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In 2001, the U.S. Geological Survey, together with several other federal and municipal agencies, began a series of field programs to determine along and cross-shelf transport patterns over the continental shelves in the central Southern California Bight. As a part of these programs, moorings that monitor winds were deployed off the Palos Verdes peninsula and within San Pedro Bay for six 3–4 month summer and winter periods between 2001 and 2008. In addition, nearly continuous records of winds for this 7-year period were obtained from a terrestrial site at the coast and from a basin site offshore of the long-term coastal site. The mean annual winds are downcoast at all sites. The alongshelf components of wind stress, which are the largest part of the low-frequency wind stress fields, are well correlated between basin, shelf and coastal sites. On average, the amplitude of alongshelf fluctuations in wind stress are 3–4 times larger over the offshore basin, compared to the coastal site, irrespective of whether the fluctuations represent the total, or just the correlated portion of the wind stress field. The curl in the large-scale wind stress tends to be positive, especially in the winter season when the mean wind stress is downcoast and larger at the offshore basin site than at the beach. However, since the fluctuation in wind stress amplitudes are usually larger than the mean, periods of weak negative curl do occur, especially in the summer season when the largest normalized differences in the amplitude of wind stress fluctuations are found in the nearshore region of the coastal ocean. Even though the low-frequency wind stress field is well-correlated over the continental shelf and offshore basins, out to distances of 35 km or more from the coast, winds even 10 km inshore of the beach do not represent the coastal wind field, at least in the summer months. The seasonal changes in the spatial structures in wind stress amplitudes suggest that an assessment of the amplitude of the responses of coastal ocean processes to wind forcing is complex and that the responses may have significant seasonal structures.

In the process of developing a tide-permitting coastal ocean forecasting system, tidal signals are added to an oceanic general circulation model for Monterey Bay, California. The model, which is configured from the regional ocean modeling system (ROMS), has three one-way nested domains with the finest resolution of 1.6 km in the horizontal direction and 32 levels in the vertical direction. In present research, the tidal simulation of the system is validated against tide gauges, ADCP current observations, high-frequency radar surface-current observations, and compared with the output from two barotropic tidal models. The results indicate that the barotropic tides from the model compare well with observations in terms of sea-surface height, with discrepancies in amplitude of less than 10% of the amplitude of the most energetic M 2 constituent. However, the discrepancy for the barotropic tidal currents can reach 30% among models with similar accuracy in sea-surface height. The generation and propagation of baroclinic tides associated with the Monterey Submarine Canyon are qualitatively reproduced by the model with weaker strength. The surface tidal current simulation is improved, especially in terms of magnitude, when the model has a more realistic stratification through changes in both barotropic and baroclinic tidal currents.

Fecal indicator bacteria concentrations measured in the surf zone off Huntington Beach, CA from July 1998-December 2001 were analyzed with respect to their spatial patterns along 23 km of beach, and temporal variability on time scales from hourly to fortnightly. The majority of samples had bacterial concentrations less than, or equal to, the minimum detection limit, but a small percentage exceeded the California recreational water standards. Areas where coliform bacteria exceeded standards were more prevalent north of the Santa Ana River, whereas enterococci exceedances covered a broad area both north and south of the river. Higher concentrations of bacteria were associated with spring tides. No temporal correspondence was found between these bacterial events and either the timing of cold water pulses near shore due to internal tides, or the presence of southerly swell in the surface wave field. All three fecal indicator bacteria exhibited a diel cycle, but enterococci rebounded to high nighttime values almost as soon as the sun went down, whereas coliform levels were highest near the nighttime low tide, which was also the lower low tide.

Three-dimensional numerical simulations of the generation and propagation of the semidiurnal internal tide in a submarine canyon with dimensions similar to those of the Monterey Canyon are carried out using a primitive equation model. Forcing with just sea level at the offshore boundary in an initially horizontally homogeneous ocean with realistic vertical stratification, internal tides are generated at the canyon foot and rim, and along portions of the canyon floor. The results compare favorably with observations, both indicating enhancement of energy along the canyon floor propagating at an angle consistent with linear internal wave theory. Due to the earth's rotation, internal tide energy is distributed asymmetrically in the cross-canyon direction, favoring the southern side. The effect of canyon floor slope is explored, with the finding that small changes in the slope result in large changes in the amount and distribution of the internal tide energy. Canyons whose floors are subcritical with respect to the semidiurnal frequency along their entire length have very little baroclinic energy, whereas canyons that are near-critical along much of their length, such as the Monterey Canyon, develop strong internal tides that propagate shoreward. Canyons that are near-critical at their mouths but supercritical further inshore generate the most internal tidal energy overall, although little of it makes it onto the continental shelf shoreward of the canyon head. The effects of internal tides within the canyons can be seen outside the canyons as well. Water is transported from depth onto the adjacent continental shelf along the canyon rims. This tidal pumping can be responsible for alongshore internal tide propagation and tidal-period surface currents with relatively small horizontal scales of variability.

From August 1993 to August 1994, six moorings that measure current, temperature, salinity, and water clarity were deployed along the axis of Monterey Canyon to study the circulation and transport of water and suspended particulate matter through the canyon system. The moorings occupied three sites that are morphologically different: a narrow transverse section (axis width 900 m) at 1450 m water depth, a wide transverse section at 2837 m, and a third site in the fan valley axis farther offshore at 3223 m that recorded for 3 yr. In addition, CTD/transmissometer casts were conducted within and near the Monterey Canyon during four cruises. Our data show a mainly biogenic, surface turbid layer, a limited intermediate nepheloid layer, and a bottom nepheloid layer. There is a consistent presence of a turbid layer within the canyon at a water depth of about 1500 m. Tidal flow dominates at all sites, but currents above the canyon rim and within the canyon appear to belong to two distinct dynamic systems. Bottom intensification of currents plays an important role in raising the near-bottom shear stress high enough that bottom sediments are often, if not always, resuspended. Mean flow pattern suggests a convergence zone between the narrow and wide site: the near-bed (100 m above bottom where the lowest current meter was located) mean transport is down-canyon at the 1450-m site, while the near-bottom transport at the 2837-m site is up-canyon, at a smaller magnitude. Transport at the 3223-m site is dominantly NNW, cross-canyon, with periods of up-canyon flow over 3 yr. A very high-turbidity event was recorded 100 m above the canyon bottom at the narrow site. The event started very abruptly and lasted more than a week. This event was not detected at either of the deeper sites. A canyon head flushing event is likely the cause.

We sampled larval fishes along cross-shelf transects off Granite Canyon, California, USA, during the upwelling seasons of 1993 and 1994 to determine whether coastal upwelling fronts affect the cross-shelf distribution of larval rockfishes Sebastes spp. during the earliest period of planktonic life. Rockfish larvae occurred in relatively high densities near surface fronts and were dis- tributed in patches oriented along sloping pycnoclines contiguous with surface fronts. Qualitative comparisons between observed distributions of larval rockfishes in relation to hydrographic structure and predictions from models of plankton distributions at convergent fronts support the hypothesis that convergent circulation contributes to observed distributions. Our results indicate that (1) coastal upwelling fronts influence larval rockfishes at an earlier life history stage than has previously been documented, and (2) the influence of upwelling fronts on distributions of larval rockfishes is similar to the influence of hydrographic fronts on distributions of larval fish reported for a variety of oceano- graphic settings. In light of the plausible effects of upwelling fronts on larval transport and ecology, our findings suggest that upwelling fronts merit further investigation for their potential role in trans- lating variability in upwelling dynamics into fluctuations in recruitment to coastal rockfish popula- tions along the west coast of North America.

Current meter data have been analyzed from seven moorings on the continental slope along the central California coast, from Point Piedras Blancas to Point Reyes. The goal was to examine the subtidal variability in the 100 m to 1000 m depth range, particularly with regard to alongshore propagating events and interactions with eddies and meanders of the California Current offshore. The 2-year time series available off Point Sur were first analyzed in conjunction with the local and remote surface wind stress and coastal synthetic subsurface pressure, and then correlated with shorter coincident current records moored at similar depths to the north and south. The poleward flowing California Undercurrent was the most prominent feature at all the moorings except at one site located well into the Monterey Submarine Canyon. The strongest poleward flows over the slope occurred in 3- to 4-month bursts, not phase locked with the seasons, with vector speeds exceeding 40 cm s−1. South of the canyon, an approximately monthly signal was identified which propagated poleward, upward, and offshore. The behavior of this signal was consistent with that of an internal coastal Kelvin wave generated at the surface by remote wind stress to the south and was likely not of equatorial origin. The wave was apparently scattered by the abrupt topography of the canyon, since its energy persisted to the north of the canyon but with unstable phase. At least three eddy-meander interaction events were observed. These warm, deep (>1000 m), anticyclonic features reversed the flow over the slope to equatorward when they moved onshore and interrupted the flow of the undercurrent. One event forced anomalously strong (>15 cm s−1) onshore flows off Monterey Bay and offshore flows off Point Sur. While quantitative transport estimates could not be made with this sparse data set, it seems apparent that such events play a significant role in the exchange of water properties between the shelf and the deep ocean.

Near‐surface currents in Monterey Bay derived from a network of shore‐based HF radars are presented for August–December 1994 and compared with those from April to September 1992. Focus is placed on the low‐frequency (2‐ to 30‐day period) motions in the remotely sensed data and on comparison of radar‐derived currents with moored current and wind observations, ship‐based acoustic Doppler current profiler observations, satellite‐based surface temperature imagery, and surface drifter velocities. The radar‐derived picture of the late summer mean flow is very similar in the two realizations and is consistent with historical data. Flow is equatorward in the outer part of the bay, poleward in a narrow band nearshore, and very sluggish in the middle of the bay. Low‐pass‐filtered time series of radar‐derived currents are highly correlated with moored current observations and with winds in the outer part of the bay. The vector time series are also coherent across a broad frequency band with currents typically in phase between 1‐ and 9‐m depths and with 1‐m currents typically 40°–60° to the right of the wind. Overall, these results confirm the utility of Coastal Ocean Dynamics Applications Radar (CODAR)‐type HF radars for the study of coastal surface currents out to ranges ∼50 km from shore, particularly for highly averaged fields. Data variability and comparison with in situ observations for high‐frequency (1‐ to 48‐hour period) motions point to the need to better characterize and minimize sources of error in the radar observations. NSF Naval Postgraduate School MBARI Office of Naval Research Office of Naval Research contract N0001495WR30022 within the Naval Ocan Modeling and Prediction Program

AVHRR and CTD data from the Monterey Bay region during spring-summer 1989 show two basic hydrographic states, upwelling and relaxation. These occur in response to local wind forcing and are modified by interaction with a California Current meander. Upwelling at Pt An˜o Nuevo, north of Monterey Bay, is identified as the source of cold, salty near-surface water frequently seen in the Bay. No evidence is found in any available data to support the commonly held belief that the Monterey Submarine Canyon is responsible for the introduction of upwelled water to the Bay's euphotic zone. During wind relaxations, upwelling ceases and a persistent California Current meander translates shoreward. Data support the idea that upwelling centers are associated with coastal headlands. The flow of upwelled water from these centers is bifurcated, with one tongue trending offshore and one equatorward. We propose a conceptual model to explain this pattern of flow and its impact on the California Current.

[1] Turbidity currents are thought to be the main mechanism to move ∼500,000 m3 of sediments annually from the head of the Monterey Submarine Canyon to the deep-sea fan. Indirect evidence has shown frequent occurrences of such turbidity currents in the canyon, but the dynamic properties of the turbidity currents such as maximum speed, duration, and dimensions are still unknown. Here we present the first-ever in-situ measurements of velocity profiles of four turbidity currents whose maximum along-canyon velocity reached 190 cm/s. Two turbidity currents coincided with storms that produced the highest swells and the biggest stream flows during the year-long deployment.

To determine the accuracy of nearshore winds from the QuikSCAT satellite, winds from three satellite datasets (scientifically processed swath, gridded near-real-time, and gridded science datasets) were compared to those from 12 nearshore and 3 offshore U.S. West Coast buoys. Satellite observations from August 1999 to December 2000 that were within 25 km and 30 min of each buoy were used. Comparisons showed that satellite–buoy wind differences near shore were larger than those offshore. Editing the satellite data by discarding observations recorded in rain and those recorded in light winds improved the accuracy of all three datasets. After removing rain-flagged data and wind speeds less than 3 m s21, root-mean-squared differences (satellite minus buoy) for swath data, the best of the three datasets, were 1.4 m s21 and 378 based on 5741 nearshore comparisons. By removing winds less than 6 m s21, these differences were reduced to 1.3 m s21 and 268. At the three offshore buoys, the root-mean-squared differences for the swath data, with both rain and winds less than 6 m s21 removed, were 1.0 m s21 and 158 based on 1920 comparisons. Although the satellite’s scientifically processed swath data near shore do not match buoy observations as closely as those offshore, they are sufficiently accurate for many coastal studies.

Beginning in July 1999 the Orange County Health Care Agency began to post and close large sections of beach located in the City of Huntington Beach, California due to bacterial contamination. Extensive studies to date have yet to definitively identify the source or sources for these bacteria. One proposed hypothesis is that the Orange County Sanitation Districts (District) submerged, offshore wastewater plume is the source. The hypothesis states that during the summer when the water column is most stratified the wastewater plume was being transported toward shore whereupon it then interacted with a local electrical power generation station's intake and/or thermal discharge plume to cause the observed chronic shoreline microbiological contamination. In response to this hypothesis, the District implemented an intensive, multi-faceted, multi-agency research study during the spring to fall 2001 to study both high and low frequency cross-shelf oceanographic transport processes on the San Pedro Shelf that could bring the plume toward shore.

The Innovative Coastal-Ocean Observing Network (ICON) is a partnership of government, academic, and industrial entities funded by the National Ocean Partnership Program (NOPP). Its goal is to bring together modern measurement technologies, to develop new technologies, and to integrate them within a data assimilating coastal ocean circulation model. The major components of the observing network include: (1) surface current maps from shore-based high frequency (HF) radar installations; (2) subsurface currents, temperature, salinity, and bio-optical properties plus surface meteorological properties from several deep-ocean moorings; (3) sea surface temperature and color from satellites; and (4) along-track temperature and temperature variances from two acoustic tomography slices through the region. The model performance is quite good at seasonal time scales, which is a validation of the one-way nesting because these variations are successfully tracked by the PWC regional-scale model. At higher frequencies, the model does not reproduce the observed level of variability. For the a long shore currents down to 150 meter depth, data assimilation resulted in greater correlation between modeled and observed currents.

Data from two shipboard experiments in 1994, designed to observe the semidiurnal internal tide in Monterey Canyon, reveal semidiurnal currents of about 20 cm s−1, which is an order of magnitude larger than the estimated barotropic tidal currents. The kinetic and potential energy (evidenced by isopycnal displacements of about 50 m) was greatest along paths following the characteristics calculated from linear theory. These energy ray paths are oriented nearly parallel to the canyon floor and may originate from large bathymetric features beyond the mouth of Monterey Bay. Energy propagated shoreward during the April experiment (ITEX1), whereas a standing wave, that is, an internal seiche, was observed in October (ITEX2). The difference is attributed to changes in stratification between the two experiments. Higher energy levels were present during ITEX1, which took place near the spring phase of the fortnightly (14.8 days) cycle in sea level, while ITEX2 occurred close to the neap phase. Further evidence of phase-locking between the surface and internal tides comes from monthlong current and temperature records obtained near the canyon head in 1991. The measured ratio of kinetic to potential energy during both ITEX1 and ITEX2 was only half that predicted by linear theory for freely propagating internal waves, probably a result of the constraining effects of topography. Internal tidal energy dissipation rate estimates for ITEX1 range from 1.3 × 10−4 to 2.3 × 10−3 W m−3, depending on assumptions made about the effect of canyon shape on dissipation. Cross-canyon measurements made during ITEX2 reveal vertical transport of denser water from within the canyon up onto the adjacent continental shelf. ONR N0001495WR30022 ONR N000149310403 ONR N0001497WR30009

Time-varying depth-dependent currents of semidiurnal frequency are identified in data collected over the northern California continental shelf during the 1982 up welling season as part of the Coastal Ocean Dynamics Experiment. An increase in semidiurnal band kinetic energy is seen during wind relaxation periods when stratification over the shelf and upper slope increases. Comparison with internal wave theory shows the vertical and horizontal structure of the semidiurnal band horizontal current and temperature fluctuations to be consistent with a first baroclinic mode internal wave with a horizontal wavelength of 20–30 km. Over the continental shelf the semidiurnal band temperature fluctuations show a phase shift which may be evidence of the influence of the subcritical bottom slope and/or the effects of bottom friction. The bottom slope is supercritical over the upper continental slope, suggesting that generation of the semidiurnal internal waves observed over the shelf may take place near the shelf break. Application of Baines' (1982) theory for the generation of internal tides underpredicts the observed baroclinic tidal currents.

The focus of this dissertation is on the description and dynamical interpretation of the tidal band current fluctuations over the continental shelf off northern California. The term "tidal band" is used here to denote fluctuations with periods from about one-half to one day, including all the major diurnal and semidiurnal tidal constituents. The semidiurnal frequency is super-inertial, and the diurnal frequency sub-inertial, at this mid-latitude location. Kinetic and potential energy are strongly peaked at the diurnal and semidiurnal frequencies. Although inertial currents are occasionally observed, particularly during the winter when internal wave energy in general is elevated in this locale, they do not contribute significantly to the current variance. Consequently, the treatment here is divided into discussion of the diurnal and semidiurnal variability. Each chapter emphasizes a process which can cause the tidal currents to deviate from what would be anticipated based solely on observations of sea level. In Chapter II, the diurnal current variability is discussed, and the role played by atmospheric forcing is examined in detail. In Chapter III, the barotropic semidiurnal tidal currents over the shelf are described, and the effect of small-scale bumps in the coastline is evaluated. The baroclinic semi diurnal tidal currents, which are dependent upon the local time-varying hydrographic conditions, are examined in Chapter IV.

As part of the Coastal Ocean Dynamics Experiment (CODE), meteorological instruments were deployed on buoys and at coastal stations and instrumented aircraft flights and coastal soundings were made to study the three-dimensional structure of the marine boundary layer over the continental shelf off northern California during the 1981 and 1982 upwelling seasons. These measurements show that after the atmospheric spring transition the airflow in the marine layer is dominated by the North Pacific high, and the surface wind field over the shelf is characterized by periods of strong (7–15 m/s), upwelling-favorable alongshelf winds lasting for up to 30 days, interrupted by shorter periods of much weaker winds directed either equatorward or poleward. These periods of weak or reversed winds typically last several days and are called wind relaxations, even though they are primarily associated with coastally trapped perturbations of the marine layer along the central and northern California coast and not with a large-scale weakening of the North Pacific high. The atmospheric boundary layer measurements made in CODE suggest a simple conceptual model which can explain much of the physiology or structure of the marine layer and associated surface wind field during periods of persistent upwelling-favorable winds. During these periods, which represent the quasi steady state regime during the upwelling season, the inversion base of the marine layer drops eastward toward the coast until it intersects the coastal mountain range at a height of several hundred meters, and the associated thermal wind produces an alongshelf wind jet which has a maximum speed just below the inversion base. Turbulent mixing tends to homogenize any stratification in the marine layer beneath the jet and couple the jet to the ocean surface, producing strong upwelling-favorable winds over the shelf. Day/night heating/cooling over the narrow coastal strip beneath the marine layer generates a weak cross-coast secondary circulation which causes the core of the alongshelf jet to drop in elevation and shift onshore. This diurnal change in the marine layer structure explains both the daytime acceleration of the surface winds observed over and near the coast and its offshore decay and the associated offshore increase in the subdiurnal alongshelf wind. Thus the quasi-steady component of the wind stress has a significant curl over the inner shelf during periods of active upwelling. This mean summer atmospheric boundary layer regime is occasionally interrupted by synoptic and/or mesoscale events or anomalous conditions. Analysis of the CODE observations suggests five types of events, two primarily synoptic-scale conditions which lead to stronger-than-normal upwelling-favorable winds over the shelf and three primarily mesoscale events which lead to wind relaxation. About half of the wind relaxation events observed in 1981 and 1982 are believed to be associated with either coastal-trapped gravity currents or internal Kelvin waves which propagate north-ward in the marine layer along the central and northern California coastal mountain range.

Current measurements obtained from a sparse array of moorings deployed over the slope and the adjacent deep-sea basin off northern California for 4 to 14 months have been analyzed for their tidal constituents. The M2 tidal currents were the strongest tidal constituent, containing 50% to 75% of the total current variance in the four major tidal bands. The semimajor axis of the M2 constituent ranged from 2 to 4 cm s−1 and tended to be aligned with the local topography. Internal tides, which were usually present in the M2 band, were often the dominant cross-slope current component on the middle slope. At least a portion of the internal tidal current field had a steady phase that slowly changed over 1 to 2 months. The semimajor axes of the diurnal barotropic ellipses had amplitudes ranging from 0.7 to 1.2 cm s−1 and were oriented both parallel and perpendicular to the local topography. These tidal current measurements suggest that the barotropic diurnal currents propagate poleward as a combination of Kelvin and continental shelf waves and that the barotropic M2 tides propagate poleward primarily as a Kelvinlike wave. Battisti and Clarke's (1982) model, which uses coastal sea level data to predict M2 tidal currents, agrees reasonably well with the observations presented here, indicating that the model can be used to predict the M2 currents seaward of the middle slope off Northern California.

Diurnal period wind stress and current fluctuations measured over the continental shelf off northern California during the Coastal Ocean Dynamics Experiment in the summers of 1981 and 1982 are examined. The diurnal currents are strongly surface intensified, with amplitudes of up to 20 cm s−1. At depths of less than 35 m, the diurnal currents are nonstationary, and their amplitude is well correlated with the amplitude modulation of the local diurnal wind stress. Beneath the surface layer, the diurnal current vectors rotate clockwise with ellipse semi-major axes of 1 to 3 cm s−1. A mixed layer model driven with diurnal wind stress and surface heat flux produces currents similar to those observed in the upper water column. The deeper currents are consistent with the combination of a freely propagating Kelvin and first-mode coastal-trapped wave and, over the inner shelf, with the interior flow forced by the diurnal wind stress in the presence of a coast.