Mapping Internal Tides From Satellite Altimetry Without Blind Directions

Satellite altimetry is one practical technique for observing internal tides on the global scale. However, it is a great challenge to extract weak internal tide signals. This paper presents a new technique for mapping internal tides from satellite altimeter data. Along‐track high‐pass filtering is needed to remove long‐wavelength nontidal noise and the barotropic tidal residual; however, the filter also removes internal tides having large angles with respect to satellite ground tracks. It thus causes blind directions in mapping internal tides from satellite altimetry: Generally west‐east propagating internal tides are missed. The new technique addresses the blind‐direction issue by replacing the problematic one‐dimensional (1‐D) high‐pass filter with a two‐dimensional (2‐D) band‐pass filter. This mapping technique is able to retrieve ubiquitous westbound and eastbound internal tides not captured in previous estimates. Long‐range westbound and eastbound waves travel over thousands of km from numerous generation sites such as the Emperor seamount chain, the Hawaiian Ridge, and the Kermadec trench. Evaluation using independent Cryosat‐2 data reveals that the new internal tide model may reduce more SSH variance than a model built in 2016 does in regions of strong internal tides. However, this mapping technique makes no improvement in strong boundary current regions, due to the dominance of mesoscale motions. Moreover, the new internal tide model contains leaked noise from westward propagating tropical instability waves (TIWs), which can be suppressed by prior along‐track high‐pass filtering. This paper suggests that better internal tide models may be constructed using both 1‐D and 2‐D filters with optimized parameters.