Directional Decomposition of Internal Tides from Satellite Altimetry.

A new global altimetry dataset (HRET) was recently published by Zaron (2019). This global dataset reveals multiple generation sites for internal tides, resulting in complicated interference patterns that make it difficult to track the propagation and evolution of individual tidal beams. To resolve the individual beams, we used a Hilbert Transform (HT) filter or complex demodulation technique. The total signal was decomposed into 4 sectors propagating in the NW, NE, SW and SE directions.

The north-easterly region of the Indian Ocean was selected as an example to demonstrate the application of the HT filter method for directional decomposition (Fig. 1). The analysis clearly demonstrates for the first time, that internal tides generated in the Lombok Strait, Indonesia, propagate in a SE direction (10³) km with only slight energy loss and thus influence the internal tide climatology on the Australian North West Shelf (NWS). In contrast, locally-generated and offshore propagating internal tides from the southern NWS lose energy within ~250 km of their generation sites. The rate of energy lost from the internal tide during propagation and an energy decay length scale can be quantitatively estimated from the HT filter method. In addition to the NE Indian Ocean example, we also applied the HT technique to the Tasman Sea, the Bay of Bengal and the South China Sea. All sites have energetic internal tides and the method provides a clear view of both semidiurnal and diurnal internal tides propagating in different directions. In summary, this new HT directional decomposition technique enables important insights into complex internal tide dynamics both regionally and globally.