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Jeffrey D. Paduan and Pearn P. Niiler

Abstract

In October 1987, 49 Lagrangian surface drifters (TRISTAR-II) were released in a 200-km × 200-km square area southeast of Ocean Station Papa as part of the OCEAN STORMS Experiment. The drifters measured temperature at the drogue level and reported their position through ARGOS approximately 11 times per day. Thirty-one of the drifters retained drogues for longer than three months, and data from those instruments are used to describe the evolving fall 1987 pattern of current and temperature structures at 15 m in the area between 46° and 49°N, 142°W and 132°W. Time variable currents were dominated by mesoscale eddies of anticyclonic rotation with horizontal radii of 53–86 km and rotational speeds of 10–20 cm s−1. These eddies persisted for at least 90 days as evidenced by successive drifter trajectories through the eddies. Currents with periods longer than 1 day had a mean to the east of 4.4 cm s−1 and a mean to the north of 0.7 cm s−1. Background eddy kinetic energy levels were 40 cm s−2. Thus, eddy kinetic energy was four times larger than mean kinetic energy. The eastward single particle diffusivity was 1100 m2 s−1 and northward diffusivity was 1600 m2 s−1. The local change of thermal energy at 15-m depth was −2.9 W m−3, while on average, flow advected cold water to the east at a rate of 0.8 W m−3. Therefore, large-scale advective processes accounted for 28% of the thermal energy balance at 15 m. This horizontal heat convergence in the open ocean is comparable in magnitude to that produced by powerful equatorial currents in the eastern Pacific cold tongue.

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Pearn P. Niiler and Jeffrey D. Paduan

Abstract

Analysis is presented of the time-dependent motion of 47 surface drifters in the northeast Pacific during fall 1987 and 16 drifters in fall and winter 1989/90. The drifters were designed at 15-m depth and were designed to have wind-produced slips less than 2 cm s−1 for wind speeds up to 20 m s−1. The coherence of velocity and local wind is presented for motions with periods between 1 day and 40 days. For periods between 5 and 20 days, drogue motion at 15-m depth is found to be highly coherent with local wind with an average phase of 70° to the right of the rotating wind vector. These results differ from analyses of FGGE-type drifters as reported by McNally et al. and Niiler in the same area. A model of wind-produced slip as a function of drifter design is used to provide a possible explanation of the differences. A linear regression, which accounts for 20%–40% of the current variance, gives water motion al 0.5% of wind speed and 68° to the right of the wind vector. Assuming an Ekman-type balance, this regression with 15-m currents yields an apparent mixing depth of 34–38 m. which is much less than the observed 60-m depth of the mixed layer. New three-parameter models for turbulent stress are presented based on these observed depth scales and regression coefficients. The model stresses rotate from downwind to crosswind at the base of the mixed layer. The model currents rotate from approximately 60° to the right of the wind vector at the surface to 180° to the right of the wind vector at the mixed layer base.

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Kenneth Laws, Jeffrey D. Paduan, and John Vesecky

Abstract

A simulation-based investigation of errors in HF radar–derived, near-surface ocean current measurements is presented. The simulation model is specific to Coastal Ocean Dynamics Application Radar (CODAR) SeaSonde radar systems that employ a compact, collocated antenna geometry. In this study, radial current retrievals are obtained by processing simulated data using unmodified CODAR data processing software. To avoid limiting the results to specific ocean current and wind wave scenarios, the analyses employ large ensembles of randomly varying simulated environmental conditions. The effect of antenna pattern distortion on the accuracy of retrievals is investigated using 40 different antenna sensitivity patterns of varying levels of distortion. A single parameter is derived to describe the level of the antenna pattern distortion. This parameter is found to be highly correlated with the rms error of the simulated radial currents (r = 0.94) and therefore can be used as a basis for evaluating the severity of site-specific antenna pattern distortions. Ensemble averages of the subperiod simulated current retrieval standard deviations are found to be highly correlated with the antenna pattern distortion parameter (r = 0.92). Simulations without distortions of the antenna pattern indicate that an rms radial current error of 2.9 cm s−1 is a minimum bound on the error of a SeaSonde ocean radar system, given a typical set of operating parameters and a generalized ensemble of ocean conditions.

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Emil T. Petruncio, Leslie K. Rosenfeld, and Jeffrey D. Paduan

Abstract

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.

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Igor Shulman, Steven H. D. Haddock, Dennis J. McGillicuddy Jr., Jeffrey D. Paduan, and W. Paul Bissett

Abstract

Bioluminescence (BL) predictability experiments (predictions of the intensity, depth, and distance offshore of the BL maximum) were conducted using an advective–diffusive tracer model with velocities and diffusivities from a fine-resolution model of the Monterey Bay, California, area. For tracer initialization, observations were assimilated into the tracer model while velocities and diffusivities were taken from the hydrodynamic model and kept unchanged during the initialization process. This dynamic initialization procedure provides an equilibrium tracer distribution that is balanced with the velocity and diffusivity fields from the hydrodynamic model. This equilibrium BL distribution was used as the initial BL field for 3 days of prognostic calculations. Two cross-shore surveys of bioluminescence data conducted at two locations (north of the bay and inside the bay) were used in four numerical experiments designed to estimate the limits of bioluminescence predictions by tracers. The cross-shore sections extended to around 25 km offshore, they were around 30 m deep, and on average they were approximately 35 km apart from each other. Bioluminescence predictability experiments demonstrated a strong utility of the tracer model (combined with limited bioluminescence observations and with the output from a circulation model) in predicting (over a 72-h period and over 25–35-km distances) the location and intensity of the BL maximum. Analysis of the model velocity fields and observed and model-predicted bioluminesence fields shows that the BL maximum is located in the frontal area representing a strong reversal of flow direction.

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