Submesoscale eddies form an important component of the circulation of the Southern California Bight (SCB), greatly impacting ecological processes. Despite their acknowledged significance in influencing ocean physics and biology, submesoscale eddies have been exceptionally hard to observe because of the technical challenges posed by both field and remote platforms. Here using a decade of high-frequency radar (HFR) surface current data we address this challenge for the SCB. Over the ten years of data, our research has mapped out the spatial distribution of submesoscale eddies and provided their seasonal and inter-annual variations. Between 2012 and 2021, a total of 235229 eddies were detected, averaging 452 eddies per week. Of these, 56% were cyclonic and 44% were anticyclonic. The contribution is roughly equal if eddies through their life spans are counted as one occurrence. This is because cyclonic eddies lived longer. The spatial distribution of eddies exhibited strong topographically related heterogeneity. Spatially coherent eddies, which reoccurred in certain locations over time, formed hotspots of eddy activity, largely in association with headlands. However, there were hotspots that did not seem to be associated with any typographic feature. Eddy temporal variations were examined at seasonal and interannual scales. On seasonal scales, eddies were found to be more numerous in the summer and early fall than in the spring. In August, the number of eddies was the highest, with 55% more observed eddies than in April, the least active month. The strong equatorward flow in the springtime seems to be linked with the reduced eddy activity at this time, likely due to the flow's suppressing effect on vortices and instabilities. At interannual scales, the eddy activity substantially increased in association with the 2014-2015 Blob event and the 2015-2016 El Niño. Observed eddies rose by 38% in 2014 compared to 2013 and remained high in 2015 and 2016. The results of this study are useful for the validation of numerical modeling studies in the SCB and could be of interest to the biological community to evaluate links between ecosystems and submesoscale activity along the highly productive coasts of the SCB.

The solar heating term in the air–sea heat budget is unique, as solar radiation penetrates the water column and directly heats beneath the sea surface. The physics of solar transmission through the upper ocean, and methods for quantifying this process in climate models are described. The relatively large amount of solar energy that passes through the air–sea interface is scattered and absorbed by sea water and its constituents. Molecular water and chlorophyll-containing phytoplankton are the primary attenuators of solar radiation. Variations in chlorophyll concentration can alter the solar flux divergence and resulting upper ocean stratification.

Horizontal relative dispersion is investigated with Lagrangian drifter and dye tracer observations on relatively small scales (~100 to 850 m) in coastal waters. Anisotropy is quantified with an aspect ratio of the spreading in two orthogonal directions. Individual observations generally appear highly anisotropic. However, the ensemble mean computed in a coordinate system aligned with bathymetry indicates only weak anisotropy due to averaging highly anisotropic observations over a wide range of principal‐axis directions. The strong anisotropy is preserved when the mean is computed in a principle‐axis coordinate system. In fact, the ensemble mean in principle‐axis coordinates gives antidispersion or convergence in the minor‐axis direction. This result suggests gradients in buoyant materials such as spilled oil and other contaminants are not necessarily smoothed as the standard eddy‐diffusivity parameter suggests. Flow kinematics computed with clusters of four drifters indicate that approximately 72% of energy in the observed dispersing flows can be attributed to organized submesoscale structures.

Coincident Lagrangian observations of coastal circulation with surface drifters and dye tracer were collected to better understand small-scale physical processes controlling transport and dispersion over the inner shelf in the Gulf of Mexico. Patches of rhodamine dye and clusters of surface drifters at scales of O (100) m were deployed in a cross-shelf array within 12 km from the coast and tracked for up to 5 h with airborne and in situ observations. The airborne remote sensing system includes a hyperspectral sensor to track the evolution of dye patches and a lidar to measure directional wavenumber spectra of surface waves. Supporting in situ measurements include a CTD with a fluorometer to inform on the stratification and vertical extent of the dye and a real-time towed fluorometer for calibration of the dye concentration from hyperspectral imagery. Experiments were conducted over a wide range of conditions with surface wind speed between 3 and 10 m s −1 and varying sea states. Cross-shelf density gradients due to freshwater runoff resulted in active submesoscale flows. The airborne data allow characterization of the dominant physical processes controlling the dispersion of passive tracers such as freshwater fronts and Langmuir circulation. Langmuir circulation was identified in dye concentration maps on most sampling days except when the near surface stratification was strong. The observed relative dispersion is anisotropic with eddy diffusivities O (1) m 2 s −1 . Near-surface horizontal dispersion is largest along fronts and in conditions dominated by Langmuir circulation is larger in the crosswind direction. Surface convergence at fronts resulted in strong vertical velocities of up to −66 m day −1 .

Kelp plants were tagged monthly with drifters in the Santa Barbara Channel between November of 2015 and December of 2017. This dataset contains GPS positions of freely drifting kelp plants (nominally) every 10 minutes. Tagged kelp plants begin at one of three kelp forests (Mohawk, Hope Ranch, or Isla Vista) off the Santa Barbara coast. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/739111

Fronts and eddies identified with aerial guidance are seeded with drifters to quantify submesoscale flow kinematics. The Lagrangian observations show mean divergence and vorticity values that can exceed 5 times the Coriolis frequency. Values are the largest observed in the field to date and represent an extreme departure from geostrophic dynamics. The study also quantifies errors and biases associated with Lagrangian observations of the underlying velocity strain tensor. The greatest error results from undersampling, even with a large number of drifters. A significant bias comes from inhomogeneous sampling of convergent regions that accumulate drifters within a few hours of deployment. The study demonstrates a Lagrangian sampling paradigm for targeted submesoscale structures over a broad range of scales and presents flow kinematic values associated with vertical velocities O(10)mh(-1) that can have profound implications on ocean biogeochemistry.

The Northern Channel Islands in California host multiple Marine Protected Areas containing kelp forest ecosystems. Little is known about the water circulation onshore of the 20-m isobath. We use water velocity recorded at 21 sites near the 15-m isobath at the Islands and mainland during 1999-2012 to describe the water circulation on time scales of days to months. The mean circulation is eastward or weak at the Islands but poleward along the mainland (speeds 0-10 cm s(-1)). The subinertial-frequency along-shelf flow is surface-intensified and reverses direction on time scales of days. In summer, the flow becomes more poleward throughout the region. The mean cross-shelf flow profiles are strikingly similar at most sites, with flow speeds 1-2 cm s(-1). The mean flow near bottom in the vicinity of the kelp forests is offshore. The time-varying, two-layered response to wind is stronger, up to 6 cm s(-1). The flushing time of the shelf onshore of the 15-m isobath is short, at most 2 dy. At a few sites exposed to the prevailing wind, up to 60% of the velocity variance is predictable from wind measured in the Santa Barbara Channel. In the lee of Point Conception or at the Islands, however, regional wind explains little of the velocity variance. During weak winds, the velocity at some mainland, but not Island, sites responds to pressure gradients measured along the mainland coast. These pressure gradients are associated with local wind relaxations at Pt. Conception, not with remotely-generated coastal-trapped waves. (C) 2015 Elsevier Ltd. All rights reserved.

Ocean gliders are autonomous underwater vehicles typically used to sample spatial variations in scalar variables (e.g., temperature, salinity, and bio-optical water properties) along transects. More recently, gliders have been equipped with ADCPs for measuring current profiles along transects. Accurate measurement of velocity profiles from a moving platform requires knowledge of the platform motion over the earth. Determination of glider motion over the earth relies on glider GPS positions available only at times of glider surfacing. Glider surfacing intervals typically range from 10's of minutes to hours, precluding accurate instantaneous knowledge of glider motion. This is a major challenge for measuring velocity profiles from ocean gliders. One approach for determining vertical velocity profiles from glider-mounted ADCPs relies on estimating depth-integrated currents averaged between glider surfacings. Once estimated, vertically averaged velocity can be combined with horizontal glider velocities relative to the water to obtain depth-resolved velocities using methods of Visbeck (2001) and Todd et al. (2011). The vertically averaged velocity is calculated from the distance the glider strays from its projected position during the time between surfacings. This distance is computed as the difference between the dead-reckoned surfacing location and the actual surfacing location as measured by GPS. There are numerous methods for computing the dead-reckoned position of glider surfacings, but these have not been evaluated to determine which best predicts surfacing locations. Here, three methods for calculating vertically averaged horizontal current velocities from gliders are evaluated. Two Slocum Coastal G1 gliders (manufactured by Teledyne Webb Research), each with an upward looking 1 MHz ADCP (manufactured by Teledyne RD Instruments), were deployed off the California coast during the summer of 2012. The gliders flew 500 m square patterns around a bottom-mounted, upward-looking 600 kHz ADCP (manufactured by Teledyne RD Instruments) moored at a depth of 26 m. The moored ADCP data are key to evaluating methods for computing vertically averaged velocity from gliders and ultimately assessing depth-resolved current velocities obtained from glider-mounted ADCPs. The first method calculates the glider's horizontal velocity using pitch and vertical velocity. Vertical velocity is calculated by taking the derivative of pressure with respect to time. It is often assumed that the flight path is in the direction of the glider's long axis and α, the angle of attack or the angle between the glider's path through the water and its long axis, is zero. This assumption can result in errors of 2.5 cm s -1 (Merckelbach et al. 2001) which are on the order of 10% of the horizontal velocity. Previous studies have estimated angle of attack (e.g. Sherman et al. 2001) using model results for internally-mounted ADCPs on Spray gliders. The gliders in this study carried externally-mounted ADCPs, so errors may also result from extra drag and non-zero values of α. The value of α is found by maximizing the r2 of vertically averaged velocities found by this method with vertically averaged velocities from the mooring. The second method directly measures currents using the ADCPs mounted on the gliders. Water velocity relative to the glider is obtained from the ADCP bin nearest the glider which is 1m long and begins 1.2 m from the glider. This velocity is calculated on glider upcasts and downcasts in east-north-up (ENU) coordinates. Velocities in ENU coordinates are derived from software provided with the ADCP. The third method uses velocities measured along the ADCP beams in so-called beam coordinates. Directly measured velocity can then be used (see Todd et al. 2011) from the component of the ADCP beams oriented in the direction parallel to the glider's long axis - this can be 2 or 3 beams depending on the transducer head on the ADCP. Experimentation with these data and this method is ongoing and will be compared with the previously mentioned methods. The three methods are used to estimate vertically averaged currents from the gliders, and these are compared with vertically averaged currents from the bottom-mounted ADCP. Of the three methods, the first produces the highest correlations (r 2 values). However, it is necessary to estimate the angle of attack when using the first method.

Knowledge of horizontal relative dispersion in nearshore oceans is important for many applications including the transport and fate of pollutants and the dynamics of nearshore ecosystems. Two-particle dispersion statistics are calculated from millions of synthetic particle trajectories from high-resolution numerical simulations of the Southern California Bight. The model horizontal resolution of 250 m allows the investigation of the two-particle dispersion, with an initial pair separation of 500 m. The relative dispersion is characterized with respect to the coastal geometry, bathymetry, eddy kinetic energy, and the relative magnitudes of strain and vorticity. Dispersion is dominated by the submesoscale, not by tides. In general, headlands are more energetic and dispersive than bays. Relative diffusivity estimates are smaller and more anisotropic close to shore. Farther from shore, the relative diffusivity increases and becomes less anisotropic, approaching isotropy similar to 10 km from the coast. The degree of anisotropy of the relative diffusivity is qualitatively consistent with that for eddy kinetic energy. The total relative diffusivity as a function of pair separation distance R is on average proportional to R-5/4. Additional Lagrangian experiments at higher horizontal numerical resolution confirmed the robustness of these results. Structures of large vorticity are preferably elongated and aligned with the coastline nearshore, which may limit cross-shelf dispersion. The results provide useful information for the design of subgrid-scale mixing parameterizations as well as quantifying the transport and dispersal of dissolved pollutants and biological propagules.

Oceanographic observations made during the Sound Predictions 2009 field experiment in Prince William Sound, Alaska, have documented rapid changes in the upper water column (0–40m) circulation. An assortment of drifting buoys, sampling four different depths, and HF radar surface current mapping, revealed three modes of circulation: anticyclonic, open cyclonic, and closed cyclonic. Each mode was observed at least once within an 18-day window, and the transition between them took as little as a day. Time-resolved hydrographic measurements show that the mass field was variable, but generally arranged such that the surface geostrophic flow should be in a closed-core cyclonic eddy configuration. Observations show that the mass field was likely influenced by relatively low salinity water flowing into Prince William Sound from the shelf, and from local freshwater input. We quantitatively examine why a closed-core circulation was not always observed by focusing on the transition between the closed and open cyclonic flow patterns. The western region of the central sound is a key area for this transition. Here, the high-frequency radar revealed that the closed circulation was established when the net flow shifted direction from northward to southward. A detailed comparison of the meridional geostrophic and wind-driven flows, using measured winds and hydrographic data from CTD profiles and two autonomous vehicles, shows that the geostrophic flow was mostly southward while the wind-driven flow was mostly northward. A net southward flow can be caused by a decrease in the northward wind-driven flow or an increase in the southward geostrophic flow.

This study offers a new method for estimating High-Frequency (HF) radar surface current velocity error in data comparisons with other types of instrumentation. A new method is needed in order to remove the zero-mean random spatial and temporal fluctuations present in surface-current measurements from all sensors. Conventional methods for calculating radar error when comparing with another instrument have included their root mean square differences and scatter plots that provide correlation coefficient and slope/intercept of the regression line. It seems that a meaningful estimate of radar error should attempt to remove both sensors’ zero mean random fluctuations, inasmuch as possible. We offer and compare a method that does this. The method was tested on data collected in the Central San Francisco Bay, where GPS surface-drifter deployments were conducted within the coverage of four 42 MHz radars over six days in October of 2008. Drifters were continuously deployed in these areas over the sampling days, providing 525 usable drifter measurements. Drifter and radar measurements were averaged into thirty-minute time bins. The three-day long-term averages from the sampling areas were then subtracted from the thirtyminute averages to remove biases associated with comparisons done with short, disjoint time-sample periods. These were then used to develop methods that give radar error or bias after the random fluctuations have been removed. Results for error estimates in this study are commensurate with others where random fluctuations have been filtered, suggesting they are valid. The estimated error for the radars in the SF Bay is low, ranging from −7.57 cm/s to 0.59 cm/s.

Relative dispersion statistics and related Lagrangian parameters, not well observed in coastal regions, are obtained from in situ surface drifter observations and presented in the context of Lagrangian stochastic models. Clusters of GPS tracked surface drifters, with initial horizontal spacing of 5–10 m, were repetitively deployed in the Santa Barbara Channel from July 2004 to June 2005. The drifters sampled their position every 10 min for 1–2 days. Mean square pair separation distance, or relative dispersion, increases approximately exponentially in time during the first ∼5 h of sampling (e‐folding time of 0.9 h). Thereafter, the dispersion increase is approximately quadratic in time. Large error bars on the observed mean dispersion, and higher‐order Lagrangian statistics that are not clearly supportive of the aforementioned dispersion curves, indicate uncertainty. The mean square relative (separation) velocity shows near‐linear growth with pair separation distance, extending from 0.3 to 85 cm2 s−2over length scales from ∼8 m to 2.2 km. The observed length scale dependency in square relative velocity is investigated in a Lagrangian stochastic model (LSM) for a cloud of particles. Modeled dispersion agrees with observations only when the velocity scale for the sub‐grid scale random normal deviate in the LSM (typically a constant) is length scale dependent, and takes into consideration the observed scaling. Occasional large (>25 cm s−1) discrepancies in grid‐scale velocities between drifters and HF radar cause general disagreement in distributions of ending positions of LSM trajectories when compared with Lagrangian observations.

The development and implementation of a three-dimensional ocean modeling system for the Prince William Sound (PWS) is described. The system consists of a regional ocean model component (ROMS) forced by output from a regional atmospheric model component (the Weather Research and Forecasting Model, WRF). The ROMS ocean model component has a horizontal resolution of 1km within PWS and utilizes a recently-developed multi-scale 3DVAR data assimilation methodology along with freshwater runoff from land obtained via real-time execution of a digital elevation model. During the Sound Predictions Field Experiment (July 19–August 3, 2009) the system was run in real-time to support operations and incorporated all available real-time streams of data. Nowcasts were produced every 6h and a 48-h forecast was performed once a day. In addition, a sixteen-member ensemble of forecasts was executed on most days. All results were published at a web portal (http://ourocean.jpl.nasa.gov/PWS) in real time to support decision making.The performance of the system during Sound Predictions 2009 is evaluated. The ROMS results are first compared with the assimilated data as a consistency check. RMS differences of about 0.7°C were found between the ROMS temperatures and the observed vertical profiles of temperature that are assimilated. The ROMS salinities show greater discrepancies, tending to be too salty near the surface. The overall circulation patterns observed throughout the Sound are qualitatively reproduced, including the following evolution in time. During the first week of the experiment, the weather was quite stormy with strong southeasterly winds. This resulted in strong north to northwestward surface flow in much of the central PWS. Both the observed drifter trajectories and the ROMS nowcasts showed strong surface inflow into the Sound through the Hinchinbrook Entrance and strong generally northward to northwestward flow in the central Sound that was exiting through the Knight Island Passage and Montague Strait entrance. During the latter part of the second week when surface winds were light and southwesterly, the mean surface flow at the Hinchinbrook Entrance reversed to weak outflow and a cyclonic eddy formed in the central Sound. Overall, RMS differences between ROMS surface currents and observed HF radar surface currents in the central Sound were generally between 5 and 10cm/s, about 20–40% of the time mean current speeds.The ROMS reanalysis is then validated against independent observations. A comparison of the ROMS currents with observed vertical current profiles from moored ADCPs in the Hinchinbrook Entrance and Montague Strait shows good qualitative agreement and confirms the evolution of the near surface inflow/outflow at these locations described above. A comparison of the ROMS surface currents with drifter trajectories provided additional confirmation that the evolution of the surface flow described above was realistic. Forecasts of drifter locations had RMS errors of less than 10km for up to 36h. One and two-day forecasts of surface temperature, salinity and current fields were more skillful than persistence forecasts. In addition, ensemble mean forecasts were found to be slightly more skillful than single forecasts. Two case studies demonstrated the system’s qualitative skill in predicting subsurface changes within the mixed layer measured by ships and autonomous underwater vehicles. In summary, the system is capable of producing a realistic evolution of the near-surface circulation within PWS including forecasts of up to two days of this evolution. Use of the products provided by the system during the experiment as part of the asset deployment decision making process demonstrated the value of accurate regional ocean forecasts in support of field experiments.

This study explores Eulerian and Lagrangian circulation during weak winds at two inner-shelf locations off the Southern California coast where the shoreline, shelf, wind, and wave characteristics differ from those in previous studies. In agreement with recent observational studies, wave-driven Eulerian offshore flow just outside the surf zone, referred to as undertow, is a substantial component of the net cross-shore circulation during periods of weak winds. Drifter observations show onshore surface flow, likely due to light onshore winds, and a consistent decrease in onshore velocity of roughly 4 cm s(-1) within a few hundred meters of the surf zone. Undertow is examined as a possible explanation for the observed Lagrangian decelerations. Model results suggest that, even when waves are small, undertow can decrease the velocity of shoreward-moving drifters by >2 cm s(-1), roughly half the observed deceleration. The coastal boundary condition also has the potential to contribute to the observed decelerations. Subtracting predicted Stokes drift velocities from the Lagrangian drifter observations improves the agreement between the drifter observations and coincident Eulerian ADCP observations.

This article presents observations of near-surface current trajectories made with water-following drifters in the Philippine archipelago. The data describe small-scale flows around obstacles and provide some snapshots of regional currents that both add insight into conceptual views of circulation on a variety of scales. The most interesting tracks are those collected in San Bernardino Strait, where the interaction of energetic tidal flows with small islands, seamounts, and headlands give rise to flows with vorticity and strain rate that can exceed 100f on scales < 1 km. The observations show some of the high Rossby number flows that challenge regional circulation models. Much of the data inform subgrid-scale motions that models must presently parameterize.

A purely Lagrangian assessment of dispersion from modeled surface current trajectories in the coastal ocean is presented. Modeled trajectories come from ROMS simulations for the Southern California Bight during the 1996 through 1999 period. Data are from surface current trajectories collected primarily in the Santa Barbara Channel with CODE style drifters. Distributions of particle positions from trajectories emanating from launch locations within 10 kilometers of the coast throughout the Santa Barbara Channel that advect for one through four days (Lagrangian PDFs) are evaluated descriptively and quantitatively. The two dimensional Kolmogorov‐Smirnov (K‐S) statistical test for comparing discrete sampled data with a known probability distribution is the quantitative basis. In general, dispersion distributions from observations are similar to Lagrangian PDFs computed from modeled trajectories and the K‐S statistic quantifies this accordingly. A few specific regions of poor model‐data agreement are indicated and discussed. The purely Lagrangian assessment, elucidates an improved understanding of model performance and ocean circulation beyond that offered in a Eulerian sense, and is necessary when modeled trajectories are utilized for applied oceanographic and marine ecology problems.

Dense arrays of surface drifters are used to quantify the flow field on time and space scales over which high- frequency ( HF) radar observations are measured. Up to 13 drifters were repetitively deployed off the Santa Barbara and San Diego coasts on 7 days during 18 months. Each day a regularly spaced grid overlaid on a 1-km(2) ( San Diego) or 4- km(2) ( Santa Barbara) square, located where HF radar radial data are nearly orthogonal, was seeded with drifters. As drifters moved from the square, they were retrieved and replaced to maintain a spatially uniform distribution of observations within the sampling area during the day. This sampling scheme resulted in up to 56 velocity observations distributed over the time ( 1 h) and space ( 1 and 4 km2) scales implicit in typical surface current maps from HF radar. Root- mean- square ( RMS) differences between HF radar radial velocities obtained using measured antenna patterns, and average drifter velocities, are mostly 3 - 5 cm s(-1). Smaller RMS differences compared with past validation studies that employ current meters are due to drifter resolution of subgrid- scale velocity variance included in time and space average HF radar fields. Roughly 5 cm s(-1) can be attributed to sampling on disparate time and space scales. Despite generally good agreement, differences can change dramatically with time. In one instance, the difference increases from near zero to more than 20 cm s(-1) within 2 h. The RMS difference and bias ( mean absolute difference) for that day exceed 7 and 12 cm s(-1), respectively.

A purely Lagrangian assessment of dispersion from modeled surface current trajectories in the coastal ocean is presented. Modeled trajectories come from ROMS simulations for the Southern California Bight during the 1996 through 1999 period. Data are from surface current trajectories collected primarily in the Santa Barbara Channel with CODE style drifters. Distributions of particle positions from trajectories emanating from launch locations within 10 kilometers of the coast throughout the Santa Barbara Channel that advect for one through four days (Lagrangian PDFs) are evaluated descriptively and quantitatively. The two dimensional Kolmogorov-Smirnov (K-S) statistical test for comparing discrete sampled data with a known probability distribution is the quantitative basis. In general, dispersion distributions from observations are similar to Lagrangian PDFs computed from modeled trajectories and the K-S statistic quantifies this accordingly. A few specific regions of poor model-data agreement are indicated and discussed. The purely Lagrangian assessment, elucidates an improved understanding of model performance and ocean circulation beyond that offered in a Eulerian sense, and is necessary when modeled trajectories are utilized for applied oceanographic and marine ecology problems.