Characterizing Non-phase-locked Tidal Currents in the California Current
System using High-frequency Radar
Abstract
Over nine 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 M$_2$ constituent
and 181 days for S$_2$, 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 M$_2$ $\pm$ 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 by 150 km, comparable to the width of the swath of the recently
launched Surface Water and Ocean Topography (SWOT) altimeter.