Role of mesoscale eddies in modulating the internal tide: observation results in the northern South China Sea

The role of mesoscale eddies in modulating the semidiurnal internal tide (SIT) in the northern South China Sea (SCS) is examined using the data from a cross-shaped mooring array. From November 2013 to January 2014, an anticyclonic and cyclonic eddy (AE and CE) pair crossed the westward SIT beam originating in Luzon Strait. Observations showed that, due to the current and stratification modulations by the eddy pair, propagation speed of the mode-1 SIT sped up (slowed down) by up to 0.7 m/s (0.4 m/s) within the AE’s (CE’s) southern portion. As a result of the spatially varying phase speed, the mode-1 SIT wave crest was clockwise-rotated (counterclockwise-rotated) within the AE (CE) core, while it exhibited convex and concave (concave and convex) patterns on the southern and northern peripheries of the AE (CE), respectively. In mid-to-late November, most of the mode-1 SIT energy was refracted by the AE away from the Dongsha Island toward the north part of the northern SCS, which resulted in enhanced internal solitary waves (ISWs) there. Corresponding to the energy refraction, responses of the depth-integrated mode-1 SIT energy to the eddies were generally in phase at the along-beam-direction moorings, but out-of-phase in the south and north parts of the northern SCS at the cross-beam-direction moorings. From late-December to early-January, intensified mode-2 SIT was observed, whose energy was likely transferred from the mode-1 SIT through eddy-wave interactions.