Sung Yong Kim

Anisotropic Response of Surface Currents to the Wind in a Coastal Region

Analysis of coastal surface currents measured off the coast of San Diego for two years suggests an anisotropic and asymmetric response to the wind, probably as a result of bottom/coastline boundary effects, including pressure gradients. In a linear regression, the statistically estimated anisotropic response explains approximately 20% more surface current variance than an isotropic wind‐ocean response model. After steady wind forcing for three days, the isotropic surface current response veers 42 86 28 to the right of the wind regardless of wind direction, whereas the anisotropic analysis suggests that the upcoast (onshore) wind stress generates surface currents with 10 86 48 (71 86 38) to the right of the wind direction. The anisotropic response thus reflects the dominance of alongshore currents in this coastal region. Both analyses yield winddriven currents with 3%‐5% of the wind speed, as expected. In addition, nonlinear isotropic and anisotropic response functions are considered, and the asymmetric current responses to the wind are examined. These results provide a comprehensive statistical model of the wind-driven currents in the coastal region, which has not been well identified in previous field studies, but is qualitatively consistent with descriptions of the current response in coastal ocean models.

Evaluation of directly wind‐coherent near‐inertial surface currents off Oregon using a statistical parameterization and analytical and numerical models

Directly wind-coherent near-inertial surface currents off the Oregon coast are investigated with a statistical parameterization of observations and outputs of a regional numerical ocean model and three one-dimensional analytical models including the slab layer, Ekman, and near-surface averaged Ekman models. The transfer functions and response functions, statistically estimated from observed wind stress at NDBC buoys and surface currents derived from shored-based high-frequency radars, enable us to isolate the directly wind-forced near-inertial surface currents. Concurrent observations of the wind and currents are crucial to evaluate the directly wind-forced currents. Thus, the wind stress and surface current fields obtained from a regional ocean model, which simulates variability of the wind and surface currents on scales comparable to those in observations, are analyzed with the same statistical parameterization to derive the point-by-point transfer functions and response functions. Model and data comparisons show that the regional ocean model describes near-inertial variability of surface currents qualitatively and quantitatively correctly. The estimated response functions exhibit decay time scales in a range of 3–5 days, and about 40% of the near-inertial motions are explained by local wind stress. Among the one-dimensional analytical models, the near-surface averaged Ekman model explains the statistically derived wind-current relationship better than other analytical models.

Wind-Driven Sea Level Variability on the California Coast: An Adjoint Sensitivity Analysis

Effects of atmospheric forcing on coastal sea surface height near Port San Luis, central California, are investigated using a regional state estimate and its adjoint. The physical pathways for the propagation of nonlocal [O(100km)] wind stress effects are identified through adjoint sensitivity analyses, with a cost function that is localized in space so that the adjoint shows details of the propagation of sensitivities. Transfer functions between wind stress and SSH response are calculated and compared to previous work. It is found that (i) the response to local alongshore wind stress dominates on short time scales of O(1 day); (ii) the effect ofnonlocalwindsdominatesonlongertimescalesandiscarriedbycoastallytrappedwaves,aswellasinertia‐ gravity waves for offshore wind stress; and (iii) there are significant seasonal variations in the sensitivity of SSH to wind stress due to changes in stratification. In a more stratified ocean, the damping of sensitivities to local and offshore winds is reduced, allowing for a larger and longer-lasting SSH response to wind stress.

Interpretation of coastal wind transfer functions with momentum balances derived from idealized numerical model simulations

The local wind-driven circulation off southern San Diego is addressed with two complementary statistical and dynamical frameworks based on observations and idealized numerical model simulations. The observations including surface currents from high-frequency radars, subsurface currents from a nearshore mooring, and wind records at a local wind station are analyzed with the idealized ocean model (MITgcm) simulations using realistic bottom topography and spatially uniform wind stress forcing. Statistically estimated anisotropic local wind transfer functions characterize the observed oceanic spectral response to wind stress separately in the x (east-west) and y (north-south) directions. We delineate the coastal circulation at three primary frequencies [low (σL=0.0767 cycles per day (cpd)), diurnal (σD=1 cpd), and inertial (σf=1.07 cpd) frequencies] with the momentum budget equations and transfer functions. At low frequency, the magnitudes of transfer functions are enhanced near the coast, attributed to geostrophic balance between wind-driven pressure gradients and the Coriolis force on currents. The response diminishes away from the coast, returning to the balance between frictional and Coriolis terms, as in the classic Ekman model. On the contrary, transfer functions in the near-inertial frequency band show reduced magnitudes near the coast primarily due to friction, but exhibits the enhanced seaward response as a result of the inertial resonance. The idealized model simulations forced by local wind stress can identify the influences of remote wind stress and the biases in the data-derived transfer functions.

A statistical description on the wind-coherent responses of sea surface heights off the US West Coast

Local and remote wind-coherent responses of sea surface heights (SSHs) off the US West Coast (USWC) are described with statistical and analytical models. The wind transfer functions are statistically derived from surface wind stress at National Data Buoy Center (NDBC) buoys, located within 50 km from the shoreline, and detided SSHs (SSH anomalies; SSHAs) at shoreline tide gauges for 15 years (1995 to 2009) using linear regression in the frequency domain. A two-dimensional analytical model constrained by the coastal boundary provides a dynamical framework to interpret the data-derived statistical model. Although both transfer functions agree well at low frequency [σ ≤ 0.4 cycles per day (cpd)], they appear to be inconsistent at high frequency (σ ≥ 0.8 cpd; e.g., diurnal and its harmonic frequencies) because of incoherent signals between wind stress and SSHAs as well as their low signal-to-noise ratios. A multivariate regression analysis using wind stress at multiple wind buoys is implemented with a modified expectation maximization. The cross-validated skill increases and becomes saturated as the number of regression basis functions increases, demonstrating the influence of local and remote winds. The skill computed from all available winds off the USWC has a maximum as 0.1 in southern California, 0.2 to 0.3 in central California, and 0.3 to 0.5 in northern California, Oregon, and Washington. The residual SSHAs, incoherent components with all available coastal wind stress off the USWC, still contain poleward propagating signals, considered as components forced by remote winds outside of the domain.

National IOOS high frequency radar search and rescue project

The U.S. Integrated Ocean Observing System (IOOS®) partners have begun an effort to extend the use of high frequency (HF) radar for U.S. Coast Guard (USCG) search and rescue operations to all U.S. coastal areas with HF radar coverage. This project builds on the success of an IOOS and USCG-supported regional USCG search and rescue product created by Applied Science Associates (ASA), Rutgers University and University of Connecticut for the mid-Atlantic region. We describe the regional product and the expanded national products two main components: optimally-interpolated velocity fields and a predicted velocity field.

Resonant ocean current responses driven by coastal winds near the critical latitude

The currents forced by wind stress near the critical latitude (30°N or 30°S) appear enhanced as a result of the combination of natural modes of ocean currents (e.g., near-inertial motions) and diurnal land/sea breeze-driven currents. Here we assess the wind-current transfer function, derived from observations of coastal surface currents and surface winds off the U.S. West Coast (32°N to 47°N), as a function of the latitude (alongshore direction) and frequency. We compare the transfer functions at the diurnal frequency derived from observations and two analytical models (e.g., the Ekman model and a near-surface averaged Ekman model). The amplitude of the data-derived transfer function decreases with increasing distance from the critical latitude, and its argument varies nearly within the range estimated from two analytical models. Our results confirm that the resonant wind-current responses near the critical latitude are 8 to 12 times stronger than the purely diurnal land/sea breeze-forced currents in other latitudes.

DISPERSED OIL TRANSPORT MODELING CALIBRATED BY FIELD-COLLECTED DATA MEASURING FLUORESCEIN DYE DISPERSION

Oil-spill fate and transport modeling may be used to evaluate water column hydrocarbon concentrations, potential exposure to organisms, and impacts of oil spills with and without dispersant use. Important inputs to transport modeling for such analyses are current velocities and turbulent dispersion coefficients. Fluorescein dye studies off San Diego, California, were used to calibrate an oil transport model by hindcasting movement and dispersion of dye. The oil spill model was then used to predict subsurface hydrocarbon concentrations and potential water column impacts if oil were to be dispersed into the water column under similar conditions. Fieldcollected data included surface currents calculated from high-frequency radar data (HF-Radar), near-surface currents from drifter measurements drogued at several depths (1m, 2m, 4m or 5m), dye concentrations measured by fluorescence, spreading and dye intensity measurements based on aerial photography, and water density profiles from CTD casts. As the dye plume quickly extended throughout an upper mixed layer (7-15m), the horizontal dye movements were better indicated by the drifters drogued to a depth near the middle of that layer than the HF-Radar, which measured surface (~top 50 cm) currents (including wind drift). Diffusion rates were estimated based on dye spreading measured by aerial photography and fluorescencedepth profiles. The model used these data as inputs, modeling of wind-forced surface water turbulence and drift as a function of wind speed and direction (based on published results of fluid dynamics studies), and Stokes law for droplet rise/sinking rates, to predict oil transport and dispersion rates within the water column. Use of such diffusion rate data in an oil fate model can provide estimates of likely dispersed oil concentrations under similar conditions, which may be used to evaluate potential impacts on water column biota. However, other conditions with different patterns of current shear (due to background currents, tidal currents, and wind stress) should be examined before these results can be generalized.

Influence of varying upper ocean stratification on coastal near‐inertial currents

The influence of varying horizontal and vertical stratification in the upper layer ( O(10) m) associated with riverine waters and seasonal atmospheric fluxes on coastal near-inertial currents is investigated with remotely sensed and in situ observations of surface and subsurface currents and realistic numerical model outputs off the coast of Oregon. Based on numerical simulations with and without the Columbia River (CR) during summer, the directly wind-forced near-inertial surface currents are enhanced by 30%–60% when the near-surface layer has a stratified condition due to riverine water inputs from the CR. Comparing model results without the CR for summer and winter conditions indicates that the directly wind-forced near-inertial surface current response to a unit wind forcing during summer are 20%–70% stronger than those during winter depending on the cross-shore location, which is in contrast to the seasonal patterns of both mixed-layer depth and amplitudes of near-inertial currents. The model simulations are used to examine aspects of coastal inhibition of near-inertial currents, manifested in their spatial coherence in the cross-shore direction, where the phase propagates upward over the continental shelf, bounces at the coast, and continues increasing upward offshore (toward surface) and then downward offshore at the surface, with magnitudes and length scales in the near-surface layer increasing offshore. This pattern exhibits a particularly well-organized structure during winter. Similarly, the raypaths of clockwise near-inertial internal waves are consistent with the phase propagation of coherence, showing the influence of upper layer stratification and coastal inhibition.

Quality Assessment Techniques Applied to Surface Radial Velocity Maps Obtained from High-Frequency Radars

AbstractThis paper presents examples of the data quality assessment of surface radial velocity maps obtained from shore-based single and multiple high-frequency radars (HFRs) using statistical and dynamical approaches in a hindcast mode. Since a single radial velocity map contains partial information regarding a true current field, archived radial velocity data embed geophysical signals, such as tides, wind stress, and near-inertial and low-frequency variance. The spatial consistency of the geophysical signals and their dynamic relationships with driving forces are used to conduct the quality assurance and quality control of radial velocity data. For instance, spatial coherence, tidal amplitudes and phases, and wind-radial transfer functions are used to identify a spurious range and azimuthal bin. The uncertainty and signal-to-noise ratio of radial data are estimated with the standard deviation and cross correlation of paired radials sampled at nearby grid points that belong to two different radars. This ...