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.

This article reviews the discovery, development, and use of high-frequency (HF) radio wave backscatter in oceanography. HF radars, as the instruments are commonly called, remotely measure ocean surface currents by exploiting a Bragg resonant backscatter phenomenon. Electromagnetic waves in the HF band (3-30 MHz) have wavelengths that are commensurate with wind-driven gravity waves on the ocean surface; the ocean waves whose wavelengths are exactly half as long as those of the broadcast radio waves are responsible for the resonant backscatter. Networks of HF radar systems are capable of mapping surface currents hourly out to ranges approaching 200 km with a horizontal resolution of a few kilometers. Such information has many uses, including search and rescue support and oil-spill mitigation in real time and larval population connectivity assessment when viewed over many years. Today, HF radar networks form the backbone of many ocean observing systems, and the data are assimilated into ocean circulation models.

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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 Ϫ1 , 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 ϫ 10 Ϫ4 to 2.3 ϫ 10 Ϫ3 W m Ϫ3 , 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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