Journal Article

Phase‐Accurate Internal Tides in a Global Ocean Forecast Model: Potential Applications for Nadir and Wide‐Swath Altimetry

Shriver, Jay F., Thakur, Ritabrata, Nelson, Arin D., Zaron, Edward D., Arbic, Brian K., Buijsman, Maarten C., Yadidya, Badarvada

2024

Overview

Internal tides (ITs) play a critical role in ocean mixing, and have strong signatures in ocean observations. Here, global IT sea surface height (SSH) in nadir altimetry is compared with an ocean forecast model that assimilates de‐tided SSH from nadir altimetry. The forecast model removes IT SSH variance from nadir altimetry at skill levels comparable to those achieved with empirical analysis of nadir altimetry. Accurate removal of IT SSH is needed to fully reveal lower‐frequency mesoscale eddies and currents in altimeter data. Analysis windows of order 30–120 days, made possible by the frequent (hourly) outputs of the forecast model, remove more IT SSH variance than longer windows. Forecast models offer a promising new approach for global internal tide mapping and altimetry correction. Because they provide information on the full water column, forecast models can also help to improve understanding of the underlying dynamics of ITs. Plain Language Summary Tidal flow over topographic features on the seafloor generates vertical displacements along the interfaces of ocean layers that have different densities. These vertical displacements at tidal frequencies are known as internal tides. Internal tide displacements are largest well below the sea surface, but also display a sea surface height (SSH) signature that is large enough to be measured by satellite altimeters. Removing internal tide signals from satellite altimeter SSH allows for a more accurate accounting of non‐tidal features, including slowly evolving ocean currents and eddies, that are also measured by altimeters. Here, we show that supercomputer ocean forecast simulations of the global internal tide field are able to remove internal tide SSH from satellite altimeter measurements with a skill level that is comparable to the skill of internal tide SSH removal based upon analysis of the satellite altimeter data itself. Thus, forecast models offer a complementary method for this important task. In addition, forecast models provide information on the entire ocean water column, not just the sea surface. Finally, the hourly outputs of forecast models allow for a greater variety of tidal analysis record lengths than can be achieved with altimeter outputs, which report sea surface height fields much less frequently. Key Points Global ocean forecast models can accurately simulate both long‐term (phase‐locked) internal tides and their short‐term modulations Ocean forecast models offer a useful complement to empirical models for mapping internal tides and correcting altimetry for internal tides In regions of strong internal tides, optimal variance reduction in nadir altimetry is attained through short‐term tidal analyses (∼60 days)

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Altimeters / Altimetry / Analysis / Eddies / Empirical analysis / Forecasting models / global ocean forecast model / HYCOM / Interfaces / internal tide corrections / internal tide mapping / Internal tides / Mathematical models / Mesoscale eddies / nadir altimetry / Ocean currents / Ocean floor / Ocean mixing / Oceans / Physiographic features / Removal / Satellite altimetry / Satellites / Sea surface / Seawater / Tidal analysis / Tidal currents / Tidal dynamics / Tidal flow / Tides / Vortices / Water circulation / Water column