Christopher N. K. Mooers

Several effects of a baroclinic current on the cross‐stream propagation of inertial‐internal waves†

Several effects of a baroclinic current on inertial‐internal waves at constant frequency are investigated, primarily through use of the method of characteristics. The special case of waves propagating transverse to a baroclinic current is considered. When the slope of an isopycnal is of the same order of magnitude as the slope of the characteristics, appreciable asymmetries are induced in the characteristics, the phase and group velocities, and the solution itself. These asymmetric effects are especially significant for waves at the low frequency end of the passband for free waves. Also, modifications occur to the passband, resulting in anomalously high and low frequency bands. The effective local inertial frequency, σf = [f(f+vx )]1/2, separates the normal and anomalously low frequency bands. Hence, the low frequency limit of the normal frequency band increases or decreases depending upon whether the horizontal shear in the mean flow is cyclonic or anticyclonic. In the anomalous frequency bands, the slop...

A technique for the cross spectrum analysis of pairs of complex-valued time series, with emphasis on properties of polarized components and rotational invariants☆

Abstract Methods are derived for the cross spectrum analysis of pairs of two-dimensional velocity vectors or pairs of complex-valued time series in general. These methods are based on a decomposition of the complex-valued series into polarized components. The advantages include: (1) the invariance of coherence under coordinate rotation, and (2) the calculation of coherence and phase for polarized components; i.e. clockwise and anticlockwise components. A (phasor) correlation coefficient as a function of frequency is derived for a pair of complex-valued series. This coefficient includes contributions from both polarized components. The methods may be implemented either by the cross spectrum analysis of a pair of complex-valued series or by combinations of quantities ordinarily computed in the cross spectrum analysis of a set of four real-valued series.

Observation and Simulation of Storm-Induced Mixed-Layer Deepening

Abstract Two observed cases of mixed-layer (ML) deepening due to storms are analyzed and simulated. The primary goal is to learn whether the relevant scale velocity in the parameterization of wind-driven deepening is U*, the wind stress friction velocity, or δV, the magnitude of the horizontal mean velocity difference across the base of the ML. ML deepening is isolated from air–sea exchange and horizontal advection by diagnosing the entrainment tendency of ML temperature. ML deepening is found to be highly intermittent on the storm time scale. Deepening in response to a wintertime atmospheric cold front occurred as δV was accelerated during the initial rise in wind stress. Deepening abruptly ceased as wind stress began to decelerate δV, though the stress magnitude continued to increase. A similar relationship between wind stress δV and ML deepening was also observed in a summertime case and is evidence that the relevant scale velocity is δV not U*. In both cases the observed phase and extent of ML deepeni...

The Dynamic Structure of the Frontal Zone in the Coastal Upwelling Region off Oregon

Abstract We studied the frontal zone of the coastal upwelling region off Oregon, from observations made in two successive years. The measurements were made between July and September in 1965 and 1966. The alongshore flow field was determined by combining direct measurements and geostrophic calculations. A near-surface southward jet and a subsurface northward undercurrent existed in the frontal zone. They were separated by an inclined frontal layer (permanent pycnocline). The frontal layer tended to intersect the sea surface about 10 km offshore, where a surface front was formed. Through a combination of direct current measurement and water mass analysis, the cross-stream flow was estimated to be seaward near the surface, shoreward at the top of the inclined frontal layer, but seaward at the bottom of the inclined frontal layer and shoreward below that. During a 25 h anchor station, a high degree of correlation existed between the vertical structure of the alongshore and cross-stream flows. An anomalously ...

Coastal-Trapped Waves in a Continuously Stratified Ocean

Abstract The theory of coastal-trapped waves is extended to include the general features of continuous density stratification, variable bottom topography, and a finite coastal wall. In the limit of a vanishing coastal wall, topographic Rossby waves are the only class of sub-inertial frequency, trapped wave motion. The stratification effect on topographic Rossby waves depends on both the local baroclinic radius of deformation and the characteristic offshore length scale of the wave motion. For intermediate density stratification, long waves are nearly depth-independent in the shelf region, and are bottom-trapped in the slope region. The topographic Rossby waves reduce to the barotropic shelf waves and the bottom-trapped waves in the limits of small and large density stratification, respectively. In the general case of comparable influences from the coastal wall and bottom slope effects, baroclinic Kelvin waves and topographic Rossby waves are eigenmodes of the system. The eigenfunctions are modified from t...

The space‐time structure and variability of the shelf water‐slope water and Gulf Stream surface temperature fronts and associated warm‐core eddies

The space-time structure and variability of three thermal features on the surface of the Northwest Atlantic—the shelf water-slope water front; the Gulf Stream front; and warm-core, anticyclonic eddies— were examined between September 1, 1975, and August 31, 1977, using weekly satellite-derived charts of surface temperature fronts. The temporal and spatial correlation scales of both the shelf-slope and Gulf Stream fronts were about 2 weeks and 80 km, respectively. The rms amplitude of both fronts increased from about 25 to 80 km in the first 1,000 km northeastward from Cape Hatteras. The dominant Gulf Stream meanders had wavelengths of about 320 km and periods of 7–8 weeks, and they propagated downstream jit a speed of about 6 cm/s. The warm-core eddies averaged about 100 km in diameter and propagated southwestward from Georges Bank at an average speed of 6 cm/s; they decreased hi diameter from about 120 to 90 km and increased in speed from about 4 to 7 cm/s in moving from the Georges Bank-to-Hudson Canyon subdomain to the Hudson Canyon-to-Cape Charles subdomain. Some large Gulf Stream meanders induced perturbations of the shelf-slope front. Eddies forced seaward perturbations of the front, which propagated southwestward with the eddies. (There were more rapidly propagating disturbances which also moved southwestward along the shelf-slope front; they may have been due to coastally trapped waves.) There was significant interannual variation in the mean position of the Gulf Stream front and the number and intensity of warm-core eddies shed by the Stream, though the correlation scales of its front did not change. Consequently, there were substantial interannual variations in the inferred entrainment of shelf waters by warm-core eddies. Furthermore, the correlation scales of the shelf-slope front had interannual variation. The results of this study agree with earlier, more limited estimates, and they extend the statistical analyses to the space-time continuum of long waves and meanders with the aid of wave number-frequency spectra and empirical orthogonal functions. These analyses quantify the broadbanded nature of the various perturbations and their interactions. Thus a holistic view of the principal surface thermal features of the Northwest Atlantic, and their interactions, is provided.

Meanders of the Gulf Stream Downstream from Cape Hatteras 1975–1978

Abstract Weekly Gulf Stream paths within 1000 km downstream of Cape Hatteras were obtained for 1975–78 from the Navys weekly EOFA charts based on satellite IR imagery. They displayed two dominant meander modes: first, a standing meander energetic over periods between 4 months and at least 4 years; and second. down-stream-propagating meanders that were most energetic at periods of several weeks. The long-period standing meander was confined between nodes located at the separation point near Cape Hatteras (i.e., where the Streams mean path turns seaward) and at a point about 600 km farther downstream. The rms amplitude was 36 km at the antinode. The amplitude of propagating meanders increased rapidly in the first 200 km downstream of the separation point, where the capture of warm-core eddies was common. Farther down-stream, the predominant meanders had a wavelength averaging 330 km, a period averaging 1.5 month, a phase speed averaging 8 cm s−1, a downstream group speed averaging 17 cm s−1, and downstrea...

Dynamical Interpolation and Forecast of the Evolution of Mesoscale Features off Northern California

Abstract The series of cruises off Northern California comprising OPTOMA11, during two months in summer 1984, were specifically designed as an ocean prediction experiment. In addition to a regional survey from Cape Mendocino to Monterey, six surveys were made of a (150 km)2 domain offshore of Pt. Arena/Pt. Arena/Pt. Reyes. During the initial phase (over about ten days) of OPTOMA11, an intense (speeds up to 50 cm s−1, relative to 450 m) jet/cyclone system propagated offshore at about 5 km day−1. The subsequent evolution (over about 40 days) of the streamfunction field was governed by the meandering of the jet and the associated changes in the intensity of the anticyclonic region to the north of the jet and the cyclonic region to the south. From quasi-geostrophic (QG) model hindcast experiments using the streamfunction data, wind stress curl was an important forcing mechanism in the later phase of the experiment. Forecast in a domain extending over the continental slope were in agreement with objective anal...

Prediction of the Dispersal of Oil Transport in the Caspian Sea Resulting from a Continuous Release

Abstract A 3-D hybrid flow/transport model has been developed to predict the dispersal of oil pollution in coastal waters. The transport module of the model takes predetermined current and turbulent diffusivities and uses Lagrangian tracking to predict the motion of individual particles (droplets), the sum of which constitute a hypothetical oil spill. Currents and turbulent diffusivities used in the model have been generated by a numerical ocean circulation model (Princeton ocean model) implemented for the Caspian Sea. The basic processes affecting the fate of the oil spill are taken into account and parameterized in the transport model. The hybrid model is implemented for a simulated continuous release in the coastal waters of the Caspian Sea. The potential of the model for the prediction of the advective and turbulent transport and dispersal of oil spills is demonstrated.

Dynamical Forecasting and Dynamical Interpolation: An Experiment in the California Current

Abstract In order to perform real-time dynamical forecasts and hindcasts, three high-resolution hydrographic surveys were made of a (150 km)2 domain off northern California, providing two sets of initialization and verification fields. The data was objectively analyzed and regularly gridded for model compatibility. These maps initially show an anticyclonic eddy segment in the northeast and part of another in the northwest. Two weeks later only the northwest anticyclonic eddy remained, with the domain center dominated by a 0.6 m s−1 jet. Two weeks after that only a larger northwest eddy with fairly weak velocities remained. Numerical forecasts with persistent boundary conditions and forecast experiments with boundary conditions linearly interpolated between surveys were performed. The real-time forecast successfully predicted the formation of the zonal jet prior to its observation. Dynamical interpolation shows unambiguously that the two anticyclonic eddies have merged and formed a single eddy. Even the fo...