rotational eddy velocities, estimated from low-pass filtered drifter and current meter velocities, respectively, also generally agree. Cross-shore rotational velocities have a midsurfzone maximum whereas alongshore rotational velocities are distributed more broadly. Daily estimates of the Lagrangian time scale, the time for drifter velocities to decorrelate, vary between 40 and 300 s, with alongshore time scales greater than cross-shore time scales. The ratio of Lagrangian to apparent Eulerian current meter decorrelation times TL/TA varies considerably, between about 0.5 and 3. Consistent with theory, some of the TL/TA variation is ascribable to alongshore advection and TL/TA is proportional to V/σ, which ranges between about 0.6 and 2.5. Estimates of TL/TA vary between days with similar V/σ suggesting that surfzone Lagrangian particle dynamics vary between days, spanning the range from “fixed-float” to “frozen-field” [Lumpkin et al., 2002], although conclusions are limited by the statistical sampling errors in both TL/TA and V/σ.

The Effect of Barotropic and Baroclinic Tides on Three-Dimensional Coastal Dispersion: DISPERSION IN COASTAL MODELS

The effects of barotropic and baroclinic tides on three-dimensional (3-D) coastal dispersion are examined with realistic, 200-m horizontal resolution simulations of the Central Californian continental shelf during upwelling. Over multiple tidal cycles, the horizontal relative dispersion and vertical dispersion of 3-D drifters are similar between simulations with no tides and with barotropic tides. In contrast, baroclinic tides, which dissipate across the shelf and induce vertical mixing, result in a factor of 2–3 times larger horizontal and vertical dispersion. The increase in horizontal dispersion with vertical mixing is qualitatively consistent with weak-mixing shear dispersion. Without shear dispersion, horizontal dispersion of surface-trapped (2-D) drifters was similar in all simulations. However, 2-D drifter trajectory differences relative to no tide simulations are 3–4 times larger with baroclinic tides than barotropic tides alone. These results demonstrate the need to include baroclinic tides and 3-D tracking for coastal passive tracer dispersion. Plain Language Summary Understanding the dispersal of material in the coastal ocean is relevant to pollutant dilution, marine ecosystem sustainability, and search-and-rescue operations. Although numerical circulation models are commonly used to predict material dispersal, these models often do not include tides. Here the tidal effect on material dispersal is compared with numerical drifters released in a realistic model without tides, with surface tides (the rise and fall of sea level), and with internal tides (the rise and fall of interior density layers). Surface tides contribute little additional dispersal in the model region, while internal tides induce 2–3 times larger horizontal and about 2 times larger vertical dispersal in comparison to models without tides. In addition, after 48 hr surface drifter trajectory differences between models with and without internal tides are 8 km. Therefore, internal tides need to be considered in models used to plan oil-spill response or search-and-rescue operations.