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.

We investigate the impact of ocean data assimilation using the Ensemble Adjustment Kalman Filter (EAKF) from the Data Assimilation Research Testbed (DART) on the oceanic and atmospheric states of the Red Sea. Our study extends the ocean data assimilation experiment performed by Sanikommu et al. (2020) by utilizing the SKRIPS model coupling the MITgcm ocean model and the Weather Research and Forecasting (WRF) atmosphere model. Using a 50-member ensemble, we assimilate satellite-derived sea surface temperature and height and in-situ temperature and salinity profiles every three days for one year, starting January 01 2011. Atmospheric data are not assimilated in the experiments. To improve the ensemble realism, perturbations are added to the WRF model using several physics options and the stochastic kinetic energy backscatter (SKEB) scheme. Compared with the control experiments using uncoupled MITgcm with ECMWF ensemble forcing, the EAKF ensemble mean oceanic states from the coupled model are better or insignificantly worse (root-mean-square-errors are 30% to -2% smaller), especially when the atmospheric model uncertainties are accounted for with stochastic perturbations. We hypothesize that the ensemble spreads of the air–sea fluxes are better represented in the downscaled WRF ensembles when uncertainties are well accounted for, leading to improved representation of the ensemble oceanic states in EAKF. Although the feedback from ocean to atmosphere is included in this two-way regional coupled configuration, we find no significant effect of ocean data assimilation on the latent heat flux and 10-m wind speed, suggesting the improved skill is from downscaling the ensemble atmospheric forcings.

Waves generated by tropical cyclones can have devastating effects on coastal regions. However, the role of ocean currents in modifying wave amplitudes, wavelengths, and directions is commonly overlooked in wave forecasts, despite the fact that these interactions can lead to extreme wave conditions. Here, we use satellite observations and wave modeling to quantify the effects of ocean currents on the surface waves generated during a tropical cyclone event in the Arabian Sea. As a case study, this paper documents beams of wave heights originating from the eyewall of a tropical cyclone caused by current-induced refraction. Alternating regions of high and low wave heights in the model simulations are consistent with observations and extend for thousands of kilometers all the way to 100 m isobath. Our results highlight the importance of accounting for wave refraction by currents in order to accurately predict the impact of tropical cyclone generated waves on coastal regions.