Temporal variability of Multimodal Internal Tides at the East China Sea Shelf

Internal tide variations and mixing properties are important to shelf dynamics and mass exchange. In the present study, spatiotemporal variability of internal tides and their modulation factors on the southern East China Sea (ECS) shelf are examined using a three-month mooring observation. Semidiurnal and diurnal internal tides are found to exhibit distinct varying trends. Specifically, the semidiurnal internal tides are quite weak at the early stage, but greatly enhanced in the last three spring-neap cycles. In contrast, the diurnal internal tides follow quasi spring-neap variability except for the strengthening in two specific periods. The enhancement of semidiurnal internal tides in late July and August can be attributed to the strengthened stratifications shelf-slope area northeast of Taiwan Island, which is identified as the generation source. While the diurnal internal tides are modulated by background circulation through the effective critical latitude. The weak critical latitude effect corresponds to the intermittent enhancement of diurnal internal tides in two specific periods. In addition, the circulation also affects the vertical modal structures of the internal tides. The proportion of higher modes internal tides increases during robust eddy activities.  The high-frequency and high-mode internal tides are of crucial significance for turbulent mixing on the shelf region.

Key word: Internal Tides; Mooring Observation; Spatiotemporal variation; Shelf dynamics