Assimilating SWOT Observations into OTIS for Improved Regional Tidal Modeling in the Canadian Arctic Archipelago

Recent work by Hart-Davis et al. (2024) has demonstrated that sea surface height anomalies (SSHAs) derived from SWOT (Surface Water and Ocean Topography) satellite measurements capture coherent tidal signals across a range of complex coastal environments, including estuaries and marginal seas. In addition, SWOT orbit is designed to minimize theoretical ocean tidal aliasing. These measurements provide a new opportunity to incorporate SWOT observations into regional tidal modeling frameworks to improve tide prediction accuracy and spatial resolution in challenging nearshore environments. Building on this foundation, we propose to extend the OTIS (Oregon State University Tidal Inversion Software) framework to assimilate SWOT SSHAs alongside conventional satellite altimetry and in situ tide gauge measurements. Our study focuses on the Canadian Arctic Archipelago (CAA), characterized by narrow straits, fjords, complex bathymetry, and a highly modulated tidal regime. The CAA represents a particularly challenging environment for tidal modeling due to sparse in situ observations, sea ice variability, and limitations of classical along-track altimetry in coastal zones. The SWOT wide-swath capabilities offer unprecedented spatial coverage in these regions, providing an opportunity to improve tidal model fidelity significantly. SWOT Level-2 SSH products will be processed to isolate the barotropic tidal signal before data assimilation. Isolating the tidal signal involves removing non-tidal variability through atmospheric pressure corrections, internal tide removal based on empirical models, and spatiotemporal filtering designed to suppress mesoscale and submesoscale noise. Harmonic analysis will be performed to extract major tidal constituents, including M2, S2, K1, and O1 using OTIS. SWOT-derived tidal amplitudes and phases will then be mapped onto the OTIS representer framework, treating each SWOT observation as a data constraint linked to the model solution at corresponding grid locations. The OTIS inversion will be adapted to accommodate SWOT measurements' higher density and twodimensional spatial structure. Modifications will be made to the representer weighting and observational error covariance structure to reflect SWOT's variable noise characteristics, particularly near complex coastlines and regions of variable land/sea ice contamination. Assimilation will proceed via a least-squares inversion that balances observational fit with hydrodynamic consistency, as enforced by OTIS's linearized shallow water equations. The resulting SWOT-enhanced tide model will be validated with tide gauge records and vertical displacement measurements from continuous GPS stations within the CAA. GPS-based estimates of tidal loading effects offer an independent means of validating the modeled ocean tide signal, particularly in regions where direct tide gauge observations are limited or seasonally unavailable. Comparisons will also be made against existing tidal models that assimilate conventional altimetry data, such as TPXO9v5 and FES2014b, to assess improvements attributable specifically to SWOT assimilation. SWOT ground tracks converge at high latitudes and frequently overlap between swath crossovers throughout the CAA. This spatial redundancy enhances tidal estimation by providing multiple independent observations of sea surface height anomalies at closely spaced locations and varying phases of the tidal cycle. Such overlap improves signal-to-noise through ensemble averaging, increases spatial resolution in narrow straits and fjords, and strengthens harmonic analysis of major tidal constituents. It also provides more robust constraints for the OTIS inversion framework, allowing better resolution of localized tidal dynamics in regions where conventional altimetry is degraded. These advantages are especially critical in the CAA, where sharp tidal gradients and limited in situ observations challenge existing modeling approaches. Based on the findings of Hart-Davis et al. (2024) and the increased spatial coverage afforded by SWOT, we anticipate that incorporating SWOT SSHAs into OTIS will reduce coastal tidal misfits and better resolve small-scale spatial variability in the CAA relative to models constrained solely by alongtrack altimetry. We expect improvements in narrow channels and near complex coastal boundaries, where traditional satellite altimetry lacks sufficient resolution to resolve key dynamical features. This work aims to demonstrate the potential of wide-swath altimetry to transform regional tidal modeling in polar regions, with implications for scientific understanding of Arctic oceanography and operational forecasting in regions vulnerable to tidal forcing and sea ice variability.

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