Daniel M. Fernandez

Measurements of upper ocean surface current shear with high‐frequency radar

Estimates of the variation with depth of both the speed and direction of near-surface ocean currents within the top meter have been made using time series data collected from a dual-frequency, high-frequency radar located on the California coast, south of Monterey. Long-term averages derived from this data set over 3 different years consistently revealed an expected reduction in the speed of the near-surface current as a function of depth, consistent with a logarithmic vertical current profile. An unexpected and previously unmeasured clockwise rotation with increasing depth of the near-surface current was also observed. Hypotheses are suggested for the apparent rotation.

Detecting upwelling along the central coast of California during an El Niño year using HF-radar

Abstract Surface currents in the vicinity of Granite Canyon, California (36°25.9′N, 121°55.0′W), were measured hourly using HF-radar in 1990–1992. The 1990 data revealed the M 2 and S 2 semi-diurnal tidal constituents. These high frequency components were removed from 6-month records taken during part of the upwelling season of 1991 and 1992. Daily and weekly variations in current speed and direction were generally similar in 1991 and 1992 even though 1992 was an El Nino year. Correlation analysis revealed that in both years daily and weekly variation in currents were mostly explained by corresponding changes in the alongshore component of the wind stress, indicating the effects of coastal upwelling. Variation in sea surface temperatures adjacent to the coast were correlated with the currents generated by coastal upwelling in 1991, but not in 1992. These observations are consistent with the hypothesis that during an El Nino event the water is upwelled to the surface from above a depressed thermocline. In 1992, at Granite Canyon, normal coastal upwelling was superimposed upon the El Nino event of that year.

Computation of ripple wave parameters: A comparison of methods

Remote sensing of oceanic and atmospheric quantities, such as the waveheight spectrum and wind velocity, depend largely on the interaction of electromagnetic radar waves with ocean surface waves in the short gravity and capillary range. The propagation of such ripple waves upon the ocean surface is strongly affected by the presence of surface films which constitute the ocean microlayer. In order to understand the impact of surface films on oceanic and atmospheric remote sensing, it is necessary to have an accurate and, if possible, convenient model relating film properties with the propagation and damping of ripple waves. This study examines two different methods of obtaining the damping coefficient of short gravity and capillary waves. One method, developed by Bock and Mann, utilizes numerical techniques based on the modified Levich characteristic equation to accurately yield complex roots governing the propagation of Laplace transverse and Marangoni longitudinal waves on the air-sea interface. The other method, developed by Cini and Lombardini, involves finding an approximate analytical solution of the Levich equations using a perturbational technique. A comparison of the two methods yields tolerance ranges of various parameters, including frequency, modulus of surface dilational viscosity, and modulus of surface dilational elasticity, over which the two methods agree to 5–10%. To make our comparison concrete, we apply both methods to a specific surface film sample collected by Garret. Both theoretical results are compared to experimental results.

Upwelling and relaxation events inferred from high-frequency radar measurements of coastal ocean surface currents

High-frequency radar offers oceanographers a powerful yet convenient means of measuring coastal ocean currents and other air/sea interaction variables from land-based sites. The paper addresses the potential such a system offers for detecting the physically significant coastal circulation events known as coastal upwelling and relaxation. A high-frequency radar was in operation from April, 1990 until September, 1992 on the California Coast (USA) overlooking the Pacific Ocean near Monterey. Periods of months of uninterrupted radar operation have allowed the production of an unparalleled data set which includes radial ocean surface currents measured at five angles and eight range bins. Correlation of low-pass-filtered current measurements with shore-based measurements of sea surface temperature and buoy-measured wind speeds shows evidence of coastal upwelling and relaxation events.<<ETX>>