Regional modeling of internal-tide dynamics around New Caledonia. Part 2: Tidal incoherence and implications for sea surface height observability

Abstract. New Caledonia, in the southwestern tropical Pacific, has recently been identified as a hot spot for energetic semidiurnal internal tides. In a companion paper, the life cycle of coherent internal tides, characterized by fixed amplitude and phase, was investigated in the regions through harmonic analysis of a year-long, hourly time series from numerical simulation output. In this study, we investigate temporal variability of the internal tide by decomposing the semidiurnal signals into coherent and incoherent components. We show that the departure from coherence, in both generation and propagation of internal tides, is linked to the presence of mesoscale eddies and attempt to identify the underlying mechanisms. Our results suggest that temporal variability of the barotropic-to-baroclinic energy conversion is largely governed by the coherent component (>90 %) through the astronomically forced fortnightly modulated spring-neap cycle, which induces biweekly conversion variations by a factor between 3 and 7. The incoherent component – negligible in the annual mean – can explain on monthly to intraseasonal scales a notable fraction of variability while reducing or enhancing semidiurnal conversion by up to 20 %. We find that it is dominated by local effects such as the work of the barotropic tide on baroclinic bottom pressure amplitude variations, and linked to mesoscale-eddy-induced stratification changes. Away from the generation sites, tidal incoherence becomes increasingly important, manifesting as refraction and altered orientation of tidal beams due to interactions with mesoscale currents and varying group and phase speeds. The incoherent sea surface height signature, with a root-mean-square amplitude of 1–2 cm, is widespread across the domain. In tidal beam propagation direction, this incoherent component introduces limitations in the spectral observability of mesoscale to submesoscale dynamics below 80 km wavelength. Along altimetry tracks that are not aligned with the predominant propagation direction, the incoherent tide tends to dominate over the coherent tide due to its more isotropic nature. Consequently, transition scales estimating the wavelength at which balanced motion becomes dominant over unbalanced motion should be interpreted with caution in regions with pronounced internal-tide activity and well-defined propagation directions.