Leslie K. Rosenfeld

Bifurcated flow from an upwelling center: a cold water source for Monterey Bay

Abstract 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 Bays 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.

Remotely sensed surface currents in Monterey Bay from shore-based HF radar (Coastal Ocean Dynamics Application Radar)

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.

Observations of the Internal Tide in Monterey Canyon

Data from two shipboard experiments in 1994, designed to observe the semidiurnal internal tide in Monterey Canyon, reveal semidiurnal currents of about 20 cm s21, 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 3 1024 to 2.3 3 1023 Wm 23, 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.

QuikSCAT satellite comparisons with nearshore buoy wind data off the U.S. West Coast

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 s 21, root-mean-squared differences (satellite minus buoy) for swath data, the best of the three datasets, were 1.4 m s 21 and 378 based on 5741 nearshore comparisons. By removing winds less tha n6ms 21, these differences were reduced to 1.3 m s 21 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 s 21 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.

Distribution and transport of suspended particulate matter in Monterey Canyon, California

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.

High resolution modeling and data assimilation in the Monterey Bay area

Abstract A high resolution, data assimilating ocean model of the Monterey Bay area (ICON model) is under development within the framework of the project “An Innovative Coastal-Ocean Observing Network” (ICON) sponsored by the National Oceanographic Partnership Program. The main objective of the ICON model development is demonstration of the capability of a high resolution model to track the major mesoscale ocean features in the Monterey Bay area when constrained by the measurements and nested within a regional larger-scale model. This paper focuses on the development of the major ICON model components, including grid generation and open boundary conditions, coupling with a larger scale, Pacific West Coast (PWC) model, atmospheric forcing etc. Impact of these components on the Models predictive skills in reproducing major hydrographic conditions in the Monterey Bay area are analyzed. Comparisons between observations and the ICON model predictions with and without coupling to the PWC model, show that coupling with the regional model improves significantly both the correlation between the ICON model and observed ADCP currents, and the ICON models skill in predicting the location and intensity of observed upwelling events. Analysis of the ICON model mixed layer depth predictions show that the ICON model tends to develop a thicker than observed mixed layer during the summer time, and while assimilation of sea surface temperature data is enough for development of observed thin mixed layer in the regional larger-scale model, the fine-resolution ICON model needs variable heat fluxes as surface boundary conditions for the accurate prediction of the vertical thermal structure. The paper targets researchers involved in high-resolution numerical modeling of coastal areas in which the dynamics are determined by the complex geometry of a coastline, variable bathymetry and by the influence of complex water masses from a complicated hydrographic system (such as the California Current system).

Moored observations of the current and temperature structure over the continental slope off central California: 1. A basic description of the variability

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.

Barotropic semidiurnal tidal currents off northern California during the Coastal Ocean Dynamics Experiment (CODE)

Barotropic semidiurnal tidal currents measured off the coast of northern California during the Coastal Ocean Dynamics Experiment (CODE) are examined. While the pressure field is consistent with the idea that the semidiurnal surface tide is dominated by a Kelvin wave, a high degree of variability over alongshore distances of the order of 25 km is observed in the velocity field. Comparison with existing models used to predict tidal velocities from sea level measurements cannot account for this spatial structure. Perturbation analysis of a Kelvin wave propagating along a coastal boundary with bumps characterized by an alongshore length scale much less than the Rossby radius of deformation shows effects on the velocity and pressure field which decay offshore with the alongshore scale of the bumps. The effect on the velocity field exceeds that on the pressure field by a factor equal to the ratio of the Rossby radius to the alongshore scale of the bumps. We conclude that the alongshore structure observed in the measured barotropic semidiurnal tidal currents may be due in part to the local variations in the coastline geometry in the CODE region.

Numerical simulations of the internal tide in a submarine canyon

Abstract 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 earths 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.

Light scattering and extinction in a highly turbid coastal inlet

Multifactor regression analysis was used to test for relationships between chemical, physical and optical properties of the water column in the organically rich, highly turbid waters of Floridas Fort Pierce Inlet. Optical measurements were made at three visible light wavelengths (445 nm, 542 nm and 630 nm). Scattering by suspended particulate material was found to be the primary optical mechanism controlling downwelling irradiance at all three wavelengths. Larger particles showed constant scattering efficiencies of 2 when their diameters exceeded 3 to 5 microns, depending upon the wavelength used for observation. Selective absorption had a definite effect on the transmission of radiant energy in the 445 nm wavelength range. High correlation between extinction at 445 nm and the cross-sectional area of the suspended particulate material indicates particulate, rather than dissolved materials, are the major water column constituents that selectively absorb short wavelength radiant energy in this inlet. Spectral distribution of the downwelling radiant energy field was found to shift dramatically over a period of several months. These shifts in downwelling spectral irradiance were attributed to seasonal and/or event related shifts in concentrations of selectively absorbing compounds within the water column.