Characterizing Non-Phase-Locked Tidal Currents in the California Current System Using High-Frequency Radar

Over 9 years of hourly surface current data from high-frequency radar (HFR) off the US West Coast are analyzed using a Bayesian least-squares fit for tidal components. The spatial resolution and geographic extent of HFR data allow us to assess the spatial structure of the non-phase-locked component of the tide. In the frequency domain, the record length and sampling rate allow resolution of discrete tidal lines corresponding to well-known constituents and the near-tidal broadband elevated continuum resulting from amplitude and phase modulation of the tides, known as cusps. The FES2014 tide model is used to remove the barotropic component of tidal surface currents in order to evaluate its contribution to the phase-locked variance and spatial structure. The mean time scale of modulation is 243 days for the M2 constituent and 181 days for S2, with overlap in their range of values. These constituents' modulated amplitudes are significantly correlated in several regions, suggesting shared forcing mechanisms. Within the frequency band M2 +/- 5 cycles per year, an average of 48% of energy is not at the phase-locked frequency. When we remove the barotropic model, this increases to 64%. In both cases there is substantial regional variability. This indicates that a large fraction of tidal energy is not easily predicted (e.g., for satellite altimeter applications). The spatial autocorrelation of the non-phase-locked variance fraction drops to zero over a distance of 150 km, a scale that is comparable to the swath width of the Surface Water and Ocean Topography altimeter. Tides in the ocean encompass both the highly predictable daily changes in sea level seen from the shore as well as a less predictable component that changes over time depending on seasonal conditions and wind. Tidal signals are detectable at the ocean surface, for example, via satellite or land-based radar antenna. The time-evolving tide signals interact with other processes in the ocean, like currents, and can become harder to predict and describe. Many studies have examined this aspect of tides. In this work, we use land-based radar observations of ocean currents off the US West Coast to examine this process with high detail, using mathematical techniques to separate the tides from everything else and then evaluating how much the tide has been altered by other processes. This is useful because ocean-observing satellites can observe a single part of the ocean only when the satellite passes overhead, approximately every 10-20 days, while the data we analyze are sampled hourly and thus more easily allow us to draw conclusions about how tides behave. In the California Current System, High-Frequency Radar resolves stationary and non-stationary tides Broadband cusps occur at tidal constituent frequencies, with M2 exhibiting annual modulation We analyze the tidal modulation to assess physics governing non-stationary tidal variability