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Robert L. Haney, Robert A. Hale, and Curtis A. Collins

Abstract

A method for extending upper ocean density observations to the deep ocean is tested using a large number of deep CTD (conductivity-temperature-depth) stations in the California Current. The specific problem considered is that of constructing the best estimate for the density profile below a certain depth D given an observed profile above that depth. For this purpose, the estimated disturbance profile is modeled as a weighted sum of empirical vertical modes (E0Fs). The EOFs are computed from the surface to 2000 m, using 126 largely independent CTP stations off Point Sur, California. Separate computations are made for the summer half-year (mid-April to mid-October) and the winter half-year (mid-October to mid-April). For each observed density profile. the EOF weights that determine the estimated profile are obtained by performing a successive least-squares fit of the disturbance density profile above D to the first N EOFs. In this study, N is taken to be 7, which is the number of EOFs that account for the “signal” in the profiles as determined by the methods of Preisendorfer et al. and Smith et al. The estimated profiles are then verified against the observed profiles to 2000 m, and the results are presented as a function of the depth D.

In general, the vertical extension method is moderately successful at estimating density fluctuations at and below 500 m from data entirely above 500 m. Observed density profiles to depths shallower than 500 m can he extended to 500 m, with a correlation that depends on the time of year as well as on the depth of the observed profile. For example, a minimum of 200 m of data is needed to perform a useful extension to 500 m, and in all cases extensions are more successful in winter than in summer. As might be expected, correlations between the estimated profiles and a seven-mode reconstruction of the observed profiles, representing the “signal” part of the observed profiles, are somewhat higher. The dynamic height of the sea surface relative to 500 m, an important integral quantity, can be estimated quite well with only 300 m of data. A practical result of this study is that data down to only 200 or 300 m, as might be acquired by a SeaSoar CTD survey, can be extended to 500 m or more using the EOF-based method with a known and useful level of skill. Tests with a small sample of independent data confirm the above results. The success of the method is attributed to the fact that in this part of the ocean the dominant EOFs represent variability in the upper ocean that is also reflected at deeper depths.

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Christopher N. K. Mooers, Curtis A. Collins, and Robert L. Smith

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 warm water mass occurred at the base of the frontal layer. We believe it was formed near the surface front and that it sank and flowed seaward along the base of the inclined frontal layer. Vertical shears in the horizontal velocity were caused by the mean baroclinic flow and the tidal and longer period baroclinic oscillations. A zone of low dynamic stability was produced near the base of the inclined frontal layer, coincident with the warm anomaly, providing a mixing mechanism for the erosion of the warm anomaly and the broadening of the frontal layer offshore. Estimates of temporal and spatial scales and of horizontal eddy viscosity coefficients are given. Internal tidal motions provided an energy flux to the mean motion. A conceptual model is presented for the mean state (averaged over a fortnight or, equivalently, over one or more upwelling “wind event cycles”) of coastal upwelling.

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Newell Garfield, Curtis A. Collins, Robert G. Paquette, and Everett Carter

Abstract

During the period 1992–95, nineteen isobaric RAFOS floats, placed in the California Undercurrent at intermediate depths (150–600 m) off Monterey and San Francisco, California, reveal a region of varying width of subsurface, poleward flow adjacent to the continental margin. The float trajectories exhibit three patterns: poleward flow in the undercurrent; reversing, but predominately alongshore, flow adjacent to the continental margin;and, farther offshore, anticyclonic motion accompanied by slow westward drift. Flow continuity of the undercurrent exists between Pt. Reyes and at least Cape Mendocino with an average speed dependent on the float depth. Speeds were variable, but common features were acceleration occurring to the south of Pt. Arena and deceleration to the north of Cape Mendocino. An important mechanism for floats, and water, to enter the ocean interior from the undercurrent is through the formation of submesoscale coherent vortices.

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